How to Grow Ageratum (Floss Flower)

Blue ageratum with blue flowers and bees

The Spruce / Autumn Wood

The Ageratum genus, a member of the Aster family, includes more than 40 annual and perennial plant species native to Central America and Mexico, but for North American gardeners, the name ageratum generally refers to Ageratum houstonianum, also known as floss flower.

This annual plant is useful in flower beds for the large masses of bright purplish-blue flowers that bloom reliably from late spring until the first frosts of fall. At one time, only very low varieties were available, making this primarily an edging plant, but now there are now more upright varieties available, growing as much as 30 inches tall, making it also useful for the middle of the flower garden. Few other plants offer the long bloom period and the unique blue color of ageratum, though cultivars in different colors are also available.

Ageratum is normally planted from nursery bedding packs in the spring after all danger of frost has passed and the ground is warm. It can also be grown from seeds started indoors; the plant will flower about 60 to 70 days after the seeds sprout.

  Botanical Name Ageratum houstonianum
  Common Name Ageratum, floss flower
  Plant Type Annual
  Mature Size 6–30 inches tall, 6–18 inches wide
  Sun Exposure Full sun to part shade
  Soil Type Moist, well-drained soil
  Soil pH 6.0 to 7.5 (slightly acidic to slightly alkaline)
  Bloom Time June through first frost
  Flower Color Purplish blue; pink and white cultivars also available
  Hardiness Zones 2–11 (grown as a true annual)
  Native Area Mexico
Toxicity Toxic to grazing animals
Ageratum plant with purple flowers in garden

The Spruce / Evgeniya Vlasova

Purple ageratum flowers in the flowerbed.
Eugene4873 / Getty Images
Butterfly on Agaratum
blue ageratum flowers blooming in the garden
In the Midwest and many other regions, ageratum is a reliable plant that will produce mounds of fluffy blue flowers. 777Anna777/Shutterstock

Ageratum Care

Ageratum is a useful plant for lending long-lasting color in the mixed flower bed. Low-growing varieties are often used as edging plants or in rock gardens. Taller plants are a good addition to cutting gardens. Ageratum will do well in sunny locations planted in moist but well-drained soil that has been amended with compost or peat moss. Space shorter varieties about 6 inches apart, taller varieties about 12 inches apart.

As they become established, ageratums need to be kept moist. Once established, they will tolerate periods of dryness but will perform best if regularly watered. Covering the soil with mulch will help preserve soil moisture.

Light

In cold climates, give ageratums full sun. In the South, the plant can profit from some afternoon shade.

Soil

Plant ageratum in well-drained but moist soil that has been amended with compost. It is not fussy about soil pH, which increases this annual's versatility.

Water

Keep plants constantly moist as they are getting established. After this, about 1 inch of water per week from rainfall and/or irrigation will keep them healthy, but don't worry about short periods of drought, especially if the soil is well-mulched. Water through ground-level soaking rather than overhead spraying.

Temperature and Humidity

These plants are native to Mexico and Central America and will do best in warm conditions. Avoid the temptation to plant them too early in the season, as they may remain stunted for the entire growing season. Humid conditions may make ageratums susceptible to fungal problems; make sure the plants have good air circulation.

Fertilizer

Ageratums don't require a lot of feeding when planted in good, rich soil, but if the leaves begin to turn yellow, it's a sign they need nutrients. Granular slow-release balanced fertilizer mixed into the soil around the plants should return them to good health and profuse blooming.

Is Ageratum Toxic?

Like many plants, ageratums contain alkaloid compounds that evolved in the plant as a defense against insect and animal pests. These alkaloids sometimes make the plant quite toxic. In the case of ageratum, the pyrrolizidine alkaloid they contain is toxic to grazing animals, such as cattle, goats, and deer. It is not considered toxic to humans or to pets such as cats and dogs.

Symptoms of Poisoning

Visible symptoms in grazing animals that ingest this plant are not readily noticeable, but ingestion of the plant can gradually cause liver lesions.

Ageratum Varieties

There are many varieties of ageratum, with new cultivars regularly introduced. Consider these favorites:

  • 'Blue Horizon' is a taller blue variety, growing to 2 feet.
  • 'Hawaiian Royal' is a traditional compact series that is known for having genuine blue flowers.
  • 'Red Top' is a tall-growing variety with burgundy flowers.
  • 'Southern Cross' is a compact form of ageratum with bi-colored flowers.
  • 'Blue Danube' is an early blooming, 6- to 7-inch plant with icy blue-purple flowers.
  • 'Red Flint' is an unusual red variety that grows 24 inches tall.
  • 'Dondo White' is a white-flowering variety that grows to 24 inches.

Pruning

When the plants are young, pinching back the growing tips will cause the plants to spread and fill out. Though not essential, deadheading the spent flowers will stimulate continued growth and blooms. These plants will die back at the first hint of frost, at which time they should be pulled from the garden.

How to Grow Ageratum from Seed

Ageratums are easy to grow from seeds, but when sown directly into the garden, they may not flower until late summer. For this reason, they are usually started indoors, six to eight weeks before the last frost.

Spread the seeds over a cell tray filled with seed starter mix and just barely cover them with a sprinkling of starter mix; the seeds need light to germinate. Place the tray in a warm, sunny location and mist frequently until the seeds germinate and sprout. Thin out the seedlings to one per cell. Keep the soil moist but not soaked as the seedlings develop true leaves. As warm weather approaches, harden the seedlings off by giving them increasing periods of outdoor time for a week or two before transplanting. They can be planted in the garden once all danger of frost has passed and soil temperatures are at least 60 degrees Fahrenheit.

Potting and Repotting

Ageratum can make a good plant to include in mixed patio containers, hanging baskets, or window boxes. It will do well in any well-draining commercial potting mix.

Common Pests/Diseases

Powdery mildew, a soil-borne fungus, can affect ageratums. Prevent the spores from splashing up on the plants by watering at ground level rather than spraying from overhead.

Spider mites can be a problem in hot, dry weather. Misting regularly can prevent these infestations. Damaged plants can be cut back and will usually sprout again.

Blue ageratum plant with blue flowers

The Spruce / Autumn Wood

Blue ageratum with blue flowers and leaves and bee in background

The Spruce / Autumn Wood

Blue ageratum with blue flowers in front of leaves

The Spruce / Autumn Wood