Ornamental · Asteraceae
Ageratum
Ageratum houstonianum
Give floss flower full sun and steady moisture, and it carries soft blue blooms from late spring to frost.
Full sun
Every 2–4 days
50–86°F · 10–30°C
55–70 days
Overview
Ageratum, or floss flower, is a low garden annual in the daisy family grown for its fuzzy clusters of soft blue, lavender, and purple flowers. The bloom heads look like little powder puffs, and they keep coming from late spring until frost with hardly a pause. It earns a spot at the front of a bed, along an edge, or in a pot where its color can spill over the rim.
Plants stay neat and mounded, with the bedding types hugging the ground and the cutting types reaching higher for the vase. It belongs to the same family as asters and sunflowers, but unlike those bold faces, ageratum reads as a soft haze of cool color. Few annuals hold a true blue this well, which is why it turns up again and again in summer plantings.
It asks for little once it's settled, but it does want even moisture and warmth. Let the soil bake dry and the plant wilts in a hurry. Give it sun, steady water, and decent air around the leaves, and it rewards you with months of bloom and very little fuss.
Care
The short version: full sun with light afternoon shade where summers run hot, rich and well-drained soil kept evenly moist, and a monthly feed through the growing season to keep the blue flowers coming.
Light
Ageratum flowers best in full sun, though it takes partial shade and even welcomes a little afternoon shade where summers turn hot. Too much shade thins the bloom and leaves the plants loose and leggy.
Water
Keep the soil evenly moist, watering roughly every 2 to 4 days, since the plant wilts fast once the ground dries out. It likes consistent moisture but not soggy feet, so good drainage matters as much as regular watering.
Soil
Plant in rich, well-drained soil with plenty of organic matter worked in to hold moisture without staying waterlogged. A pH between 6.0 and 7.5 suits it, so most garden ground works with a little compost added.
Temperature
Ageratum grows happily from about 50°F to 86°F (10°C to 30°C) and wants warmth to thrive. It has no tolerance for frost, which ends the plant, so wait for settled spring weather before setting it out.
Humidity
Average outdoor humidity is fine and there's no figure to aim for. Air movement counts for more, since still, muggy conditions around crowded leaves are what invite powdery mildew.
Feeding
Feed with a balanced general-purpose fertilizer about once a month through the growing season. That steady, light feeding keeps the foliage green and the flowers coming without pushing soft, floppy growth.
Pruning
No real pruning is needed. Deadheading is optional rather than required, but snipping off spent flower clusters keeps the planting tidy and encourages fuller, more uninterrupted bloom.
Size & spacing
Plants range from 6 to 30 inches (15 to 76 cm) tall depending on the type, the dwarf bedding kinds staying low and the cutting kinds reaching higher. Give each one 6 to 18 inches (15 to 46 cm) of room so air moves freely and mildew stays down.
When to plant
Ageratum is a frost-tender annual that waits on warm weather. Start seeds indoors in late winter, about 6 to 8 weeks before your last frost, set the plants out only after the danger of frost has passed, and the flowers 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 traces back to either too little air around the leaves or too much water at the roots. Catch the early signs and a fix is usually simple. Match what you see below, then act before it spreads through the planting.
Plants wilting and yellowing, then collapsing
When a plant yellows, wilts, and falls apart despite watering, the cause is usually root rot from soggy, poorly drained soil. Dig at the base and you'll find soft, brown, mushy roots. There's no saving a plant this far gone, so pull it and fix the drainage before anything else goes in that spot.
Fix:
- Improve soil drainage before or at planting
- Reduce watering frequency and avoid waterlogging
- Remove severely affected plants
Dusty white film coating the leaves and stems
This is powdery mildew, a fungus that takes hold in hot, muggy weather when air sits still around crowded plants. Open the planting up for better airflow, water at the base instead of over the leaves, and strip off the worst-hit foliage. Full sun and steady moisture keep it from coming back.
Fix:
- Improve air circulation by spacing plants properly
- Avoid overhead watering and water at the base
- Remove and discard heavily infected foliage
Sticky leaves, twisted growth, and clustered insects
Sticky honeydew and warped new growth point to aphids or whiteflies feeding on the foliage and flower buds. Check the undersides of leaves for soft-bodied aphids or tiny white flies that scatter when disturbed. A strong jet of water knocks most of them off, and insecticidal soap handles the stubborn cases.
Fix:
- Rinse pests off with a strong spray of water
- Apply insecticidal soap if infestations persist
- Encourage natural predators
Pale speckled leaves with faint webbing underneath
Fine yellow stippling and thin webbing are the mark of twospotted spider mites, which run rampant on dry, drought-stressed plants in hot weather. Tap a leaf over white paper and you'll see the specks crawl. Keep the plant well watered to take the stress off, rinse the mites away, and reach for a miticide only if they hang on.
Fix:
- Keep plants adequately watered to reduce stress
- Rinse mites off leaves with water
- Apply miticide or insecticidal soap if needed
Toxicity
This is not a plant to let animals graze. Ageratum contains pyrrolizidine alkaloids, natural compounds that damage the liver, and they build up with repeated feeding rather than passing through. Pets that nibble a leaf usually get no worse than an upset stomach, but keep horses, rabbits, and grazing livestock away from it entirely, and don't treat any part of the plant as food.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
A curious cat that chews a leaf usually shows nothing worse than drooling, vomiting, diarrhea, or low energy. Keep larger nibbles off the menu, since the liver-damaging alkaloids add up over time.
Dogs Mild
Most dogs that sample the plant get away with a mild stomach upset, perhaps some vomiting, loose stools, or a dull mood. Repeated or heavy ingestion is the real worry, because the alkaloids slowly harm the liver.
Horses Toxic
Horses are highly sensitive grazers, and the pyrrolizidine alkaloids build up to scar the liver over weeks of feeding. Watch for weight loss, jaundice, and odd, dull behavior, and keep this plant well out of their reach.
Rabbits Toxic
As steady plant-eaters, rabbits are at real risk from the same liver toxins. Don't offer ageratum trimmings, and keep it out of any run or hutch where a rabbit might browse.
Livestock Severe
Cattle, sheep, and goats face the worst of it. Chronic alkaloid poisoning brings on poor appetite, depression, jaundice, swelling, fluid in the belly, and lasting liver damage, so grazing animals must be kept off it entirely.
Birds Toxic
The same hepatotoxic alkaloids that harm grazing animals are not safe for birds either. Treat the plant as off-limits for poultry and pet birds rather than risk repeated exposure.
People Caution
No part of ageratum is food. The pyrrolizidine alkaloids it carries are known to injure the human liver, so keep children from putting it in their mouths and don't brew or eat any part 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 comes in tall cutting types for the vase and low mounding types for bedding, and the named selections sort neatly along those lines. These are the ones you'll see most often.
Blue Horizon
A tall cutting type that reaches about 30 inches with medium-blue flowers held high enough for the vase.
Blue Danube
A compact dwarf bedding cultivar that stays low and tidy under a haze of lavender-blue flowers.
Hawaii series (e.g. Leilani Blue)
Low mounding bedding ageratums grown for their dense, even blanket of blue flower coverage.
How to propagate
Ageratum is grown from seed, and the seed is easy to handle once you know its one quirk. Start a tray indoors ahead of the season and you'll have plenty of plants to fill 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.
Start ageratum indoors in late winter, about 6 to 8 weeks before your last frost, then transplant or direct sow once frost danger has passed. The seed is tiny and needs light to germinate, so press it onto the surface and don't bury it. Expect sprouts in 1 to 2 weeks and the first flowers about 8 to 10 weeks after sowing.
Myths
Ageratum is completely safe and non-toxic to pets and people.
It isn't. The plant carries pyrrolizidine alkaloids that damage the liver, so a pet that chews it can get an upset stomach and grazing livestock can suffer lasting harm. Treat ageratum as toxic and keep animals from feeding on it.
You have to deadhead ageratum constantly or it stops flowering.
The plant blooms on its own all season whether or not you snip the spent heads. Deadheading is optional, a tidiness habit that nudges fuller, more even bloom rather than a chore the plant depends on.