Ageratum: A Guide to Growing and Caring for the Floss Flower

Updated April 8, 2021
purple floss flower

Ageratum, also known as floss flower, is an easy-to-grow summer annual with fluffy purple flowers. It is one of those special plants that will re-seed itself without becoming particularly invasive, making it a great choice for cottage gardens.

Ageratum Flower Species Description

The first thing to know about ageratum is that the named cultivars and the basic species each behave somewhat differently in the garden. The floss flower provides a unique and vibrant color splash for your garden.

Basic Ageratum Species

The basic species - in other words, any ageratum seed packet that does not have a cultivar name - is the one that reseeds itself readily. It grows to about two feet tall. Ageratum leaves resemble mint leaves, although the flowers look like tiny asters, to which the plant is related. The basic species is best when scattered sporadically among other annuals and perennials in unstructured plantings.

Ageratum leaves

Ageratum Cultivar Colors

The named cultivars tend to be much shorter and compact plants than the two-foot basic species, and come in various shades of blue, purple, pink, and even white and red. The cultivars are also much less likely to seed themselves, which could be seen as a pro or a con, depending on how you want to use them. The low-growing cultivars are great for creating a mass of purple or blue in large flower beds.

Ageratum Facts and Growing Guide

Ageratum grows equally well in full sun or partial shade. It doesn't require the richest garden soil, yet it isn't a plant for poor rocky soils either. Good drainage is important.

Ageratum in the garden

Sowing Seeds for Ageratum Flowers

Ageratum grows quickly from seed planted in spring and will bloom until the first fall frost. The roots don't like to be transplanted, so it's best to sow ageratum directly where it is to grow. You can sow seeds directly on the surface of the soil without covering them with soil, since the seeds need light to germinate. Ageratum shorter varieties are especially suited to be sown to grow in a container or flowerpot.

Grow Ageratum Seedlings

You can use peat pots to give ageratum an early start indoors. The ageratum seedlings can be planted directly in the ground with the pot. You don't need to remove the peat pot since it will decompose.

Establishment and Care for Ageratums

Ageratum requires very little care other than a modest amount of watering during dry spells. The flowers can be dead-headed, but it's generally not the best use of a gardener's time. The spent blossoms all but disappear on their own and will provide self-seeding. If anything, it's helpful to cut the tall varieties back about 25 percent in midsummer to keep them from getting too leggy.

Common Ageratum Pests and Diseases

Ageratum is generally free of pests and disease. In humid climates, powdery mildew can be an issue toward the end of the growing season. While you can always treat powdery mildew, many gardeners find that since it is at the end of the season, it's better to pull the plants out.

Ageratum Varieties for Your Garden

Purple and blue ageratum varieties abound. There are many shades of each available, along with a few more exotic forms.

  • Hawaii is a mix of blue, white, and pink varieties.
  • Blue Danube has electric blue flowers on dwarf plants just six or eight inches tall.
  • Bavaria has white flowers with bright blue edges and grows about 12 inches tall.
powder blue ageratum

Floss Flower for Light Blue Flowers

The floss flower (Ageratum houstonianum) is also known as Blue Mink, Mexican Paintbrush, Blueweed, and Blue Horizon. It is grown in the garden of The Jefferson Monticello, President Thomas Jefferson's home. The floss flower has light blue flowers on bushy and compact 6" to 30" tall plants, depending on the type of floss flower.

Perennial Ageratum Can Be Invasive

The most commonly known Mistflower or Blue Mistflower (Conoclinium coelestinum) is also called Ageratum Blue, Wild Ageratum, and Hardy Ageratum. Like other ageratums, mistflower is self-seeding, but unlike others, it is also a perennial. The mistflower has a very aggressive rhizome production. Propagation can be done using clump division when the plants first emerge in early spring. The perennial ageratum can be invasive thanks to its rhizomes, unlike self-seeding annuals.

Ageratum Self-Seeding Summer Annual

Ageratum is a favorite of butterflies and comes in a striking range of colors. The plant grows quickly and has a long bloom period with the ability to self-sow, making it indispensable if you want cottage garden attributes.

Ageratum: A Guide to Growing and Caring for the Floss Flower