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Scientific: Ageratum houstonianum
Common: floss flower
Family: Asteraceae
Origin: Mexico

Pronounciation: A-ger-A-tum Hou-STON-i-an-um

Hardiness zones

Landscape Use: Floral accent where the color blue is desired in garden borders or as an edging plant, cool season bedding plant in Phoenix, warm season annual in the eastern United States, and a short-lived landscape perennial along the west coast; great for container gardens.

Form & Character: Short, diminutive, delicate, recessive, tender.

Growth Habit: Evergreen, herbaceous perennial mostly treated as a cool season annual in Phoenix. Medium growth rate to 6- to 24-inches tall. Most cultivars form a dense mound.

Foliage/Texture: Small, opposite, light green leaves, ovate to 1 inch long, crenate margins; medium fine texture.

Flowers & Fruits: Terminal clusters of 1/4- to 1/2-inch flower heads in cymose clusters without ray flowers. Flower colors range from white, pink, burgundy, blue, mauve, and purple, fruit inconspicuous.

Seasonal Color: Colorful flowers when in bloom.

Temperature: Optimal cardinal temperature range is 40o to 85oF.

Light: Full sun

Soil: In Phoenix, soils must be well drained and heavily amended with organic matter. Easily becomes chlorotic in sandy soils or when pH is alkaline.

Watering: Regular

Pruning: None, except to remove spent flowers (if one feels like they have to be a busy gardener).

Propagation: Seed which germinate in 5 days, cutting.

Disease and Pests: Red spider mites, white fly, powdery mildew and botrytis on flowers.

Additional comments: Floss flower takes 600 to 100 days after germination to bloom. Therefore, always buy nursery transplants. Plant out in late September in Phoenix for winter show. There are so many named cultivars with flower colors ranging from white, pink, powder blue (like the little gutty bruins), deep blue, lavender to deep burgundy red.

Taxonmic musings: The genus name, Ageratum means not growing old (referring to its persistent flowering habit), though the day of its popularity in Phoenix as a winter bedding plant seems to have 'died' several years ago. Facts are that floss flower is not as popular a Phoenix bedding plant as it once in the middle and later 20th century. The species name, houstonianum was named after botanist and Scottish surgeon, Dr. William Houstoun who brought floss flower plants to Scottland from Mexico and the West Indies.