Ornamental · Asteraceae
Ageratum
Ageratum houstonianum
Floss flower lifts fuzzy blue powder puffs from late spring to frost on full sun and steady moisture.
Full sun
Every 2–4 days
50–86°F · 10–30°C
55–70 days
Overview
Ageratum, the floss flower, is a low annual in the daisy family whose flowers gather into soft, fuzzy clusters of blue, lavender, and purple. Each bloom head sits like a tiny powder puff, all frayed edges and no hard center, and they crowd the foliage from late spring until frost. The plant rides low at the front of a bed, traces a soft line along an edge, or breaks over the rim of a pot in a wash of color.
The plants build into rounded, cushiony mounds, the bedding kinds pressing close to the soil while the cutting kinds lift their heads higher for the vase. It shares a family with asters and sunflowers, but it skips their wide open faces and instead settles into a soft haze of cool color that blurs at the edges. A true blue rarely shows up in an annual, and ageratum holds that cool note as well as any.
Settled in, the plant asks for little beyond even moisture and warmth. Let the soil go bone dry and the soft stems sag and the powder-puff heads droop within hours. Give it sun, steady water, and open air moving through the leaves, and it answers with months of that fuzzy blue bloom.
Care
The short version: ageratum wants full sun softened by light afternoon shade in hot summers, rich well-drained soil kept evenly moist, and a monthly feeding through the growing season to keep the fuzzy blue heads coming.
Light
Ageratum opens the fullest, densest bloom in full sun, though it takes partial shade and even welcomes some afternoon shade where the summer sun bears down. In deep shade the powder-puff heads grow sparse and the soft stems stretch out loose and leggy.
Water
Keep the soil evenly moist, watering about every 2 to 4 days, since the soft foliage wilts and the heads droop soon after the ground dries. The plant wants steady moisture but not waterlogged roots, so drainage matters as much as a regular hand at the hose.
Soil
Plant it in rich, well-drained soil dark with organic matter, the kind that holds moisture in its crumb without turning to mud. A pH between 6.0 and 7.5 suits it, a range most garden ground reaches once a little compost is worked through.
Temperature
Ageratum runs warm and content from about 50°F to 86°F (10°C to 30°C). A single frost blackens and ends it, so wait until spring weather settles before the plants go out into the open.
Humidity
Ordinary outdoor humidity is fine, and there is no figure to chase. Moving air matters more, since still, heavy air pooling among crowded leaves is what coats them in powdery mildew.
Feeding
A balanced general-purpose fertilizer about once a month through the growing season is all it wants. Light, steady feeding keeps the leaves a deep green and the fuzzy heads coming, while heavier doses push soft, floppy growth.
Pruning
No real pruning is needed. Deadheading is optional, not required, but pinching off the spent, faded heads keeps the mound looking crisp and draws fuller, more even bloom.
Size & spacing
Plants stand 6 to 30 inches (15 to 76 cm) tall depending on the type, the dwarf bedding kinds low and cushiony, the cutting kinds reaching higher. Set each one 6 to 18 inches (15 to 46 cm) apart so air slips between the mounds and mildew finds no damp pocket to settle in.
When to plant
Ageratum is a frost-tender annual that holds back until the air warms. Start seeds indoors in late winter, about 6 to 8 weeks before your last frost, set the plants out only after frost has passed, and the fuzzy heads carry from late spring through the first hard frost.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Late winter, about 6 to 8 weeks before last frost
- Plant: After all danger of frost has passed in spring
- Harvest: Late spring or June through frost
Hardiness
USDA hardiness zones: 2–11
Temperature: down to -46°C
Troubleshooting
Most ageratum trouble shows up plainly on the leaves or stems, tracing back to either still air around the foliage or too much water at the roots. Caught early, the look of the plant tells you what to do. Match what you see below, then act before it creeps through the planting.
Blades drooping, yellowing, and slumping to the ground
When the leaves yellow, the stems sag, and the whole plant slumps despite watering, the trouble is usually root rot down in soggy, airless soil. Lift the base and the roots come up soft, brown, and slick with rot. Nothing brings a plant back from that, so pull it and dry out the drainage before anything new takes root in that spot.
Fix:
- Improve soil drainage before or at planting
- Reduce watering frequency and avoid waterlogging
- Remove severely affected plants
A floury white crust filming leaf blades and stems
A powdery, dust-gray film clinging to the leaves and stems is powdery mildew, a fungus that creeps in during hot, sticky weather when air sits dead around crowded plants. Open the planting so air moves, water at the base rather than across the foliage, and strip away the worst-coated leaves. Full sun and steady moisture keep the film from creeping back.
Fix:
- Improve air circulation by spacing plants properly
- Avoid overhead watering and water at the base
- Remove and discard heavily infected foliage
Glossy sticky leaves, kinked shoots, and knots of insects
A sticky, glazed sheen and curling new shoots give away aphids or whiteflies clustered on the foliage and flower buds. Turn the leaves over and you find pinhead aphids packed along the veins, or pale flies that rise in a small cloud when brushed. A hard jet of water knocks most loose, and insecticidal soap finishes the ones that cling on.
Fix:
- Rinse pests off with a strong spray of water
- Apply insecticidal soap if infestations persist
- Encourage natural predators
Yellow-flecked leaf surfaces laced with silky webbing
Leaves freckled with pale yellow dots and strung with the faintest webbing point to twospotted spider mites, which swarm dry, heat-stressed plants. Hold a leaf over a sheet of white paper, tap it, and watch the dust specks crawl. Soak the plant to lift the drought stress, hose the mites off the undersides, and turn to a miticide only if they hold their ground.
Fix:
- Keep plants adequately watered to reduce stress
- Rinse mites off leaves with water
- Apply miticide or insecticidal soap if needed
Toxicity
Soft as it looks, this plant is not for animals to graze. Ageratum carries pyrrolizidine alkaloids, natural compounds that damage the liver and gather with repeated feeding rather than passing through. A pet that mouths a fuzzy leaf usually suffers no worse than an upset stomach, but keep horses, rabbits, and grazing livestock away from it entirely, and treat no part of the plant as food.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
A curious cat that mouths a leaf usually shows nothing harsher than drooling, a bout of vomiting, loose stools, or a flat, sleepy mood. Steer it off the bigger bites, since the liver-damaging alkaloids gather quietly over time.
Dogs Mild
A dog that nips at the plant tends to come away with a mild churn in the stomach, maybe some vomiting, soft stools, or a dull spell. The deeper worry is heavy or repeated ingestion, because the alkaloids slowly eat at the liver.
Horses Toxic
Horses are tender grazers, and the pyrrolizidine alkaloids stack up to scar the liver across weeks of feeding. Watch for the body thinning, the eyes yellowing, and a dull, off temper, and keep this plant well past their reach.
Rabbits Toxic
Always nosing for greens, rabbits stand at real risk from these same liver toxins. Hand them no ageratum trimmings, and keep it clear of any run or hutch where soft noses might browse.
Livestock Severe
Cattle, sheep, and goats bear the heaviest toll. Drawn-out alkaloid poisoning hollows the appetite, drops the spirits, yellows the skin, swells the body, pools fluid in the belly, and scars the liver for good, so grazing stock must stay well off it.
Birds Toxic
The same liver-wrecking alkaloids that fell grazing animals offer birds no safety either. Keep the plant well out of reach of poultry and pet birds rather than gamble on repeated exposure.
People Caution
No part of ageratum is food. The pyrrolizidine alkaloids packed in its tissue can injure the human liver, so keep small hands and mouths off it, and brew or eat none of it.
Keep grazing animals away; contains liver-damaging pyrrolizidine alkaloids.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Ageratum splits by shape: tall stems cut for the vase and low mounds laid down for bedding, and the named selections fall on one side or the other. The few below are the ones most often seen.
Blue Horizon
A tall cutting type rising to about 30 inches, its medium-blue powder puffs carried high on stems long enough for the vase.
Blue Danube
A compact dwarf bedding cultivar that stays low and cushiony beneath a soft cloud of lavender-blue flowers.
Hawaii series (e.g. Leilani Blue)
Low mounding bedding ageratums grown for the dense, even blanket of blue they spread across the soil.
How to propagate
Ageratum comes from seed, and the seed is easy to work once you know its one habit. A tray started indoors before the season fills out into plenty of plants for a bed.
SeedEasy
Start indoors in late winter, about 6 to 8 weeks before the last frost; transplant or direct sow outdoors after frost danger passes. · Roughly 1 to 2 weeks to germinate; flowering in about 8 to 10 weeks from sowing.
Sow ageratum indoors in late winter, about 6 to 8 weeks before your last frost, then move the seedlings out or direct sow once frost danger has passed. The seed is little more than dust and needs light to germinate, so press it onto the surface and leave it bare. Green shows in 1 to 2 weeks, and the first fuzzy heads open about 8 to 10 weeks after sowing.
Myths
Ageratum is a perfectly safe little bloom, harmless to every pet and person.
It is not. Behind the soft look, the plant carries pyrrolizidine alkaloids that damage the liver, so a pet that chews it can get an upset stomach and grazing livestock can take lasting harm. Treat ageratum as toxic and keep animals from feeding on it.
Let the spent flower tufts stay and ageratum quits blooming.
Not so. The mound keeps throwing fresh heads all season whether or not the spent ones are trimmed. Deadheading is optional, a tidy touch that draws fuller, more even bloom rather than a chore the plant leans on.