Ageratum is a genus of 40 to 60 tropical and warm-temperate flowering plants in the family Asteraceae (tribe Eupatorieae), comprising annuals and perennials. Most species are native to Central America and Mexico, with four found in the United States; the group is informally known as "whiteweed" in North America.
Plants typically form low tussocks or small mounds, reaching up to about 76 cm in height. The opposite leaves are cordate or oval in shape, hairy or tomentose in texture, with slightly toothed or serrate margins. The flowers are the genus's most distinctive feature: small, fluffy heads in shades of lavender-blue, pink, lilac, or white, arranged in compact compound umbels. The fruit is small and dry.
Ageratum houstonianum (floss flower) is by far the most widely cultivated species, prized as a bedding annual for its long-flowering season from early summer to first frost. Several species, including A. houstonianum and A. conyzoides, contain pyrrolizidine alkaloids and are toxic to livestock, causing liver lesions.
Etymology
The common name "whiteweed," used in the United States, reflects the pale or whitish appearance some species take on in certain growth stages. The genus name Ageratum derives from the Greek ageratos ("non-ageing"), a reference to the long-lasting nature of the flowers.
Distribution
Most Ageratum species are native to Central America and Mexico, with four species extending into the United States. The genus is concentrated in tropical and warm-temperate zones of the Americas.
Ecology
Several Ageratum species, notably A. houstonianum and A. conyzoides, contain pyrrolizidine alkaloids that are hepatotoxic and tumorigenic to livestock. These same species have become widespread weeds outside their native range.
Cultivation
Ageratum houstonianum is the most commonly grown species, available in compact varieties of 15–20 cm and taller forms reaching about 46 cm. Flower colours include blue, violet, pink, and white; blue cultivars are most popular. Plants suit rock gardens, bedding schemes, and containers, performing well in full sun or partial shade. They bloom from early summer through the first frost and require minimal care.
