It is not so easy to find deep blue flowers in the summer flower bed, but ageratum is an easy to grow annual that helps satisfy a gardener’s desire for true blue flowers all season.
A compact plant that grows only eight inches high, ageratum can be grown from seeds or purchased in the spring as small plants from garden centers. The puffy, azure flowers help satisfy a need when other blue flowers have faded in the heat.
“Blue Danube” is a variety of ageratum have a lovely lavender color. “Hawaiian White” is a white variety. Ageratum also comes in purplish and rose colors.
The flowers grow as clusters on the tips of the stems and have numerous tiny frills around the center of the bloom. The leaves are somewhat heart shaped, deeply veined, and are an attractive medium green.
Plant the seeds when the frost season has passed and the soil has started to warm. Place them about a eighth of an inch deep and firm the soil over them. Ageratum seeds need some light to germinate. Keep them consistently watered until the seeds sprout in about one week. Warm water will encourage sprouting. Thin or transplant four inch tall seedlings eight to ten inches apart.
Ageratum will grow in full sun to light shade, though half day of sun is best, especially in hotter climates. A bed that gets morning sun and afternoon shade is ideal. Ageratum does well in medium rich soils. The plant needs about an inch of water per week when no rain falls, though it is slightly drought resistant.
The flower looks good in rock gardens or containers. Its short height make it great for growing in the front border of a flower bed. Plant in groups to give a better effect and make the blue color more noticeable.
Ageratum has few diseases.
Give ageratum only light feedings of liquid fertilizer about 60 days apart. Keep the dead flowers cut in order to encourage continuous blooms. Insects and diseases usually leave ageratum alone, though spider mites can be a problem in warm, humid weather. If one plant does look unhealthy, pluck it out before it has a chance to infect its neighbors.
Butterflies are frequently attracted to ageratum for its nectar.
Any hard freeze will kill ageratum in the fall though it might survive a light frost, especially if it is covered overnight.
The scientific name for the flower is Ageratum houstonian. One common name is “floss flower” because of the frilled blooms.