The appeal of ageratum is its blueness. The well-mannered plants are mostly 6 to 12 inches tall. In summer, the puffy flowers come in some of the garden’s truest shades of blue.

Use ageratum in containers (click here for more on container gardening) or as an edging for borders. Set out transplants in spring, when temperatures begin to warm up and the danger of frost has passed.

Provide full sun unless you live in a hot-summer climate, where part shade is best. Nursery plants are often sold in bloom. After you plant them, pinch back the flowers to encourage bushy growth.

Make a special effort to cut off faded flowers, especially the white ones, which turn an unsightly brown.