Ageratum (floss flower)

Ageratum houstonianum · Aster family (Asteraceae)

At a glance
Hardiness zones	2 to 11
Sun	full sun to part shade
Water	moderate
Life cycle	annual
Frost	Tender: killed by frost
Spacing	6 to 12 inches
Days to harvest	84 to 105 days

About Ageratum (floss flower)

Ageratum, or floss flower, is a neat, mounding annual from Mexico and Central America with fuzzy, powder-puff flower clusters. It's one of the few annuals with truly blue flowers, and it also comes in lavender, pink and white. Short types edge beds and pots, taller ones make good cut flowers, and butterflies visit it all season. The species reseeds freely, and UF/IFAS lists it as a landscape weed, though not a problem in Florida's natural areas.

Grown as a summer annual in every zone (NC State lists 2-11); frost kills it.

When to plant

Timing is counted from your average last spring frost (and first fall frost). Sow & Tell works out the actual dates for your ZIP code.

Start seeds indoors: 8 to 10 weeks before the last frost
Set out plants: up to 2 weeks after the last frost

Days to harvest count from sowing: Wisconsin gives 12-15 weeks to bloom for the tall cultivar 'Blue Horizon'. Start seed indoors 8-10 weeks before the last frost; press it onto the surface without covering, since it needs light to sprout, about 7-21 days at 70-75 F (Wisconsin, Iowa State). Harden off and plant out after frost, 6-12 inches apart (Wisconsin, NC State).

Soil

Rich, moist, well-drained soil with plenty of organic matter; it handles short dry spells (NC State, Wisconsin).

Give it rich, moist, well-drained soil with organic matter (NC State); no source we found gives a feeding schedule, so follow a soil test.

Feeding

Spider mites build up in hot, dry weather, so water during dry spells (Wisconsin).

Harvest

Blooms from late spring until frost (May to October). Old flower heads often turn brown and hang on, so deadhead to keep plants tidy and blooming; tall types can be cut for bouquets or pressed (Wisconsin, NC State, Missouri).

Varieties (17)

Variety	Type	Days	Notes
Artist series	Bedding		Early, compact, upright mounds 8-10 in tall; Basso Blue, Rose and White.
Blue Mink	Bedding		Powder-blue flowers on 12-14 in plants.
Summer Snow	Bedding		Fluffy white flowers.
Aloha series	Dwarf bedding		6-8 in tall and 8-12 in wide, in blue and white.
Blue Blazer	Dwarf bedding		More uniform, vigorous and earlier than older open-pollinated types.
Blue Danube	Dwarf bedding		Uniform, early, mid-blue flowers on 6-8 in plants; excellent Phytophthora resistance.
Hawaii series	Dwarf bedding		Under 8 in, in soft pink, royal blue and white; blooms earlier and longer than many.
Monarch Magic	Spreading		Spreading plants 6-12 in tall and 8-16 in wide; very attractive to butterflies.
Blue Horizon	Tall cut-flower	84 to 105 days	Vigorous triploid hybrid with purplish-blue clusters about 3 in wide on 30 in stems; excellent Phytophthora resistance.
Red Flint	Tall cut-flower		Upright, 24 in, with reddish-purple flowers; good cut or dried.
Timeless Mix	Tall cut-flower		24-30 in, with long-lasting blue, pink, red-purple and white flowers.
White Bouquet	Tall cut-flower		18-24 in, with snow-white flowers for cutting.
Leilani Blue
Neptune Blue
Redtop			Raspberry-red to magenta flowers.
Summer Stars
True Blue

Pests to watch for

Aphids, Whiteflies, Twospotted spider mite

Diseases and problems

Powdery mildew, Phytophthora root rot

In the kitchen

Safety: Not an edible flower. UF/IFAS lists ageratum as toxic to grazing animals, and in a 2022 study goats fed large amounts over several weeks developed liver and kidney damage (Bhatta and others).

Pets: Not listed by the ASPCA (no entry on its toxic plant lists for dogs or cats); UF/IFAS notes it is toxic to grazing animals, so keep pets and livestock from eating it.

Sources

Photo: Photo: The Cosmonaut / Wikimedia Commons, CC BY-SA 2.5 CA (CC BY-SA 2.5 CA). Photo resized for the web.

Planting dates and advice are general guidance; check with your local Cooperative Extension office for your area.