Floss Flower: How To Care For Ageratum Houstonianum

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Best known as a floss flower, Ageratum houstonianum (a-ger-AE-tum ho-stone-ee-AE-num) has a reputation for being popular in the New World and infamous in Hawaii and everywhere else.


This annual goes by many names, including:

  • Ageratum
  • Blue maudlin
  • Bluemink
  • Flossflower
  • Blueweed
  • Mexican ageratum
  • Mexican paintbrush
  • Pussy foot

The Floss Flower originated in Mexico and much of Central America but has since spread worldwide.

It is a warm-season annual hailing from the Asteraceae family that resembles regular asters but lacks ray flowers.

Instead, the tufted flowers come in a range of colors, most famously natural blue tones. A close relative, the blue billygoat weed (Ageratum conyzoides), is often considered synonymous with the floss flower.

Floss Flower Ageratum Care

Size & Growth

The majority of cultivars in this species range from 6 to 24” inches tall clumped in a dense mound 6 to 18” inches wide.

A few cultivars of floss flowers, such as ‘Blue Horizon’ and classic ‘Blue Danube’ achieve a height of 30″ to 36” inches. It generally requires 60 to 100 days to bloom after germinating.

The toothed leaves are oval in shape, measuring up to 2” inches long. Its stems are round and green with soft hairs.

Flowering and Fragrance

Ageratums are famous for their true blue blooms, which last from May to November in parts of the US.

It gets its common name from the feathery, pom-pom-shaped blue flowers which have a pleasant fragrance and tend to cover the plant.

Ageratums produce fuzzy, tufted flowers in rounded, dense flower heads adding a desirable blue flower color to the garden, even in partial shade.

Between 5 and 15 florets grow on each flower cluster. They can grow from 12″ to 39″ inches tall.

The original medium blue has been expanded through cultivation to include bicolor, burgundy, lavender, mauve, pink, powder blue, purple, red, and white.

Its seeds are tiny, black or brown, and have five whitish scales that allow them to be wind-sown.

Light & Temperature

While floss flower can tolerate partial shade, it thrives in full sun. An excess of shade may reduce the number of blooms, cause legginess, and fade the flower color.

Light shade may prove beneficial in especially hot regions.

While it’s sensitive to frost, it will grow anywhere in the United States.

It prefers temperatures between 40° and 85° degrees Fahrenheit. In USDA hardiness zones 10 to 11, it is often grown as a perennial.

Watering and Feeding

Ageratum houstonianum requires a moderate amount of water due to its shallow roots. Regular watering will encourage growth and help prevent wilting.

Blue Ageratums do not do well if conditions are too dry and wilt fast if this is the case.

Ageratum plants prefer warmth over a cold. This brings some people to suggest watering the plant using warm water as opposed to cold.

This is important when the plants are young since warm water seems to speed up plant growth.

Keep the soil moist throughout spring and summer by watering floss flowers regularly.

Floss flowers need about 2.5″ inches of water a week.

For daily irrigation use a soaker hose or use a sprinkler for watering twice weekly. Reduce the amount of water you give if plants receive more than 1″ inch of rainfall in a particular week.

Fertilizing Ageratum Floss Flowers

Ageratum houstonianum requires regular feeding. Use a granular, slow-release fertilizer mix, applying at planting and mid-season.

Or, feed your floss flower ageratum every two weeks with a water-soluble liquid blooming plant fertilizer.

Follow the instructions provided on the packaging for proper dosage and application.

A protective layer of organic mulch or compost helps keep the soil from drying out. Try to avoid contact with the base of the plant to reduce the risk of rot.

Soil & Transplanting

Ageratum needs a well-drained, rich organic soil to thrive. Acidity over 7.0 pH will kill seedlings but has less effect on established plants. Avoid sandy soils, which may make the plant chlorotic.

When transplanting, space the plants 8 to 10” inches apart in holes big enough for the root ball. Be sure to cover the root ball completely and water thoroughly to compact the soil.

Around the base of your plants apply a 2″-inch layer of mulch to ensure the soil does not dry out. For best results, use grass clippings, organic compost, or bark mulch.

Be sure to refresh the mulch layer as it decomposes to always ensure you maintain it not less than 2″ inches.

Grooming And Maintenance

You may think this fast-spreading ornamental means it’s low maintenance. But it may actually need more care than many other plants in your garden.

Ageratum needs regular feeding and watering and may suffer if neglected. Taller plants may need support.

As a flower head dies, it will drop off and be replaced by new flowers. But, you may choose to deadhead spent flowers to encourage more prolific blooming.

The new blooms form on top of the old ones. Pinch off the browned blossoms to encourage new growth.

Remove the browned blossoms using your fingers and as close to the stem as possible.

Additionally, you will want to be careful of seeds throughout the blooming cycle. They can spread quickly to take over the rest of the garden.

Ageratum doesn’t generally need pruning. But you may wish to trim them back if they begin spreading or start wearing out.

As these plants are frost-tender, most will die off at first frost. But, protecting the roots and covering them at night may extend their lifespan. 

How To Propagate Ageratum Houstonianum

Start floss flower seeds indoors about 6 to 8 weeks before the last frost. Germination takes 7 to 21 days on average.

Take your young plants outside after all the danger of frost passes. Transplant after last frost.

After the risk of frost is over, you can plant directly outdoors although the bloom time will be shorter. Aim for a pH of 6.0 to 7.0 during germination.

Plant these annuals under 12″ inches in a sunny to the light-shaded area of the garden in moist, well-drained soil. Ageratum is not choosy about soil conditions but does not do well in soggy conditions.

For improved health and growth of Ageratum, add 4″ to 6″ inches of organic compost to the planting location. Using a garden tiller incorporate the compost into the soil. The compost helps improve soil fertility.

Floss Flower Pests and Diseases

When cared for properly, the ageratum flower is generally hardy. However, you need to look out for a few pests.

Several fungal and bacterial infections target Ageratum houstonianum. The good news is – that these infections are usually treatable.

Powdery mildew, Botrytis, and root rot will attack when soil moisture or humidity gets too high.

Insect infestations may include:

  • Aphids
  • Fungus gnats
  • Plant lice
  • Red spider mites
  • Thrips
  • Whiteflies can weaken the plant and cause the pointy leaves to yellow

Lure away whiteflies using an old-fashioned fly trap. Make one by painting a post bright yellow and covering it with a sugar solution, honey, or any other sticky substance.

Your Ageratum probably succumbed to a bacterial infection if you discover:

  • Severe wilting, yellowing, or dying Agregatum leaves
  • Find any lesions on the stems

You need to remove the unhealthy plant to ensure that the infection does not spread.

You may find mold on the flowers and stems or brown spots on the foliage of your plants. They will probably survive the problem through the season.

It is advisable to thin your plants to allow more airflow and air circulation around the plants.

Sunlight will keep:

  • The flowers and leaves drier
  • Prevent molds from damaging plants more

Generally, Ageratums are rabbit and deer tolerant. Deer may munch on the leaves when there’s a shortage of other food resources.

Be warned that Ageratum houstonianum contains pyrrolizidine alkaloids. Grazing animals may develop liver lesions from consuming this plant.

The plant’s ability to spread has given it a reputation in much of the world as an invasive weed. Seeds can travel not only by wind or water, but by fur, clothing, vehicles, and produce.

While naturalized throughout much of North America, it is considered invasive in Hawaii.

Suggested Ageratum Uses 

One of the best qualities of this popular ground cover is its ability to repel mosquitoes while attracting a range of pollinators and butterflies. It’s known to be a key nectar source for the vulnerable Eumaeus atala butterfly of Florida.

This annual flower is great for a wide range of displays, from edging borders and rock gardens to window flower boxes, pots, and as a bedding plant.

Its natural, long-lasting blue flowers of the Houstonianum Ageratum draw attention in flower beds, while the color variants and pleasant smell give a reason to pause during a long day. The fragrance also makes these great cut flowers for table displays.

Another interesting value of the plant is its ability to reduce the presence of certain nematodes and other infectious pests while attracting beneficial insects.

When used in complementary farming, it has displayed the ability to reduce fungal infections in tomato plants. Some evidence also suggests essential oils may help repel some tick species.

It is one of the few annual plants tolerant of light shade. Ageratum blooms from late spring through late summer and all the above qualities make this species of flowering plant a fine choice for outdoor containers.

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