Ageratum (Ageratum Houstonianum)

Fundamental Facts

HARDINESS: Tender
PREFERRED SOIL pH: Near neutral
PREFERRED SOIL TYPE: Well-drained, fertile
PREFERRED LIGHT: Sun to partial shade
ATTRIBUTES: Blue, white, and bicol-ored flowers; for edgings, pots, cut flowers
SEASON OF INTEREST: Early summer through early fall
FAVORITES: 'Capri' dwarf variety; taller 'Blue Horizon'
QUIRKS: Thrives in warm, humid weather
GOOD NEIGHBORS: Campanulas, catmint, dahlias, sulfur cosmos, salvias, zinnias
WHERE IT GROWS BEST: Moist, well-drained soil, where summer nights stay above 55'F (13'C)
POTENTIAL PROBLEMS: Spider mites, whiteflies
CRITTER RESISTANCE: Good
SOURCE: Bedding plants, seeds
DIMENSIONS: Dwarfs 6 in (15.2 cm) tall, 12 in (30.5 cm) wide; tall types 2 ft (0.6 m) tall, 1 ft (0.3 m) wide

Ageratum in the Landscape

Known to old-time gardeners as floss-flower or pussy foot, ageratum is grown for its tiny, usually soft powder blue blossoms, which are packed into small, fluffy balls. While the clusters are small, new hybrid varieties are so packed with flowers that it's often difficult to see the leaves, making these low, spreading plants look like a carpet of blue flowers.

Willing to adapt to full sun or partial shade, dwarf ageratums are ideal plant partners for filling containers, edging beds, and planting in front of taller flowers. For a bold effect, try massing these fluffy bloomers in groupings of 10 or more, spacing the plants 6-8 in (15.2-20.3 cm) apart. Ageratums are easy to mix with annuals of any color, or you can tuck them between perennials to echo the shades of blue and purple flowers of campanulas and catmint. Or, use them to visually cool down the hot bright orange and yellow flowers of sulfur cosmos or yellow dahlias.

Expanding the Color Range

Back in the 1800s when ageratums first came into cultivation, flower colors were limited to blue. Blue ageratums remain garden treasures, but now there are white ageratums, such as the compact, 10 in (25.4 cm) tall 'White Ball', and innovative bicolors, such as the Capri series, which has medium blue flowers with contrasting white centers. Older, long-stemmed varieties are well worth seeking out if you grow cut flowers and are willing to grow the plants from seeds. 'Blue Horizon' and 'Blue Bouquet' produce a summer-long parade of blue flowers on 2 ft (0.6 m) tall plants.

Growing Ageratum

It's easy to find ageratums in garden centers, discount stores, and nurseries as spring bedding plants. But you can easily start seeds indoors 6 weeks before the last frost. Use moistened, commercial sterile, soilless seed starting mix and lighdy press the seeds, which need light to germinate, into the mix without covering them. Keep the soil moist and set the pots in a sunny window. When seedlings have two or three mature leaves, transplant them to individual pots and grow on. Set plants out after the last frost has passed, and keep the soil consistendy moist for 3 weeks.

Ageratums thrive in hot weather. In locations where summers are very hot and humid, though, they prefer a little light shade. But watch out for too much shade, which can make these compact plants leggy. While prolonged droughts can wilt and weaken them, a deep drench of water will quickly revive them. Long-stemmed varieties bloom longer if you prompdy remove old flowers, but vigorous dwarf varieties flower nonstop with occasional pinching off of old blooms. These natives of Mexico and Central America truly resent a chill, and their leaves respond to frost by turning from green to black overnight. To get them through an early frost, cover them with blankets.

Ageratums are usually pest free but can fall prey to two types of tiny sucking insects. Barely visible spider mites feed on the backs of leaves, giving the foliage a pale, stippled appearance. You may even be able to see their fuzzy webs. Whiteflies look like white gnats and will fly up in a cloud when disturbed. Either pest can be dispatched by spraying the plant with a strong blast of water from a hose or by applying insecticidal soap, as directed, in the evening or on a cloudy day to prevent sun-scorched leaves.