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Disease - Deciduous Woody Ornamental, Fact Sheet

Eastern Filbert Blight

What is Eastern filbert blight?  Eastern filbert blight is a potentially serious fungal disease found throughout the United States, including Wisconsin.  It affects only Corylus species, commonly known as hazelnuts or filberts.  On hazelnuts native to Wisconsin such as American hazelnut (Corylus americana) and beaked hazelnut (Corylus cornuta), the disease causes little significant damage, but on the commonly grown European hazelnut (Corylus avellana), including Harry Lauder’s walking stick (Corylus avellana ‘Contorta’), the disease is lethal.  Turkish filbert (Corylus colurna) also appears to be highly susceptible.

Eastern filbert blight can cause small black cankers that form in rows, or deep gouges in the bark of severely infected trees/shrubs.
Eastern filbert blight can cause small black cankers that form in rows, or deep gouges in the bark of severely infected trees/shrubs.

What does Eastern filbert blight look like?  Eastern filbert blight causes cankers (i.e., dead, collapsed areas of bark) on branches or main trunks.  Easily visible within the cankers are black, football-shaped stromata (the reproductive structures of the causal fungus).  The stromata often form in rows of two.  Cankers first appear on new twigs and expand over time.  American hazelnut trees/shrubs are able to live almost indefinitely with Eastern filbert blight, forming a small number of slowly-expanding cankers (if any cankers form at all) that lead to limited branch dieback.  On European hazelnut however, cankers will expand anywhere from one inch to three feet in a year, and can eventually form long, deep gouges or grooves on severely affected trees/shrubs.  European hazelnuts typically die due to girdling from Eastern filbert blight within five to 10 years.

Where does Eastern filbert blight come from?  Eastern filbert blight is caused by the fungus Anisogramma anomala.  Stromata formed by the fungus produce spores that are spread short distances by water splash and over longer distances by wind.  Humans also can spread Anisogramma anomala on their hands and clothing, on gardening tools, and by transporting wood from infected trees/shrubs.  Unlike other canker fungi that can infect through wounds, the Eastern filbert blight fungus primarily infects through immature tissue on actively growing shoots.  Cankers appear 12 to 18 months after infection.  Eastern filbert blight does not affect hazelnut leaves, fruits or nuts.

How do I save trees/shrubs with Eastern filbert blight?  There is no cure for Eastern filbert blight.  If only a few branches on a tree/shrub are affected, prune these branches two to three feet below each canker.  Disinfest tools after each cut by dipping them for at least 30 seconds in a 10% bleach solution or (even better) a 70% alcohol solution.  Alternatively, use a spray disinfectant containing roughly 70% ethanol, spraying tools until they drip and then allowing them to air dry.

If a tree/shrub is severely affected by Eastern filbert blight (e.g., when there are so many cankers on multiple branches that the tree/shrub would look ugly if pruned, when branch pruning would require removing part of the trunk, or when trunk cankers are present), removal of the tree/shrub is the preferred management strategy.

Pruned branches and removed trees/shrubs should be burned (where allowed by local ordinance), deep-buried, or chipped (as long as the chips are allowed to dry to kill the fungus).

How do I avoid problems with Eastern filbert blight in the future?  Consider planting native species of hazelnut (e.g., American and beaked hazelnut) that are naturally resistant to the disease.  If you decide to plant European hazelnut, select cultivars that have been bred for resistance.  ‘Jefferson’, ‘Santiam’, ‘Yamhill’, and ‘Theta’ are resistant, nut-bearing cultivars.  ‘Red Dragon’ is a resistant, ornamental cultivar.  Note that these cultivars are not hardy in all hardiness zones in Wisconsin.  Hybrid hazelnuts (crosses between American and European hazelnut) are becoming increasingly available, but should be used with caution because their susceptibility to Eastern filbert blight has not been adequately tested.

Once hazelnut trees/shrubs are established in your yard, routinely inspect the plants for infection and remove infected branches as they occur.  Watch for dying branches in the summer and cankers (particularly on or near the youngest growth) in the winter.  Inspecting trees during the winter is very important, because cankers are more visible at that time.

Fungicides can be used for management, but should only be used as a last resort.  Not all fungicides that are approved for Eastern filbert blight control are particularly effective, but chlorothalonil has been shown to be an effective preventative treatment, although it will not cure existing infections.  Note that not all formulations of chlorothalonil are approved for use on nut-bearing hazelnuts; many formulations can only be used on ornamental hazelnuts.  Therefore, if you decide to use chlorothalonil, be sure to select the appropriate formulation for your particular situation.  Apply the first treatment at bud break (i.e., when half the buds show a separation of leaves) and additional treatments (up to three) every two weeks thereafter.  If you plan to eat nuts from your hazelnut tree, make sure that your last fungicide treatment is applied at least 120 days before anticipated nut harvest.  For further details about recommended fungicides, spray rate recommendations, and diagrams of bud stages, see “Pest Management Guide for Hazelnuts in the Willamette Valley”, Oregon State University Extension Bulletin EM8328 available at https://catalog.extension.oregonstate.edu/em8328.

For more information on Eastern filbert blight:  Contact your county Extension agent.

Fact Sheet

Cane Blight

What is cane blight? Cane blight is a fungal disease that affects the health of canes (i.e., stems) of cultivated and wild Rubus species (e.g., raspberries and blackberries), wherever they are grown. Black and purple raspberries appear to be more susceptible to cane blight than red raspberries, but all commonly cultivated raspberry cultivars can get the disease. Although cane blight is not typically fatal, it may cause significant fruit yield losses if left unmanaged.

Cane death on a thornless blackberry caused by cane blight. Look for a dark brown infection line and dead/dying shoots above the point of infection. (Photo courtesy of Michael Ellis, The Ohio State University)
Cane death on a thornless blackberry caused by cane blight. Look for a dark brown infection line and dead/dying shoots above the point of infection. (Photo courtesy of Michael Ellis, The Ohio State University)

What does cane blight look like? Symptoms of cane blight usually first appear in early summer after blossoming and leaf emergence, and in association with wounds caused by pruning or harvesting of fruit. Look for sudden death of side branches and tips of fruit-bearing canes, as well as dark brown or purple spots (called cankers) on the canes below the dieback. In wet weather, cankers may produce a grey ooze. In dry weather, the cankers may appear fuzzy or powdery. Dead canes may become brittle and snap off in windy conditions.

Where does cane blight come from? Cane blight is caused by the fungus Leptosphaeria coniothyrium which survives the winter in infected canes. During wet periods, the fungus produces windborne spores and infects through open wounds on canes caused by pruning, harvest damage, insect damage, or abrasions from canes rubbing together. As the disease develops, the fungus produces additional spores that can spread to other wounds by wind and splashing water, leading to additional infections.

How do I save plants with cane blight? DO NOT prune infected canes during the growing season as pruning wounds will provide the cane blight fungus easy entry into healthy tissue. Label symptomatic canes as you see them and prune the canes to the ground during the dormant season (i.e., mid- to late winter). Also prune any older cane stubs at this time to remove them as a source of abrasion and wounding for newer canes. Pruning when plants are dormant allows ample time for wounds to close at a time of year when spores of the cane blight fungus are not being produced. Use only sharp tools for pruning, and disinfest pruning tools after each cut by dipping them for at least 30 seconds in 10% bleach or 70% alcohol (e.g., rubbing alcohol). Spray disinfectants that contain approximately 70% alcohol can also be used. Dispose of any canes that you prune by burning them (where allowed by local ordinance), deep burying them, or arranging to have them hauled away through municipal brush collection. Be patient as you attempt to get cane blight under control; it may take two or more years of pruning and good sanitation to reduce cane blight to negligible levels.

How can I prevent cane blight in the future? When establishing a new raspberry patch, choose a site that is well-drained and sunny, and make sure that the distance between rows is approximately 18 inches. Also be sure to keep weeds under control. Proper site selection, row spacing and weed control will promote good airflow and drainage, and will reduce excessive moisture that is favorable for spore production by the cane blight fungus.

In addition, maintain optimum soil fertility. If you underfertilize plants, they will produce weak canes that are less able to fend off infections by the cane blight fungus. If you overfertilize plants (particularly with nitrogen), they will produce succulent new growth that is more prone to breakage and more prone to wounding by certain insects that will use the new growth as food. Remember that wounds of any kind can provide entry points for the cane blight fungus. For details on properly fertilizing raspberries, see University of Wisconsin-Extension bulletin A1610, “Growing Raspberries in Wisconsin” (available at http://learningstore.uwex.edu/Growing-Raspberries-in-Wisconsin-P750.aspx).

Disease - Conifer, Fact Sheet

Cedar-Apple Rust – Juniper

What is cedar-apple rust? Cedar-apple rust is the name of a group of closely related diseases caused by fungi that infect both junipers and woody rosaceous plants such as apple, crabapple, hawthorn and quince.

Cedar-apple rusts form slimy, orange fruiting body on junipers in early spring.
Cedar-apple rusts form slimy, orange
fruiting body on junipers in early spring.

What does cedar-apple rust look like? On junipers, the cedar-apple rust fungus causes formation of irregularly-shaped brown galls (roughly 1∕2 to two inches in diameter). During moist periods in spring, these galls produce a distinctive orange, gelatinous slime. Symptoms on rosaceous hosts appear in late May as circular, yellow-orange areas on leaves. The undersurfaces of these diseased areas often have a fringed appearance.

Where does cedar-apple rust come from? Several fungi in the genus Gymnosporangium cause cedar-apple rust. These fungi overwinter as galls on junipers.

How do I save a tree or shrub with cedar-apple rust? Junipers can easily be treated for cedar-apple rust by pruning branches about four to six inches below the galls. Clean
pruning shears between cuts by dipping them for at least 30 seconds in a 10% bleach solution or alcohol (spray disinfectants that contain at least 70% alcohol can also be used). This will prevent movement of the fungus from branch to branch, or from plant to plan during pruning. To prevent future infections, you can apply fungicides containing triadimefon or ferbam every seven to 21 days from early July through August. You can control cedar-apple rust on rosaceous hosts using fungicides containing chlorothalonil, ferbam, dithiocarbamates, mancozeb, metiram, sulfur, thiram, triadimefon, triforine, or zineb. Apply treatments when flower buds first show color, when half of the flowers are open, at petal-fall, seven to 10 days after petal fall and again 10 to 14 days later. Be sure to read and follow all label instructions of the fungicide that you select to insure that you use the fungicide in the safest and most effective manner possible.

How do I avoid problems with cedar-apple rust in the future?  The best way to avoid cedar-apple rust is to plant trees and shrubs that are resistant to the disease. Check at your local nursery for resistant varieties of juniper, apple, crabapple, hawthorn, and quince that are available in your area and that will satisfy your landscaping needs.

For more information on cedar-apple rust:  See UW-Extension Bulletins A2598 and A8KS711 or contact your county Extension agent.

Fact Sheet, General Horticulture/Weeds

How to Properly Prune Deciduous Trees

Why should I prune my trees? Pruning is important for a variety of reasons. Pruning can help control the size of a tree, direct growth, influence flowering or fruiting, or maintain plant health and appearance. Pruning can also increase the safety of a tree by removing broken, diseased, dead, or dying branches. In addition to pruning, selecting plants that are suited to your environment and location are very important. The ultimate height and spread, in addition to location of overhead power lines, should be taken into account when selecting trees for landscaping.

The three step method of pruning large limbs.
The three step method of pruning large limbs.

What should I prune?

  • Newly planted trees: Newly planted trees should not be pruned unless a branch is broken, diseased or dead. These trees need foliage to produce carbohydrates (sugars) that are then transported to the root system for initiation of new roots.
  • Young trees: After a young tree is established for two to five years, the tree can be pruned to encourage a well-branched canopy. Lower branches can be removed to raise the canopy, if desired. Scaffold branches to be maintained in the tree should be selected such that they are 12-18 inches apart, are evenly distributed around the trunk and have wide crotch angles. Remove no more than 1∕3 of the total crown of a tree at one time. Young trees also need corrective pruning to remove crossing branches, double leaders, watersprouts, and root suckers.
  • Older trees: Older, established trees, if properly trained when young, require little pruning. These trees should never be topped as this leads to poor branch structure and increased limb breakage. Use the three-point method of limb removal for pruning large branches (see diagram above and description below). This method ensures proper pruning and closure of wounds. Contact a certified arborist to prune larger limbs and remove trees, particularly if the tree is close to power lines or buildings.

The 3-point method of proper pruning of large limbs

When doing any type of pruning, always use a sharp pruning saw for making pruning cuts. Also, be sure to disinfect your pruning tools with alcohol or a 10% bleach solution after each pruning cut to avoid spreading diseases.

  • Step one: Select the branch that you want to remove. On large limbs, the first cut should be 12 to 18 inches from the limb’s point of attachment. The pruning cut should be an undercut made 1∕2 way through the branch (see diagram). This pruning cut is very important because it relieves weight from the branch collar and prevents accidental tearing of bark from the tree’s trunk when the limb is removed.
  • Step two: The second pruning cut should be made on the outside of the first cut (i.e., farther from the trunk). Cut all the way through the limb from the top down, thus removing the weight of the branch.
  • Step three: The final cut should be made next to the tree’s trunk outside of the branch collar. Cut from the top down and cut all the way through the remaining branch stub. The branch collar should be left intact. DO NOT cut the branch flush with the tree’s trunk. A proper cut avoids large wounds, and allows the tree’s wound to close quickly.

Should I use wound treatments? In general, wound treatments, such as tree paint or wound dressing, are not recommended. These compounds slow down wound closure and promote decay. One exception when wound treatments are recommended, is the case of oak trees that are pruned during the growing season. Using wound treatments on oaks is important to keep out insects that transmit the oak wilt fungus (see University of Wisconsin Garden Facts X1075).

When should I prune trees? Most deciduous trees should be pruned in late fall to winter. At this time of year, you can see the overall branch structure easily, and most insects and disease causing organisms are not active. Late fall/winter pruning is especially important for oak trees to help prevent spread of the fungus that causes oak wilt (see University of Wisconsin Garden Facts X1075). Late spring and summer are usually not good times of year to prune because disease pathogens are present and wound closure is slower. If you prune in late winter, some trees may bleed or ooze sap excessively in the early spring. The bleeding may be unsightly, but does not harm the tree. Examples of trees that bleed excessively are maple, willow, birch, walnut, beech, hornbeam, elm, and yellowwood.

Definitions:

  • Branch collar: the ring of trunk tissue that surrounds a lateral branch at the point of attachment to the stem.
  • Double leaders: two major, terminal growing points located at the top of the tree.
  • Root suckers: vigorous, upright, adventitious shoots that arise from latent buds below the graft union or at the base of the tree.
  • Scaffold branches: the large branches that form the main structure of the crown of a tree.
  • Topping: an improper pruning technique that reduces the height of a tree by removal of large branches back to larger primary branches. This technique is not recommended.
  • Watersprouts: vigorous, vertical, adventitious shoots that arise from latent buds above the ground or graft union on older wood.

For more information on pruning: See UW-Extension bulletins A1817, A1771, A1730 and University of Wisconsin Garden Facts XHT1013, XHT1015, or contact your county Extension agent.

Fact Sheet, Insect, Pest Alerts

Viburnum Leaf Beetle

The viburnum leaf beetle (VLB), Pyrrhalta viburni, is an invasive insect that feeds exclusively on and can significantly damage Viburnum species. VLB is native to Europe and was detected in Canada in 1947. The first report of VLB in the United States was in New York State in 1996. VLB is now found scattered across much of the northeastern US. In Wisconsin, an isolated infestation of VLB was discovered in Dane County in 2009, but was successfully eradicated. In 2014, VLB was detected on a mature viburnum bush in northern Milwaukee County and other nearby infestations were detected in June 2015. At present, all active infestations of VLB in Wisconsin are in northern Milwaukee County and southern Ozaukee County.

Viburnum leaf beetles adults (left) and larvae (right). (Photos courtesy of Paul Weston, Cornell University, Bugwood.org)
Viburnum leaf beetles adults (left) and larvae (right). (Photos courtesy of Paul Weston, Cornell University, Bugwood.org)
Adult viburnum leaf beetle feeding damage (left) and egg-laying sites (right). (Photos courtesy of Paul Weston, Cornell University, and Bruce Watt, University of Maine; Bugwood.org)
Adult viburnum leaf beetle feeding damage (left) and egg-laying sites (right). (Photos courtesy of Paul Weston, Cornell University, and Bruce Watt, University of Maine; Bugwood.org)

Appearance: Adult VLB’s are approximately ¼ inch long and yellowish-brown in color. VLB larvae can be up to ⅓ inch long and range in color from yellowish-green to light brown with a series of black spots and dashes on their bodies.

Symptoms and Effects: VLB larvae chew holes in viburnum leaves in the spring creating a lace-like (i.e., skeletonized) pattern. VLB larvae feed individually or in small groups and can cause significant damage to viburnum shrubs. This damage can resemble the feeding damage of Japanese beetles (see University of Wisconsin Garden Facts XHT1062 “Japanese Beetle”). In late June and early July, VLB adults begin to feed, chewing oblong holes in leaves. Severe VLB infestations can cause complete defoliation of a viburnum shrub, which weakens the plant over time and can eventually lead to death.

Life Cycle: There is only one generation of VLB per year. VLB’s overwinter as eggs and development from eggs to adults takes approximately eight weeks. Larvae typically appear in early to mid-May and feed for several weeks, passing through three stages (instars) as they grow. In early to mid-June, larvae pupate in the soil and adults emerge by late June or early July. VLB females lay eggs during the summer and into October. They chew small pits in twigs, deposit five to eight eggs into each pit, and then cover the pits with tiny pieces of chewed wood to protect the eggs. Each female can deposit up to 500 eggs. Eggs remain in place through the winter until they hatch the following spring.

Control:

Cultural: When selecting viburnum plants for the landscape, DO NOT use arrowwood viburnum (Viburnum dentatum), European cranberrybush viburnum (Viburnum opulus), or American cranberrybush viburnum (Viburnum opulus var. americanum) as these types of viburnums are strongly preferred by VLB. Instead use resistant viburnums such as doublefile viburnum (Viburnum plicatum f. tomentosum), Judd viburnum (Viburnum x juddii), or Koreanspice viburnum (Viburnum carlesii). In addition, between October and the following spring, examine viburnums for twigs where VLB’s have laid their eggs. Prune and destroy these twigs to reduce VLB numbers. During the growing season encourage natural VLB predators in your area (e.g., lady beetles, spined soldier bugs, assassin bugs, green lacewings) that can reduce VLB numbers.

Chemical: Prior to bud break, apply horticultural oil to twigs where VLB eggs have been laid. This will significantly reduce the number of eggs that will hatch. Control any surviving larvae with contact insecticides such as acephate, bifenthrin, carbaryl, cyfluthrin, deltamethrin, lambda-cyhalothrin, and permethrin. Horticultural oil, insecticidal soap, pyrethrins and spinosad can also be effective. To achieve the best results, apply insecticides when larvae are small and before they have caused significant damage. VLB adults can be managed with contact insecticides, if needed, but are mobile and more challenging to control. Systemic products (e.g., clothianidin and imidacloprid) applied as soil drenches can also be effective, but apply these products after flowering (to minimize any risks to pollinators), but before VLB damage occurs to achieve the best protection.

For more information on viburnum leaf beetle: Contact your county Extension agent.

Fact Sheet, General Horticulture/Weeds

Zinnias

What are zinnias? Zinnias are one of America’s most popular annual bedding plants, but have humble origins as descendants of nondescript wildflowers native to the southwest U.S., Mexico and Central America. The zinnia was named after the 18th century German botanist Dr. Johann Gottfried Zinn, who wrote the first description of small, weedy plants that he discovered in the Mexican deserts. These plants had dull purplish-red, daisy-like flowers with single petals surroundng a protruding cone. Early varieties of garden zinnias were introduced in the U.S. in 1796, with double forms appearing in the mid-1800’s. Interest in zinnias increased in 1920 when Bodger Seeds Ltd. introduced the dahlia-flowered varieties ‘Giant Dahlia’ and ‘California Giant’. These varieties had large, flat-flowered heads and multiple colors. ‘California Giant’ eventually won a gold medal from the Royal Horticulture Society of England.

Zinnias can be a bright-colored addition to any garden, particularly when planted in mass.
Zinnias can be a bright-colored addition to any garden, particularly when planted in mass.

Today, zinnias come in a wide variety of flower forms: single, semidouble, or double. Single-flowered zinnias have one row of petals and the center of the flower is exposed. Semidouble-flowered zinnias have many rows of petals and the center can still be seen. Dahlia-type zinnias (typically semidouble) have large, flat blossoms. Double-flowered zinnias have so many rows of petals that flower centers are hidden. There are several types. Beehive-types have small blooms with stacks of flat petals resembling small beehives. Button-types are similar but have flatter flowers. Cactus-types have twisted, bent petals with rolled edges.

Although there are more than a dozen species of zinnias, only a few species are regularly planted in gardens. Zinnia elegans is the most common. It grows up to three feet tall and has single or double flowers in pink, rose, red, cherry, lavender, purple, orange, salmon, gold, yellow, white, cream or light green. Flowers range in size from one to seven inches in diameter and can be solid-colored, multicolored or zoned. Flower shapes include round, domed or ball-shaped, as well as dahlia-like or chrysanthemum-like. Tetraploid varieties (having four sets of chromosomes, rather than the normal two) were developed in the 1950’s, and these varieties produce larger flowers on stronger stems, grow more vigorously and have increased disease resistance. Hybrid varieties were developed soon thereafter. There are many varieties of Z. elegans available at garden stores, including the following.

  • The Border Beauty series has plants that grow up to 20 inches tall and have 3½-inch-wide semidouble to double, dahlia-like flowers.
  • The Peter Pan series has dwarf hybrids that grow up to 12 inches tall and have very large (up to five-inch-wide), slightly curled, double flowers. Seven separate colors in this series have been recognized as All-American Selections (AAS) winners.
  • The Ruffles series was developed for cut flower production. Plants grow up to 30 inches tall and have 2½-inch-wide, ball-shaped flowers with ruffled petals on stiff, upright stems. ‘Scarlet Ruffles’ was named an AAS winner in 1974, ‘Cherry Ruffles’ and ‘Yellow Ruffles’ AAS winners in 1978.

Z. angustifolia (synonym Z. linearis) has small, single, golden-orange flowers with yellow stripes, and narrower foliage than Z. elegans. These compact plants grow eight to 12 inches high, and can spread to two feet. The variety ‘Crystal White’ has pure white flowers with yellow centers and was an AAS winner in 1997. A cross of Z. elegans and Z. angustifolia yielded the Profusion series of zinnias which has compact plants with two to three-inch-wide, single flowers. Members of this series have superior heat and humidity tolerance, and tend to be disease resistant. Two members of this series ‘Cherry Profusion’ and ‘Orange Profusion’ won gold medals from AAS in 1999 (the first awarded to flowers in 10 years).

Z. haageana (Mexican zinnia) grows up to 18 inches tall and has small 1½ to two-inch-wide flowers on long stems. Flowers may be single or double, solid or bicolor, in red, mahogany, yellow and orange. Two popular varieties may be available at your local garden center.

  • ‘Persian Carpet’ (an AAS winner in 1952) grows up to 15 inches tall and has two-inch-wide, double, bicolored flowers of gold, maroon, purple, chocolate, pink or cream;
  • ‘Old Mexico’ (an AAS winner in 1962) has bushy, compact, 18-inch-tall plants with double, 2½-inch-wide blooms of deep, rich mahogany highlighted with yellow-gold.

Z. pauciflora (synonym Z. peruviana) grows up to 30 inches tall and produces 1½-inch-wide, single, red or yellow flowers with button-like centers. This zinna is good for cutting and drying, and has powdery mildew resistance. Only varieties ‘Bonita Red’ and ‘Bonita Yellow’ are readily available.

Zinneas can be effectively used in gardens as an edging plant.
Zinneas can be effectively used in gardens as an edging plant.

Where do I get zinnias? Zinnias can be purchased as bedding plants, but local garden centers often have only a limited selection of varieties. As an alternative, zinnias can be started from seed, either indoors four to six weeks before the last expected spring frost, or directly in the garden when the soil warms sufficiently. Germination takes five to seven days. Many references warn that zinnias do not like being disturbed and should not be moved after seeding. However, plants can be seeded individually in cells of seed-starting trays or even transplanted into these cells from mass plantings. When transplanting into trays, wait until the first true leaves have emerged. Separate roots as carefully as possible and place seedlings into their new cells up to the first set of leaves (the seedling leaves). Select appropriately sized tray cells based on the predicted size of the varieties being grown. Otherwise, tall varieties may outgrow their cells before it is time to plant them outdoors. Once the danger of frost has passed, zinnias can be transplanted outside. However, do not expect substantial growth until temperatures are above 50°F.

How do I grow zinnias? Zinnias do best in full sun in fertile, well-drained soil. Space plants four to 24 inches apart depending on variety. When growing zinnias for cut flowers, crowd plants to encourage longer stems. To produce bushier plants, pinch the tops out of plants when they are four to six inches high. Remove faded blossoms to encourage new blooms. Plants should be kept well-watered and fertilized two times per month for optimum bloom. Most varieties begin to bloom when very young and continue to bloom until frost.

Zinnias have few insect pests, but occasionally can have problems with aphids (see University of Wisconsin Garden Facts XHT1043), four-lined plant bugs (see University of Wisconsin Garden Facts XHT1101) and spider mites. Common diseases of zinnias include powdery mildew (see University of Wisconsin Garden Facts XHT1005), Alternaria leaf spot and bacterial leaf spot.

How do I use zinnias most effectively in my garden? Zinnias make good edging plants, but are also effective when grown in masses. Taller varieties make good background plants in flowerbeds, while smaller varieties are suitable for container plantings. Zinnias are great additions to butterfly gardens and many varieties make excellent cut flowers.

For more information on zinnias: Contact your county Extension agent.
Fact Sheet, Insect

Zimmerman Pine Moth

Zimmerman pine moth (Dioryctria zimmermani) was first detected in the US in 1879, and has subsequently been found and is established throughout the northern US east of the Rocky Mountains. Austrian and Scots pines are preferred hosts of Zimmerman pine moth. However Eastern white and mugo pines are also attacked.

Symptoms of Zimmerman pine moth. Tunneling by larvae in branch whorls leads to formation of masses of pitch (left). Sap from feeding sites often runs down branches and trunks (right). Left photo courtesy of the Minnesota Department of Natural Resources Archive, Minnesota Department of Natural Resources, Bugwood.org
Symptoms of Zimmerman pine moth. Tunneling by larvae in branch whorls leads to formation of masses of pitch (left). Sap from feeding sites often runs down branches and trunks (right). Left photo courtesy of the Minnesota Department of Natural Resources Archive, Minnesota Department of Natural Resources, Bugwood.org

Appearance: Adult Zimmerman pine moths are midsized with gray and red-brown wings, marked with zigzag lines. Larvae are generally dirty white to light grey and up to one inch long. They can only be found in pitch masses, under bark or in new shoots.

Symptoms and Effects: Zimmerman pine moth larvae tunnel into new growth causing shoot dieback, or into whorl areas causing masses of pitch to form at the wound site. Repeated attacks by the larvae cause a weakening at the area of the infestation and make the branches and trunk susceptible to breakage.

Life Cycle: Zimmerman pine moth has a one-year life cycle and spends the winter as a young caterpillar underneath bark scales of infested trees. In mid to late April, larvae become active and they migrate to the base of branches or shoots and burrow inside. Larvae continue to feed into July and then pupate within a chamber in a mass of pitch. Adult moths emerge from infested trees in late July and August, and lay eggs near wounds or preexisting masses of pitch. Eggs hatch in approximately one week and larvae feed for only a brief time before preparing to overwinter under bark scales.

Zimmerman pine moth pupa (left) and larva (right) embedded in masses of pine pitch.
Zimmerman pine moth pupa (left) and larva (right) embedded in masses of pine pitch.

Control: Avoid plant injury (e.g., construction injury) and environmental stresses (e.g., drought stress) that can make trees attractive to Zimmerman pine moth adults. Also, consider removing heavily infested trees before July to reduce the number of adults that will emerge in the vicinity of, and potentially infest, healthy trees. If opting to use insecticides to control Zimmerman pine moth, applications must be made in the spring, before larvae migrate into tree trunks. Preventive insecticide sprays should be applied as a drenching spray to trunks in mid to late April. Spraying branches and foliage is not necessary. Permethrin or bifenthrin are preventative sprays that are available for use by homeowners. Chlorpyrifos (sold as Dursban) can be used in nurseries and Christmas tree plantations. Sprays applied in early September to kill small larvae are not recommended as they tend to have only spotty success.

For more information on Zimmerman pine moth: Contact your county Extension agent.

Fact Sheet, General Horticulture/Weeds

Zebra Iris

What is zebra iris? Zebra iris (Iris pallida), also known as sweet iris, Dalmatian iris or variegated iris, is a very old garden plant. It is native to rocky areas of northern Italy and the eastern Mediterranean including Dalmatia, a province of Croatia (hence one of the plant’s common names). Zebra iris was one of the primary species used in the development of the tall bearded iris. Dried zebra iris root (along with the roots of other species of iris) is a source of orris root powder. This powder was used medicinally (and for its supposed magical and alchemical properties) in medieval times, as well as a perfume and potpourri fixative for many centuries. Roots may require several years of drying before fully developing their fragrance. Orris oil (derived from fresh roots) is used as a flavoring in soft drinks, candies and chewing gum.

Zebra iris produces clumps of sword-like leaves. Several varieties have striped foliage.
Zebra iris produces clumps of sword-like leaves. Several varieties have striped foliage.

Zebra iris produces low clumps of sword-like leaves that remain nearly evergreen in areas with mild winters, but die back to the ground in colder climates. The original color of the foliage of this species was a solid bluish-green. Newer cultivars of zebra iris have leaves with vertical stripes of blue-green and either silvery-white (varieties ‘Alba-variegata’ and ‘Argentea Variegata’) or creamy yellow to pale gold (varieties ‘Aurea-variegata’ and ‘Variegata’).

Zebra iris cultivars are grown primarily for their attractive striped leaves, although in early summer they do produce pretty, lavender-blue flowers with small, yellow beards on three-foot-tall scapes. The tall branched flower stems may need staking in windy, exposed areas. The highly fragrant flowers have a distinctive scent that has been variously described as reminiscent of grape jelly, orange blossom, or vanilla. Zebra iris is hardy in zones 4 through 9.

Where do I get zebra iris? Zebra iris plants can be purchased at local nurseries and garden centers. Once established, clumps should be divided as needed every three to four years after flowering has occurred, just as you would a bearded iris.

How do I grow zebra iris? Grow zebra iris in full sun and well-drained soil for best results. However, zebra iris will tolerate more shade than many bearded irises do, and will also thrive in heavy clay and medium clay loam soils. When planting zebra iris, place rhizomes partly above the soil and keep newly transplanted plants well-watered. Once established, zebra iris is somewhat drought tolerant and require little maintenance. Simply remove old foliage before new leaves emerge in early spring. Note that varieties with gold leaf coloring appear to be more vigorous than those with white leaf coloring.

Zebra iris has few pests. Rabbits and deer rarely bother this plant and iris borer (see University of Garden Facts XHT1041) is much less of a problem than on bearded iris.

Zebra iris produces lavender-blue flowers with small yellow beards.
Zebra iris produces lavender-blue flowers with small yellow beards.

How do I use zebra iris most effectively in my garden? The striking foliage of zebra iris makes a great accent in a sunny perennial border, especially near the front. The stiff upright form of zebra iris offers good contrast to mounded shapes, while the dramatic coloring of striped varieties stands out among solid-colored foliage, particularly purple-leaved plants. Interplant with low, open perennials or taller ramblers such as pincushion flower (Knautia macedonica). Zebra iris also pairs nicely with traditional perennials such as fern leaf yarrow (Achillea ‘Moonshine’), purple coneflower (Echinacea spp.) and ‘Husker Red’ beard-tongue (Penstemon ‘Husker Red’), and is a good companion with Arkansas bluestar (Amsonia hubrechtii). Or use zebra iris as a neat edging in larger plantings. Zebra iris is a natural around water and really stands out in rock gardens. It can also be stunning in suitable containers.

For more information on zebra iris: Contact your county Extension agent.

Fact Sheet, General Horticulture/Weeds

Yellow Corydalis

What is yellow corydalis? Yellow corydalis (Corydalis lutea = Pseudofumaria lutea) is a shortlived perennial in the bleedingheart family (Fumariaceae). The genus Corydalis has approximately 300 species, and the name derives from the Greek ‘korydalis’ meaning crested lark, a reference to the fact that the plants’ flowers resemble a lark’s head. Other common names of yellow corydalis include yellow fumitory, hollowort, and yellow larkspur. Native to the southern Alps of Europe, this plant is hardy in zones 4 through 8.

Yellow corydalis plants have finely cut foliage with yellow, tubular flowers.
Yellow corydalis plants have finely cut foliage with yellow, tubular flowers.

Yellow corydalis plants form neat mounds of finely cut, delicate-looking light-green to blue-green foliage, somewhat resembling the foliage of maiden hair ferns or bleeding hearts. The pinnately compound leaves have three lobes, and are pale green above and glaucous (i.e., waxy) below. The leaves are borne on weak, hollow and fleshy stems. Plants grow up to 18 inches tall, but often are much shorter.

In mild climates, yellow corydalis remains evergreen (although not necessarily attractive), but in climates with colder winters, dies back to the ground. In very hot summers or during drought, plants may also die back, but they often resume growth in the fall when cooler, moister conditions prevail. In Wisconsin’s relatively cool climate, when planted in a moist site, yellow corydalis does not die back at all. This is in contrast with the majority of other Corydalis species that are more easily heat-stressed and prone to dieback.

As its name suggests, yellow corydalis produces bright, golden-yellow flowers. It blooms over a long period, from late spring through frost. The one to two inch long flowers have four petals arranged in an irregular tubular shape with a spur in the back. Flowers are borne in racemes of six to 16 flowers on stems that rise above the foliage. Slender, dehiscent capsules (i.e., seed pods that naturally break open along a seam) follow the flowers. Capsules eventually burst to scatter their seed.

In addition to C. lutea, there are other Corydalis species with yellow flowers that are suitable for use in the garden.

  • C. aurea (hardy to zones 3 through 8) is known as scrambled eggs and is a biennial that produces small flowers in the spring of the second year of growth.
  • C. cheilanthifolia (hardy to zones 4 through 9) has fern-like leaves and upright, butter-yellow flower clusters. The leaves become bronze in the fall.
  • C. ochroleuca (hardy to zones 5 through 8) has blue-green leaves, with creamy yellow-white flowers with green lips and yellow throats.

While useful as an ornamental, yellow corydalis does have one potentially problematic characteristic. It is toxic to horses, causing mouth sores, gingivitis, colic and sudden death (if enough is ingested). Therefore, yellow corydalis should not be allowed to invade pastures.

Yellow corydalis makes an excellent border plant.
Yellow corydalis makes an excellent border plant.

Where do I get yellow corydalis? Yellow corydalis plants are available at many nurseries and garden centers that stock perennials. Once established in the garden, this plant tends to self-seed prolifically, and can become somewhat weedy in certain conditions, although it is easily removed where not wanted. Yellow corydalis commonly establishes around stone walls or in gravelly soil. Seedlings can be transplanted in early spring. However, established plants often do not perform well if moved. If you do transplant yellow corydalis, plants will require extra watering and will often struggle for the remainder of the season. Yellow corydalis plants are also not easily divided, but divisions can be made in early spring. Although it self-seeds readily, yellow corydalis is not easy to germinate indoors due to its complex dormancy requirements. To germinate the seeds, sow them in a moist potting medium, then place them in a plastic bag and keep at room temperature (approximately 70ºF) for six weeks. Move the seeds to near freezing conditions (28 to 38ºF) for six to eight weeks, then return to cool conditions (50 to 60ºF). If there is no germination after several weeks, repeat the heating and cooling steps a second time.

How do I grow yellow corydalis? Yellow corydalis grows well in either sun or light shade. It prefers well-drained soil and does best with good moisture during hot weather. However, it does not tolerate wet soils during winter. Yellow corydalis likes ordinary to rich, humusy soil, but tolerates drier, gravelly soil and sandy loam to clay as well. In heavier, wetter soils, shallow planting may help yellow corydalis survive Wisconsin’s harsh winters. Yellow corydalis is relatively insect pest and disease-free.

How do I use yellow corydalis most effectively in my garden? Yellow corydalis can be extremely attractive when used to edge borders or walkways. In rock gardens, cottage and woodland gardens, it is a good filler and may naturalize. It will grow well in stone walls in cool conditions. Yellow corydalis combines well with bleeding heart (Dicentra spectabilis), lady’s mantle (Alchemilla mollis), lungwort (Pulmonaria spp. – See University of Wisconsin Garden Facts XHT1174), foam flower (Tiarella spp.), hosta (Hosta spp.), leopard plant (Ligularia spp.), and many other perennials in a shade garden. Try yellow corydalis in sunny areas under taller plants like peonies (Paeonia spp.) or daylilies (Hemerocallis spp.).

For more information on yellow corydalis: Contact your county Extension agent.

Fact Sheet, General Horticulture/Weeds

Wood Mulch and Tree Health

What are the benefits of wood mulch? Wood mulch is typically available as chipped wood, or shredded or chunked bark, and can contribute to tree health in many ways. When high quality, composted mulches are applied two to four inches deep in a ring three to six feet in diameter (or greater) from the trunk of a tree, mulch can help preserve moisture, control weeds, limit damage to the trunk from mowers and string trimmers and moderate the soil temperature. Use four inches of mulch when soils are light and well-drained, and two inches of mulch on heavier, clay soils.

Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.
Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.

Can wood mulch harm trees? Use of improperly composted mulches (some-times called “sour mulches”), can lead to tree nutrient deficiencies. Sour mulches can also produce gases like methane and ammonia that can be toxic to plants. Foliage on trees surrounded by sour mulches may initially turn yellow, then brown, die and fall off. If your mulch smells like vinegar, ammonia or sulfur, it is likely a sour mulch, and should be removed. Replace the sour mulch with a high quality, composted mulch and consult with your local UW-Extension agriculture/horticulture about testing the soil for nutrient deficiencies. Fertilize appropriately based on the results of these tests.

Improper application of mulch can also lead to problems. Piling wood mulch up against the trunk of a tree can keep the bark underneath excessively wet. This wetness can contribute to bark decay. In addition, use of thick mulch layers (greater than four inches) can lead to overly wet soils that are favorable for development of root rots (see University of Wisconsin Garden Facts X1070). To avoid these problems, make sure mulch is applied at least one to two inches away from the trunk of the tree and that the mulch layer is the appropriate thickness for the soil type in your landscape (see above).

Does woody mulch harbor or attract insects? Insects such as earwigs (see UW-Extension bulletin A3640), centipedes (see University of Wisconsin Garden Facts X1113), millipedes (see University of Wisconsin Garden Facts X1108) and sowbugs (see University of Wisconsin Garden Facts X1110) can feed on decaying organic matter in mulches. While these insects are often only nuisances, earwigs can feed on and cause damage to a variety of ornamentals, particularly to flowering plants. If mulch is used near entrances to a home or around basement windows, these unwanted insects may get inside.

Termites ingest wood and can be attracted to wood mulch, but new termite colonies are not likely to become established due to use of wood mulches. Typically termites are not a problem in Wisconsin, and when colonies are found, they occur only in the southern half of the state.

Carpenter ants (see UW-Extension bulletin A3641) and powderpost beetles (see University of Wisconsin Garden Facts X1053) are unlikely to utilize mulch as a food source because conditions required for their development would not be satisfied by wood mulch. Carpenter ants do not ingest wood as a food source; instead, they chew non-living wood (in trees or landscape timbers, etc.) to excavate galleries in which they live and raise their young. Since wood mulch is composed of small wooden pieces, it would not serve as a home. To avoid potential insect problems, keep mulch as far away from the foundation of your home as possible and seal all holes and crevices where insects might use as entry points. Also, periodically inspect landscape timbers and the house for termites.

Does woody mulch harbor tree pathogens? Wood mulch may come from many sources, including trees and shrubs that have died from a wide range of diseases. To be harmful to your trees, disease-causing organisms (pathogens) would have to survive in mulch and these organisms would have to move from the mulch either directly, or through the soil, to their new host – your tree. There is currently very little research on this topic.

Elm trees killed by Dutch elm disease (see University of Wisconsin Garden Facts X1076), can serve as breeding areas for native and European elm bark beetles. Bark beetles that breed in logs or firewood from these trees can pick up the fungi that cause Dutch elm disease (Ophiostoma ulmi and Ophiostoma novo-ulmi) and carry these fungi from tree to tree. Chipping infected elm trees creates an unfavorable environment for bark beetles yet there is no scientific literature that describes the level of risk of transmitting the Dutch elm disease fungi from wood chips or bark chunks to healthy elms.

Oak trees killed by oak wilt (see University of Wisconsin Garden Facts X1075) can be attractive to several sap-feeding beetles that can potentially pick up the oak wilt fungus (Ceratocystis fagacearum) and move it in the landscape. This process is affected by moisture and temperature and would likely be disrupted by the chipping and composting process yet there is no scientific literature that describes the level of risk of transmitting the oak wilt disease fungus from wood chips or bark chunks to healthy oaks.

Recent research at the University of Wisconsin-Madison suggests that wood chip mulches produced from trees suffering from Verticillium wilt (see University of Wisconsin Garden Facts X1008) can serve as a source of the fungus (Verticillium dahliae) that causes the disease. These studies show that Verticillium can survive for at least one year in mulch and that use of this contaminated mulch can lead to Verticillium wilt in both woody and herbaceous plants. Therefore use of mulches produced from trees with Verticillium wilt should be avoided.

For more information on proper tree care: See UW-Extension Bulletin A1817, or contact your county Extension agent.