Category Archives: General Horticulture/Weeds

Fruit and Frost

Because of our climate, Wisconsin always has a risk of spring frosts.  Temperatures below 28°F will injure or kill flowers or immature fruit.  This sheet will briefly outline critical temperatures at different stages of flower development, and describe how a gardener might protect against late season frosts.

As temperatures warm in the spring, bud scales in fruit trees begin to open and new leaves and flowers emerge.  During this process the ability to withstand cold temperatures decreases.  The cold “tenderness” of different fruit crops is related to the stage of flower development.  Tables have been developed that show these relationships very clearly.  The tables for apples and cherries are provided here.  This information is also available in the back of UW-Extension Bulletin A3314 “Commercial Tree Fruit Spray Guide.”

It is clear to see that as individual flowers are exposed to the environment, their ability to withstand frost diminishes.  It is also clear that when the flowers are open, temperatures below 28°F will begin to cause injury.  Similar critical temperatures apply to small fruits such as strawberries as well.  Once flower buds are exposed from the crown, temperatures below 28°F will cause injury.

Apple
Stage of
development

Critical temperature (°F)
10% kill

Critical temperature (°F)
90% kill

Silver tip

15

2

Green tip

18

10

Half-inch green

23

15

Tight cluster

27

21

First pink

28

24

Full pink

28

25

First bloom

28

25

Full bloom

28

25

What can be done to protect against frost?  Strawberries and raspberries can be protected by covering the planting with a tarp at sundown and then removing it in the morning after the air temperatures are above freezing.  Rocks or pipe can be used to hold the tarp in place on breezy nights.  The tarp will retain sufficient heat radiating from the soil to protect the flowers from injury.

Tree fruits are slightly more difficult to protect.  Small trees can be covered with a tarp with the corners secured to the trunk with twine.  Again, a tarp will keep heat within the tree canopy.  If colder weather is predicted, heat can be added by running an extension cord to the tree and hanging a burning 60 watt light bulb within the tree crown.  A light bulb will often add sufficient heat to keep the canopy above the critical temperature.

Hopefully the weather will hold, but if not, these techniques should offer some protection to tender flowers.

For more information on fruit and frost:  Contact your county Extension agent.

Extending the Gardening Season

Cold frames and hot beds, hoop houses, cloches, and floating row covers allow gardeners to grow plants earlier in spring and later in fall. Although these structures are used primarily for growing vegetables, they may be used for growing ornamentals, including flowering plants, as well.

Hoop houses, cold frames, hot beds, cloches and floating row covers can extend the growing season by six to eight weeks.
Hoop houses, cold frames, hot beds, cloches and floating row covers can extend the growing season by six to eight weeks.

Cold frames: Cold frames are by far the most useful and versatile of the structures that extend the gardening season. They are simple, easy-to-make structures that can be used year-round to provide warmth from the sun, and prevent water loss and damage from the wind. Many gardeners use cold frames to harden off transplants in the spring. In addition, cold frames are great places to grow salad crops such as lettuce, radishes, and spinach before and after their regular outdoor planting season. In winter, cold frames can be used for forcing bulbs (see University of Wisconsin Garden Facts XHT1144 – “Forcing Bulbs”), storing root vegetables (e.g., carrots or parsnips), or propagating trees and shrubs using hardwood cuttings.

Permanent cold frames should be sturdy enough to withstand years of sun and adverse weather. Most are constructed of wood and have a hinged cover. Glass windows make great covers, but are heavy and breakable. Alternative covers made of plexiglass or double-layered clear plastic weigh less and can be more durable. Commercially available cold frames are often built of lighter-weight materials and are often portable, allowing for movement of the frame to different sun exposures as seasons and plants change. Portable frames can also be set on concrete blocks or bricks to add height for taller plants.

Cold frame lids should be hinged so that they can be easily propped open to allow cool air to enter the frame. This can be important on sunny days when temperatures inside cold frames can get extremely hot, causing plants to wilt. Some mail order catalogs offer temperature controlled cold frame hinges that automatically open and close to vent the frame.

Hot beds: Cold frames become hot beds when heating of some kind is added to the ground under a cold frame. The modern, high-tech version of a hot bed involves burying a waterproof, thermostatically-controlled heating cable in a layer of sand two inches beneath where plants will grow. The bottom heat of hot beds encourages root growth in the plants. An older, low-tech method of creating hot beds has been to place fresh manure in the bottom of a cold frame, with decomposition of the manure providing the heat. This technique is not recommended due to potential safety issues surrounding the use of fresh manure, particularly for food crops (see University of Wisconsin Garden Facts XHT1143 – “Safely Using Manure in the Garden”).

A hot bed is ideal for starting vegetable and flower seeds in sunlight rather than under artificial light.  Many seedlings require constant warm soil temperatures to germinate, exactly the sort of environment that a hot bed can provide.

Floating row covers protect tender plants from wind and rain damage.
Floating row covers protect tender plants from wind and rain damage.

Hoop houses:  Hoop houses are similar to cold frames, only larger.  Metal or plastic pipes are bent into a series of hoops that are stuck into the ground or attached to raised beds.  The hoops are covered with four to six millimeter polyethylene that is buried in the soil at the base of the hoops.  Hoop houses can extend the growing season by an additional six to eight weeks.  Like cold frames, hoop houses must be ventilated on warm days.  To ventilate a hoop house, simply lift the plastic on the ends of the house, or make openings in the plastic at the top or sides of the house.

Cloches and hot caps:  Frequently used for tomatoes and peppers, cloches and hot caps add three to four weeks to the spring growing season.  There are many variations on cloches, but generally they are any transparent (not opaque) structure that covers a single plant (e.g., a water wall or a clear juice jug with the bottom cut out).  Hot caps tend to be conical in shape.  Like cold frames and hoop houses, cloches and hot caps should be ventilated to allow hot air to escape on sunny days.

Floating row covers:  Floating row covers are made of spun polyester or polypropylene and look like fabric.  They are laid over plants and are permeable to light, water, and air.  Floating row covers have many uses in the garden.  Row covers keep covered plants five to ten degrees warmer than uncovered plants and provide frost protection to temperatures as low as 28°F.  They also protect tender plants from wind and rain damage.  In addition, row covers are excellent insect barriers. 

For more information on extending the gardening season:  Contact your county Extension agent.

 

 

 

Forcing Bulbs

Wisconsin gardeners do not have to give up growing flowers during the long winter months.  Many plants grown from bulbs (also those grown from corms or rhizomes) can be forced to bloom indoors during the winter by giving them the combination of cold and dark conditions that they require to break dormancy, develop roots, and flower.  If you can spare a corner of your refrigerator, you can enjoy flowering hyacinths, narcissus, tulips, crocus, or other spring-flowering plants during the winter.

Hyacinths, tulips and narcissus forced into winter bloom indoors.
Hyacinths, tulips and narcissus forced into winter bloom indoors.

What plants should I select?  Look for varieties that are specifically recommended for forcing.  Select top quality bulbs with no soft spots.  DO NOT purchase bulbs that are damaged.  When selecting bulbs to force, keep in mind that late-blooming varieties need longer cold treatments than early-blooming varieties.  Also remember that taller varieties may need to be staked, and thus may not be appropriate in all settings. 

How do I plant my bulbs?  Plant bulbs from September through November for indoor bloom.  Use plastic or preferably clay containers with drainage holes.  Proper drainage is crucial for forcing bulbs.    Use a potting mix that is equal parts soil, sharp builder’s sand, and peat moss.  Partially fill containers with potting mix, arrange bulbs so that their necks will be at or just below the surface, and gently add more potting mix around the bulbs.  Amaryllis bulbs can be planted with the top 1/3 of the bulb above the level of the potting mix.  Smaller bulbs like grape hyacinth and crocus may be covered by one half to one inch of the potting mix.  Separate bulbs by about half a bulb width.  A container that is six inches wide and six inches deep will hold roughly 10 to 12 crocus, five tulip, four hyacinth, or three to five narcissus bulbs.  Plant bulbs that have one flat side (e.g., tulip bulbs) with the flat side facing the outside of the pot.  This will yield a display of large leaves towards the outside of the pot, surrounding the flower stalks.  Fertilizing bulbs is not necessary, but be sure to water your bulbs if the potting mix is very dry and add potting mix as needed if settling occurs.  Bulbs require some moisture during their cold treatment, but too much moisture can promote growth of rot fungi.  If the potting mix in your pots is overly wet, allow it to dry a bit before beginning the cold treatment.

Table 1.
Recommended cold period for forcing bulbs

Flower (Latin Name)

Required cold period

Amaryllis (Hippeastrum)

None

Crocus (Crocus)

15-17 weeks

Daffodil (Narcissus)

15-17 weeks

Dwarf Iris
(Iris reticulata/Iris danfordiae)

15-16 weeks

Glory of the Snow (Chionadoxa)

15 weeks

Grape Hyacinth (Muscari)

15-17 weeks

Hyacinth (Hyacinthus)

12-15 weeks

Lily-of-the-Valley (Convallaria)

14-15 weeks

Paperwhite narcissus (Narcissus)

None

Squill (Scilla)

15-16 weeks

Tulip (Tulipa)

15-17 weeks

Hyacinths and paperwhite narcissus can be grown without potting mix.  Hyacinths are often grown in special glass containers shaped to hold a single bulb, but any container with a neck narrow enough to suspend a bulb over water will work.  Paperwhite narcissus bulbs can be grown on sand, gravel, pebbles or marbles.  Place whatever material you select around the bulbs high enough to support them and keep them upright.  Space bulbs as described above.  For both hyacinths and paperwhite narcissus grown without potting mix, add water to the container, keeping the water level just below the bottom of the bulbs.  If water touches the bulbs, they will rot. 

How do I get my bulbs to bloom?  Most bulbs require cold and dark conditions that simulate winter conditions in order to bloom.  However, amaryllis and paperwhite narcissus bulbs do not require a cold treatment.  Cold treatment guidelines for several bulb species are given in Table 1.

Amaryllis bulbs do not require a cold period to bloom.
Amaryllis bulbs do not require a cold period to bloom.

Ideally, temperatures should decrease gradually.  Bulbs should spend the first three to five weeks developing roots at 45 to 50ºF, followed by about three weeks at 38 to 42ºF, and the remainder of their cold period at about 35ºF.  However, the constant temperature of a home refrigerator (typically between 35 and 45ºF) also works.  Be sure NOT to store fruit (particularly apples) in the refrigerator while forcing bulbs as ethylene gas released from fruit impairs flower development.  Also be sure to shield the bulbs from light.  Check moisture every few weeks.  If the potting mix is very dry, add a small amount of water.  When the cold period is complete, the bulbs should have shoots that are one to two inches tall.  Bring sprouted bulbs out of refrigeration into a sunny spot where the temperature is between 55 and 60ºF.  Shoots initially will be white, but will turn green when the bulbs are brought into the light.  Most bulbs will bloom about three to four weeks after they are removed from the cold.  Amaryllis bulbs will bloom six to eight weeks after planting.  At warmer temperatures, bulbs may grow faster, but flowers will not last as long.  Keep bulbs moist, but not overly wet.  Bulbs will require more water as leaves begin to grow rapidly. 

What do I do with my bulbs after they have bloomed?  Hyacinths, narcissus, tulips, and crocus cannot be forced two years in a row.  However, bulbs that have been grown indoors in potting mix can be planted in the garden.  Keep the bulbs in pots through the spring and summer, watering and fertilizing them regularly until they naturally die back.  In the autumn, plant the bulbs in your garden at the recommended depth.  The bulbs may bloom the following spring, but most likely will need a year of leaf growth before they rebloom.  Bulbs grown without potting mix exhaust their resources and often won’t bloom again, even in the garden. 

To get an amaryllis to bloom again, it is important to allow the bulb to store food during its growing season and then give it a dry dormant period.  After bloom, remove the flowers, give the plant plenty of sunlight, and fertilize it.  Stop watering in early September and allow the plant to dry out completely.  The foliage will die back.  If taken outside during the summer, bring the plant inside before frost.  Starting in November or December, begin to water the bulb again to repeat the blooming cycle.  You may want to consider repotting your amaryllis bulb every few years, but only into a pot no more than one to two inches wider than the diameter of the bulb. 

Keeping a log.  Keep records of the varieties that you force, the length and temperature of cold treatment, and weeks until bloom after cold treatment.  This information can provide a valuable reference for your future winter horticulture endeavors.

For more information on forcing bulbs:  See Bryan, John E.  2002.  Bulbs.  Revised edition. Timber Press, Portland, Oregon, or De Hertogh, August.  1996.  Holland Bulb Forcer’s Guide.  5th edition.  Hillegom, The Netherlands, or Hays, Robert M. and Janet Martinelli, eds.  1996.  Bulbs for Indoors.  The Brooklyn Botanic Garden, or contact your county Extension agent.

Dodder

What is dodder?  Dodder is the name of several species of parasitic plants that are widely distributed in North America and Europe.  Plants parasitized by dodder include alfalfa, carrots, onions, potatoes, cranberries, a variety of herbaceous and woody ornamentals, and many weed species.  Parasitized plants become weakened, have reduced yields (in the case of agronomic crops), and can potentially die.

Spaghetti-like dodder plants parasitizing carrots.
Spaghetti-like dodder plants parasitizing carrots.

What does dodder look like?  Dodders lack roots and leaves, and also lack chlorophyll, the green pigment found in most plants.  Dodders have slender, yellow-orange stems that cover infected plants in a spreading, tangled, spaghetti-like mass.  From May through July, dodders produce white, pink, or yellowish flowers. 

Where does dodder come from?  Dodders produce large numbers of seeds that germinate in the spring to produce shoots that attach to suitable host plants.  Dodders penetrate host tissue, and absorb nutrients via specialized structures called haustoria.  Once established on a host, the bottom of a dodder plant dies (thereby severing its connection with the soil), and the dodder plant becomes dependent on the host plant for water and nutrients.

How do I save plants parasitized by dodder?  On woody ornamentals, simply prune out dodder-parasitized branches.  When small patches of dodder occur among herbaceous plants, apply contact herbicides such as 2,4-D early in the season, preferably before dodder seedlings have parasitized host plants.  Keep in mind that use of contact herbicides will likely also kill host plants.  Alternatively, cut or burn dodder and parasitized plants to keep dodder from spreading, and to prevent seed production.  For widespread dodder infestations, a combination of frequent tilling, burning and herbicide applications may be needed to achieve control.  Be sure to read and follow all label instructions of the herbicide that you select to ensure that you use the product in the safest and most effective manner possible. 

How do I avoid problems with dodder in the future?  Dodder’s wide host range and ability to survive as dormant seeds in soil make eradication difficult.  Preventing introduction of dodder is the best method of control.  Use dodder-free seed, and be sure to clean equipment after working in a dodder-infested area.  Try to restrict animal movement between infested and non-infested areas as well.  Depending upon the specific crop or location, use of pre-emergent herbicides containing DCPA, dichlobenil, propyzamide, or trifluralin may be possible to prevent germination of dodder seeds.  Destroy actively growing dodder and any parasitized plants before the dodder produces seeds.  In agricultural settings where dodder has been a problem, rotate away from susceptible crops and grow non-host crops (e.g., corn, soybeans, or small grain cereals).  In conjunction with rotation, adequate control of weed hosts is critical to achieve control.

For more information on dodder:  Contact your county Extension agent.

Degree Day Calculation

Indicator plants are not always suitable for the timing of pest management practices. You may not have the critical indicator plant nearby to time local activities, or there may not be a good indicator plant for a particular pest’s crucial life stage. Another way of achieving the same result is to use degree day calculations. Degree days (also known as heat units or thermal units) are a way of incorporating both temperature and time into one measurement to quantify the rate of plant or insect development. All plants and insects develop in response to temperature. The warmer the weather, the more quickly they develop, and the cooler the temperature, the slower they develop. All species have a cutoff temperature below which no development occurs. This base temperature, or developmental threshold, differs depending on the species. As the ambient temperature increases above the threshold, development occurs. The higher the temperature, the faster the rate of development. The most common developmental threshold is 50oF. This is the temperature at which dormancy is broken in plants and when many insects become active. But there are a number of insects, particularly those that are active early in the spring, that have a lower threshold (38o or 43oF), and many cool-season vegetables such as cole crops and peas have a base temperature of 40oF. The base temperature is often noted as a subscript number following the abbreviation for degree day (e.g., DD50). Degree days are accumulated whenever the temperature exceeds the predetermined developmental threshold. A certain number of units are added each 24 hour period, depending on how much the temperature is above threshold, to produce a cumulative total of degree days. The mathematics for calculating actual degree days can be very complex (determining the area under the curve for the graph of time vs. temperature), but easy mathematical equations yield approximations satisfactory for practical applications. The basic procedure for calculating degree days is very simple, and all you need is a maximum/minimum thermometer. Each day, record the high and low temperatures. Determine the average daily temperature by adding the daily high temperature and low temperature together and dividing by two:

Avg. Daily Temp. = (High Temp. + Low Temp.) / 2

Example:

Avg. Daily Temp. = (60 + 50) / 2 = 55

Next, subtract the base temperature from the average daily temperature to get the number of degree days for that day:

Daily DD50 = Avg. Daily Temp. – Base Temp.

Example:

Daily DD50 = 55 – 50 = 5 DD50

If the average degree day value for a given day is less than zero, just record zero, not a negative number. Finally, to keep track of degree day accumulation, keep a running total of all degree days accumulated from the first of the year. In Wisconsin, we often don’t accumulate many degree days before April 1, so if you wish to take a break from the math from January to April, go ahead. The equation above is a very rough estimate of degree day accumulation. There is also an upper threshold for development in insects, above which there is no appreciable increase in the rate of development. This is obviously of more concern later in the season than in the spring. To compensate for the reduced growth rate at high temperatures, modified degree days use an upper threshold of 86oF as well as the lower threshold. Whenever the actual temperature is below the base temperature, the base temperature is substituted as the low temperature for the day. Similarly, whenever the daily high is above 86o, 86 is used as the high temperature for the day. Example:

Daily High = 90 and Daily Low = 45

Avg. Daily Temp. = ( 86 + 50 ) / 2 = 68

Daily DD50 = 68 – 50 = 18 DD50

If you are not inclined to dig out your calculator and record daily temperatures, you can visit the University of Wisconsin’s Soil Science website at http://www.soils.wisc.edu/wimnext/asos/SelectDailyGridDD.html and use their computerized degree day calculator. All you need to know is your latitude and longitude coordinates.

For more information on phenology: See University of Wisconsin Garden Facts X1085, X1087 and X1088, or contact your county Extension agent.

Dame’s Rocket

Invasive Plant Series What is dame’s rocket? Dame’s rocket (Hesperis matronalis) is a Eurasian biennial belonging to the mustard family. It was introduced to North America in the 1600’s and has naturalized itself in moist, wooded areas, but can also invade open areas. It may be sold in garden centers as a perennial and is often included in “wildflower” seed mixes. The plant’s three-month-long blooming period and ability to set abundant seed have contributed to its spread, as well.

Dame’s rocket leaf.
Dame’s rocket leaf.
Dame’s rocket flowers (enlarged).
Dame’s rocket flowers(enlarged).
Dame’s rocket seed pods.
Dame’s rocket
seed pods.

What does dame’s rocket look like? Dame’s rocket bears loose clusters of attractive, fragrant, pinkish-purple to white four-petaled flowers on 2 to 4 ft. stems. Flowers are produced from May–August, and the plant can produce seeds and flowers on any flower cluster at the same time. Leaves are slightly hairy and lance-shaped with toothed margins. Leaves are arranged alternately on the stem, and the basal rosette of leaves remains semi-evergreen through winter. The plants spend their first year as a rosette of basal leaves. They produce a flowering stem the second or third year, bloom, and then die. Seed pods are about 11∕2 inches long and very narrow. Dame’s rocket is often confused with garden phlox (Phlox paniculata), because the flower colors, clustered blooms and bloom time are similar. However, garden phlox has flowers with five petals (dame’s rocket has four) and opposite, untoothed leaves (dame’s rocket has alternate, toothed leaves).

How can I control dame’s rocket? Check any “wildflower” seed packets you may purchase to ensure that they do not contain dame’s rocket seeds. You can cut the flower heads off established plants after bloom so the plants do not set seed, or hand pull the plants. If plants are pulled while in bloom, do not compost them, as the seeds can still ripen and spread. Bag the plants for landfilling, or burn them. Do not allow the plants to dry before burning, as seedpods may burst open and spread seeds when dried. If appropriate, burn infested areas where allowed. Finally, glyphosate-containing herbicides can be applied in late fall when native plants are dormant, but the dame’s rocket basal leaf rosettes are still green and vulnerable to sprays. Avoid getting the herbicide on other plants. Repeat control measures for a few years until seeds in the soil are depleted.

For more information on dame’s rocket: See the DNR publication ER-090-97 – “Wisconsin Manual of Control recommendations for Ecologically Invasive Plants”, or contact your county Extension agent.

Cupflower

What is cupflower? Nierembergia, commonly known as cupflower, is a genus of tender, heat-loving perennials in the nightshade family (Solanaceae) that is valued for its long blooming period. The genus, which is native to Argentina, is named for Juan Eusebio Nieremberg, a 17th century Spanish Jesuit theologian and naturalist. The taxonomy of Nierembergia is somewhat confused, and the species names for cupflower have been variously listed as N. caerulea, N. frutescens, N. hippomanica or N. scoparia. While hardy only in USDA zones 7 through 10, cupflower can easily be grown as an annual in colder climates.

Cupflower produces mounds of blue or white, star-shaped, saucer-like flowers.
Cupflower produces mounds of blue or white, star-shaped, saucer-like flowers.

Cupflower plants form neat, compact, spreading mounds that are approximately 12 to 15 inches in diameter. Plants have multiple stems with fine-textured, stiff, linear, ½ inch-long leaves, and. eventually become covered with one inch diameter, white or bluish-purple star-shaped, saucer-like flowers with lemon yellow centers. The color of cupflower blossoms does not fade even in the brightest sun. There are several varieties of cupflower that may be available at greenhouses and garden centers in your area.

  • ‘Blue Mountain’ grows six to nine inches tall and has azure-blue flowers.
  • ‘Mont Blanc’ grows six inches tall and 12 inches wide and is a profuse bloomer with small, brilliant white flowers. This variety was an All-America Selection winner in 1993.
  • ‘Purple Robe’ grows six to eight inches tall and has violet-blue flowers. This variety was an AAS winner in 1942.
  • ‘Starry Eyes’ grows 10 to 16 inches tall and 15 to 18 inches wide. This variety has white flowers with pale lavender highlights.
  • ‘Summer Splash Compact Blue’ grows 10 to 14 inches tall and has large, blue flowers.
  • ‘Summer Splash Compact White’ grows 10 to 14 inches tall and has large, white flowers.
  • ‘White Robe’ is an early blooming variety that grows six to eight inches tall and approximately eight inches wide, and has white flowers.
White cupflower combines nicely in the garden with tall blue salvia and pink globe amaranth.
White cupflower combines nicely in the garden with tall blue salvia and pink globe amaranth.

Where do I get cupflower plants? Cupflower plants can be purchased at most commercial greenhouses and garden centers. Alternatively, you can start cupflower plants from seed. Sow seeds indoors eight to 10 weeks before the predicted date of last frost in your area. Barely cover the seeds at planting. Seeds should germinate within two to three weeks if grown at 70°F. Keep the soil slightly moist until the seeds germinate, then water the seedlings only after the soil begins to dry out. Young plants will grow very slowly, but will grow more quickly once transplanted outdoors.

How do I grow cupflower in my garden? Transplant purchased cupflower plants or homegrown cupflower seedlings outdoors after the last spring frost. Be sure to harden off the plants before transplanting. Place plants six to 12 inches apart in organically rich, well-drained soil in full sun. Mulch the plants to help retain moisture and keep the soil cool. Cupflower is drought tolerant once established, typically does not have severe insect pest or disease problems, and usually is not bothered by deer. Stem cuttings can be taken in late summer and rooted to grow plants indoors over the winter.

How do I use cupflower most effectly in my garden? Use cupflower as an edging, and in borders, rock gardens, or containers, including hanging baskets and window boxes. White varieties combine nicely with tall blue salvia (Salvia spp.) and pink globe amaranth (Gomphrena globosa – see University of Wisconsin Garden Facts XHT1171). Cupflower thrives in hot environments, such as along walkways or walls that reflect heat, where it can spill out to soften sharp landscape edges.

For more information on cupflower: Contact your county Extension agent.

Creeping Charlie

What is creeping Charlie? Creeping Charlie (Glechoma hederacea) is an herbaceous perennial plant that spreads by seed and by creeping stems (called stolons) that grow along the ground. Creeping Charlie was introduced into North America from Europe by early settlers who thought it would be a good groundcover for shade. A variegated form of the plant is sometimes used in hanging baskets. Creeping Charlie is also known as ground ivy, gill-on-the-ground, and creeping Jenny.

What does creeping Charlie look like? Creeping Charlie produces bright green, round or kidney-shaped leaves that have scalloped edges. The leaves are produced opposite each other on square (i.e., four-sided), creeping stems that root at the nodes. In spring, small, bluish-purple, funnel-shaped flowers appear. When the plant is crushed, it produces a strong mint-like odor. Creeping Charlie is often confused with henbit (Lamium amplexicaule), which is a winter annual.

Creeping Charlie produces kidney-shaped leaves with scalloped edges on creeping stems (left) and small, bluish-purple, funnel-shaped flowers (right).
Creeping Charlie produces kidney-shaped leaves with scalloped edges on creeping stems (left) and small, bluish-purple, funnel-shaped flowers (right).

How can I control creeping Charlie? Creeping Charlie thrives in moist, shady spots such as under trees and shrubs, and along the north sides of buildings. Altering these moist, shady conditions can discourage growth of creeping Charlie. If possible, improve soil drainage or water less frequently to dry the soil, and prune trees to open the canopy and increase light levels. If creeping Charlie is invading a thin lawn, try to improve turf health and density to get weeds under control. This can be accomplished by mowing regularly (to a height of two to three and one-half inches), fertilizing and watering appropriately, and overseeding in the fall. Also, make sure to grow the most suitable type of turfgrass for the location (e.g., plant shade tolerant turfgrass varieties under trees). See University of Wisconsin-Extension Bulletin A3700 for additional information on lawn establishment and maintenance. Alternatively, consider removing grass and growing shade-loving plants such as vinca, English ivy, pachysandra or hosta that compete well with weeds. In areas where creeping Charlie has become established, try removing plants by hand. This is the control method of choice in vegetable or flower gardens. However, this may not be a viable option in heavily infested areas, as the extensive spreading stems of creeping Charlie can be difficult to completely remove. Once plants are pulled, make sure to dispose of the plants in such a way that they cannot re-root. An alternative (and oftentimes more effective) means of controlling creeping Charlie is with a postemergence broadleaf herbicide. The best choice for homeowners is a weed killer that contains triclopyr. This active ingredient is found in many commercially available homeowner lawn care products, oftentimes in combination with other herbicides such as dicamba (3, 6-dichloro-o-anisic acid), 2,4-D (2, 4 dichlorophenoxyacetic acid) and mecoprop or MCPP [2-(2-methly-4-chlorophenoxy) propionic acid]. Products containing 2,4-DP can also provide adequate control. All of the products listed above can be used for treating lawns, but cannot be used in vegetable or flower gardens as many common vegetables and ornamentals are broadleaf plants that are very susceptible to these herbicides. In areas of a lawn with an extensive creeping Charlie infestation, it may be easier to use a broad-spectrum herbicide (e.g., glyphosate) to kill all of the vegetation in the area and then reseed the lawn.

When using an herbicide for

Creeping Charlie rapidly invades lawns, crowding out and replacing turf.
Creeping Charlie rapidly invades lawns, crowding out and replacing turf.

creeping Charlie control, be sure to read and follow all label instructions of the product that you select to ensure that you use the product in the safest and most effective manner possible. A general rule of thumb is to make applications when temperatures are in the mid 60s to low 80s, there is no rain expected for 24 hours following application, and there is little or no wind. DO NOT mow the treated area for several days before and after an herbicide application. Dicamba, triclopyr, MCPP, 2,4-D or 2,4-DP applications for creeping Charlie control should be made when plants are actively growing. A mid to late autumn herbicide application (after the first frost) is often particularly effective. During this period, plants are drawing nutrients from their leaves and into their roots for storage over the winter, and herbicides are more effectively moved into the roots as well, resulting in better control. A second application can be made in the fall if needed. Herbicide applications can also be made in the spring, but should be timed to correspond to creeping Charlie’s blooming period (typically April to June). Plants are more sensitive to herbicides during this time. Again, a second application may be necessary to obtain adequate control. Note that any herbicide containing dicamba should not be used in a given area more than twice per year. Finally, borax has been touted as an organic control for creeping Charlie. However, research at both the University of Wisconsin and Iowa State University has shown that borax does not provide long-term control of creeping Charlie, and can injure turf and other plants, causing stunting and yellowing. Thus borax is not recommended for creeping Charlie (or other broadleaf weed) control.

For more information on creeping Charlie: Contact your county Extension agent.

Ageratum

What is ageratum?  The genus Ageratum includes approximately 60 species of annual and perennial herbs and shrubs in the aster family (Asteraceae) that are all native to Central and South America.  One species that is commonly used as a bedding plant, Ageratum houstonianum, is from Mexico, and is named after William Houston (1695-1733), a Scottish physician who collected the first ageratum plants.  The name ageratum is derived from the Greek “a geras”, meaning non-aging and refers to the long-lasting nature of ageratum flowers.

Ageratum plants have soft, fuzzy flowers that can be blue, pink, lavendar or white.
Ageratum plants have soft, fuzzy flowers that can be blue, pink, lavendar or white.

Wild species of ageratum can grow to over two feet in height and typically reseed themselves liberally.  Varieties offered by nurseries and garden centers however are almost all hybrids that are more compact and better behaved.  Commercially available ageratum cultivars grow in neat mounds, flowering from late spring through fall.  They are one of the more dependable flowering annuals. Ageratum plants have oval to heart-shaped leaves that are up to two inches long.  Flowers of ageratum are typically some shade of blue, but can be pink, lavender or white.  The soft, fuzzy flowers are dainty and feathery, often delightfully fragrant, and usually cover plants completely.  Each flower cluster consists of five to 15 tubular florets. There are many different cultivars of ageratum, including attractive dwarf, tufted plants as well as tall, upright types that can be used as cut flowers.  Most cultivars are propagated from seed, and are predominantly F1 hybrids (i.e., offspring from crosses of two plants of closely related species or strains of a single species).

  • ‘Blue Blazer’ was the first commercial F1 ageratum hybrid.  This cultivar has better plant uniformity and vigor, and blooms earlier than open-pollinated cultivars.
  • ‘Blue Danube’ has compact six to eight-inch-tall plants covered with mid-blue flowers.  This cultivar is one of the best varieties for uniformity, earliness and general performance.
  • ‘Blue Horizon’ is an F1 hybrid that grows 30 inches tall and produces three-inch clusters of purplish-blue flowers on long stems.  This cultivar is great as a cut flower.
  • ‘Blue Mink’ is an open-pollinated cultivar that grows 12 inches tall and has powder-blue flowers.
  • ‘Hawaii’ is a series of F1 hybrids, each of which is dwarf (up to eight inches tall) and compact, with soft pink, royal blue or pure white flowers.  Members in this series flower earlier and longer than other varieties.
  • ‘Pinky’ produces salmon pink flowers on bushy and compact eight-inch-tall plants.
  • ‘Purple Fields’ is an F1 hybrid that produces compact, mounded plants that spread up to 12 inches across.  This cultivar is covered with unusual, purple flowers.
  • ‘Summer Snow’ is an F1 hybrid with fluffy white flowers;
  • ‘Trinidad’ has a unique, early-blooming blend of white, blue, violet and pink flowers on six-inch-tall plants.

Where do I get ageratum?  Ageratum transplants can be purchased at local garden centers.  However, ageratum can also be grown very easily from seed.  Start ageratum seeds eight to 10 weeks before you would like to transplant them into your garden.  Surface sow the seeds, barely covering them with vermiculite or potting mix.  Be sure that the seeds receive light to stimulate germination.  Germination usually takes seven to 21 days.  Transplant the seedlings into trays or pots when they are large enough to handle.  Move potted plants outside to harden off and transplant them into the garden when all risk of frost has passed.

Blue-flowered ageratum pairs well in the garden with yellow-flowered ornamentals.
Blue-flowered ageratum pairs well in the garden with yellow-flowered ornamentals.

How do I grow ageratum?  Transplant ageratum plants six to eight inches apart in a sunny spot.  Ageratum prefers a moist, well drained soil, but will also tolerate dry conditions.  Ageratum plants generally do not have insect or disease problems, although spider mites can be an issue, especially in hot, dry weather.  On most cultivars, old flowerheads turn brown and remain on the plants.  Deadhead regularly to improve the appearance of plants and prolong blooming.  Some cultivars are self-cleaning (i.e., the flowers fall off naturally).  Ageratum plants are sensitive to cold temperatures, so be sure to cover plants on cold nights in the fall to extend their survival.

How do I use ageratum most effectively in my garden?  Because of their short stature, most varieties of ageratum are best used for edging or borders of flowerbeds, in rock gardens, or in containers.  Blue varieties are particularly attractive when combined in the garden with pink-flowering plants.  Combine soft blue-flowering varieties of ageratum with pink begonias (Begonia spp.) for a low, pastel ground cover, or mass them with short yellow marigolds (Tagetes spp.) or cockscomb (Celosia spp.) for greater contrast.  Taller blue varieties of ageratum go well with yellow cosmos (Cosmos sulphureus).  Also try mixing the powder blue varieties with white petunias (Petunia spp.), lamb’s ear (Stachys spp.) or dark blue lobelia (Lobelia spp.).

For more information on ageratum:  Contact your county Extension agent.

African Violets

What are African violets? African violets (Saintpaulia spp.) are popular flowering houseplants in the Gesneriad family (Gesneriaceae), native to Tanzania in East Africa. Their compact forms make them ideal for use on tabletops, windowsills, and hanging baskets.

The blossoms of African violets can provide year-round color for your home.
The blossoms of African violets can provide year-round
color for your home.

There are many varieties of African violets, including trailing and miniature varieties. African violets usually form rosettes of rounded, velvety leaves with scalloped edges on short fleshy leaf stems. Leaves are often dark green with red-tinted undersides, and sometimes are variegated. Small clusters of flowers surround the foliage in shades of pink, red, white, violet, purple, blue and bicolor. Flowers can be single, double, semi-double, fringed, star-shaped, and Geneva (edged in white). African violets have a reputation for being difficult to grow, but under suitable conditions they will thrive, producing long blooming flowers throughout much of the year.

How do I care for African violets? African violets prefer locations with bright, indirect light.  In a south or west exposure, plants need to be protected from direct sunlight during peak hours, or foliage will burn.  If African violets do not form flower buds, they are likely not receiving enough light and should be moved to a sunnier location, or placed under artificial light.  In particular, supplemental fluorescent or full spectrum lighting may be necessary in winter months to encourage year-round flowering.  Position lights eight to 12 inches above plants, allowing 14 to 16 hours of light per day.  African violets prefer 70-75°F days and 60-65°F nights.  Place plants in a location with good air circulation, but keep them away from cold windows and cool drafts as sudden changes in temperature can harm the plants. African violets should be watered moderately from spring until fall, allowing soil to dry slightly between waterings.  Reduce water slightly in winter months.  Bottom water plants to avoid water splash on foliage, as cold water can damage leaves causing brown spots.  Fill saucers with warm water, allow plants to soak up water for approximately 30 minutes, and drain off excess water once the soil is sufficiently moist.  African violets are extremely susceptible to crown rots, which can rapidly kill the plants.  To prevent crown rots, avoid overwatering, avoid watering directly into the crown, and avoid watering at night. African violets grow best when potted in a well-drained, soilless potting mix or pre-packaged African violet mix.  Keep plants slightly pot-bound to encourage flowering.  Use a pot that is approximately one half the width of the plant’s spread.  When plants are in flower, apply a specially formulated African violet fertilizer once a month, following the label instructions of whatever fertilizer you select.  Remove older flowers and leaves as they begin to wither to improve the aesthetics of the plant, and to prevent problems with Botrytis cinerea, the gray mold fungus (see University of Wisconsin Garden Facts X1122).

For more information on African violets:  Contact your county Extension agent.