The Ageratum genus contains about 60 species. These may be annuals or perennials, but even perennials tend to be grown as annuals in gardens. They are native to Central America and the US.
Although this is a member of the Daisy family, Ageratum plants have distinguished fluffy flowers; these arise because their flowers lack the rays that are typically seen in the Asteraceae family.
Ageratum houstonianum photograph by Nemo's great uncle CC.
Garden grown Ageratum plants tend to be short and are largely grown for their flossy flowers. Flowers are mainly blue, but some species and cultivars may have purple, pink, or white flowers. Unfortunately this plant can become a weed in areas that it does not belong to.
Ageratum Artist Purple photograph by Serres Fortier CC.
Some common names for the half hardy annual Ageratum include Flossflower, Garden Ageratum, and Tropical Whiteweed. They typically flower from June to late September.
Ageratum can grow up to 24 inches (60 cm) in height. The plants may have downy white, pink, deep violet or pale blue flowers. Their height and color makes them very suitable for edging as they often form compact mounds, they are also ideal border plants and look attractive in containers.
Members of the Ageratum genus are native to central and northern America.
There are approximately sixty members of this herbaceous genera. Some of the most commonly grown species include Butterfly Mist (Ageratum corymbosum); Flossflower, Bluemink and Garden Ageratum (Ageratum houstonianum); Tropical Whiteweed (Ageratum conyzoides); Cape Sable Whiteweed (Ageratum littorale).
Ageratum houstonianum Growing Guide. This annual plant is better known as the Flossflower. It blooms in early summer with fluffy medium blue flowers. The plant is tender to frost so start off indoors in colder areas with short growing seasons or grow in containers. It reaches from 4 to 18 inches (10 to 45 cm) in height. Flossflower makes a great edging plant, and can also be used in a rockery, or as a border or bedding plant. This plant is fairly tolerant to deer.
Ageratum conyzoides (Chick weed/ Billygoat weed). This South American native reaches up to 3 1/3rd feet (1 m) in height. It carries ovate leaves up to 2 inches (6 cm) in length. It is not recommended that you grow this species as it is invasive in Australia, The USA, Africa, and SE Asia (where it can interfere with rice production).
Ageratum littorale (Cape Sable Whiteweed). This is a native of Florida. It reaches an height of 20 inches (50 cm) and grows well in beach sand. It can bloom at any time of the year. Flowers maybe Blue, purple, or white.
Common Names: Whiteweed, Flossflower, Butterfly Mist, Bluemink, Garden Ageratum, Tropical Whiteweed, Billy Goatweed, Cape Sable Whiteweed.
Life Cycle: Hardy annual.
Height: 6 to 48 inches (15 to 120 cm).
Native: Central and Northern America.
Growing Region: Zones 2 to 11. UK Hardiness H2 (Do not let temperatures drop below 32°F (0°C).
Flowers: Summer and autumn.
Flower Details: Blue, lavender, purple, white, pink. Umbels. Fluffy.
Foliage: Opposite. Cordate. Oval. Serrate.
Sow Outside: Surface. Following the last frost. Spacing 6 to 16 inches (15 to 40 cm).
Sow Inside: Germination time: one to two weeks. Temperature 70 to 75°F (21 to 24°C). Seven or eight weeks in advance. Transplant outdoors a few weeks after the last frost and in summer.
Requirements: Full sunlight or partial shade. Good drainage. Soil pH 5 to 7. Rich soil. Provide plenty of water in the summer, dead heading, monthly application of fertilizer.
USDA-NRCS PLANTS Database
It is best to sow Ageratum species such as Flossflower on the soil surface, with a spacing of 15 to 25 cm for smaller species and 30 cm apart for larger species after the last frost of spring. Ageratum should be grown full sunlight, but with partial shading in the hottest months of hot areas. Ageratum grows on most soils, though most species prefer a pH between 5 and 6.
They require between 5 and 14 days to germinate. If initiating growth indoors then it is best to sow at a temperature of 21 to 24 degrees centigrade six to eight weeks before transferring outdoors in spring; after the last frost, or in late summer.
Ageratum species such as ageratum conyzoides are easy to look after and requires watering only at the height of the summer, it is advisable to fertilise monthly and to remove Ageratum flower by deadheading.
Flossflower (Ageratum houstonianum)
The Flossflower is an annual Ageratum species, that can range in height from 12 to 39 inches (30 to 100cm). The leaves of the plant are oval at the stem and triangulate out, reaching a length of 1 to 3 inches (2.5 to 7.5 cm). The flowers are usually blue, but as there are many varieties of floss flower available other colors are common, especially purple, white, or pink. The plant takes its name from the floss like flowers that it carries.
As a native to Mexico the Flossflower should be grown in areas that receive lots of sun; In very hot areas they also grow well in partially shaded conditions. It usually flowers throughout summer through to the first frost of autumn.
Butterfly Mist (Ageratum corymbosum)
Ageratum houstonianum photograph by Rexness under creative commons licence.
This is a perennial garden plant that likes tropical conditions, in the USA it can grow in zones 8 to 11. The plant reaches an height of between two to four feet (60 to 120cm) depending on species. The seeds should be sown on the soil surface at a distance of 3 to 4 feet (90 to 120cm) and require a soil pH of 5.5 to 7.5. Grow in full sunlight, or in partial shade in very hot areas. The flowers of butterfly mist are light blue in color.