UW Plant Disease Facts


Authors: Susan Mahr, UW Horticulture
Last Revised: 02/19/2009
D-number: XHT1167

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.

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