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How to Grow: Ageratum
in Ageratum houstonianum
full sun, part sun
Bloom Period and Seasonal Color
Ageratum bloom summer to frost in colors such as blue, white, violet and pink
Mature Height x Spread
6 to 24 inches x 6 to 12 inches
Ageratum attracts butterflies, drought tolerant, deer resistant
Ageratum is a workhorse annual flower in many New England gardens. It’s a low growing, low maintenance annual that blooms all summer in sun or part shade with little care. The small mounding plants tolerate dry, infertile soil once established. Small, button-like ageratum flowers are produced in clusters on these plants. Although various shades of blue are the most common colors of ageratum, pink and white selections are now available to broaden the color scheme and allow gardeners to mix and match ageratum with a wider variety of plants. They look particularly attractive used along the front edge of a border of annual or perennial plants or in a container with taller annuals. In my garden butterflies love to alight on patches of ageratums.
Where, When and How to Plant
Ageratum are not fussy about soil conditions as long as it’s well-drained. Purchased transplants from the local garden center and set them out into the garden planted 6 inches apart in spring after all danger of frost has passed. To produce a larger quantity of ageratum for less cost, start seeds indoors 8 to 10 weeks before the last frost date in your area. Barely cover the seed with potting soil, as they need light to germinate. Plant in full sun in cooler parts of New England. Plant in part shade in warmer regions, or if grown a warm microclimate such as near a south-facing stone wall.
Keep plants well-watered and weeded once transplanted. Although they can tolerate dry conditions, young plants need moist soil to get established. Pinch back the growing tips of young plants to stimulate them to branch out and form a bushier plant. This will lead to more flowers. Deadhead the flowers to keep the blooming all summer and for a tidier appearance.
Regional Advice and Care
During hot, dry spells spider mites can attack ageratum causing them to dieback. To prevent spider mite infestations, mist the plants during dry spells with water. Spider mites don’t like humid conditions. Cut back damaged plants if you miss the infestation and they should regrow and flower again.
Companion Planting and Design
Grow ageratum in front of walkways or in flower beds. They look great planted in masses of one color. Pair ageratum with silver foliaged plants such as artemesia. Petunias and another low growing annuals look great next to ageratum. Plant contrasting colored flowers, such as rudbeckia or tall marigolds, behind ageratum to create a nice color combination.
The ‘Hawaii’ Series’ of ageratum features white, pink and blue flowered selections that only grow 6 inches tall. ‘Hawaiian Royal’ is particularly attractive for its true blue color. ‘Blue Horizon’ is a taller growing ageratum that can reach 2 feet tall. ‘Red Top’ is another tall grower that features burgundy colored flowers. ‘Southern Cross’ features bi-colored flowers on compact plants.
Excerpted from my book, New England Getting Started Garden Guide.