The daisy-like blossoms of the aster led Swedish botanist Carl Linnaeus to name the plant family after the Greek word “astron,” meaning star. Asters are also known as Michaelmas daisies, because their bloom time coincides with the September 29 feast day of St. Michael. Most varieties of aster bloom in late summer and fall, providing color when many other garden plants are done.
The two main types of asters are New England asters (Aster novae-angliae) and New York asters (Aster novi-belgii), according to the University of Vermont Extension. The New York asters tend to be shorter, and serve to brighten the front of the border; most New England asters are three feet or more in height. Some classic aster cultivars are “Alma Potschke,” which has rose-pink flowers on a three-foot high plant; and “Purple Dome,” at 18 to 24 inches, a compact grower with bright purple blossoms. Frikart's Aster is a summer-bloomer with lavender-blue flowers.
Asters are sometimes mistaken for daisies. Like other daisy-type flowers, the aster’s bloom is made up of two types of flowers, says the University of Illinois Extension. What we think of as petals are really ray flowers. The central flat button of the flower is made up of hundreds of small disk florets. The ray flowers of the aster may be white, pink, lavender, mauve, blue, red or purple, but the center is always golden or orange in color.
Asters prefer a location in full sun and plenty of air circulation. They will look their best if planted in a well-drained soil to which rich organic material has been added. It is important to keep the soil moist, since dryness make asters more susceptible to powdery mildew. Watering at the base of the plant, or using drip irrigation, is best. Asters are exuberant growers, benefiting from division every other year.
Discard the worn-out centers, replanting the healthy young growth on the outsides of the clump. Most asters should be divided in the spring to ensure they have adequate time to establish good root growth before they bloom in fall or late summer; divide summer-blooming asters after flowering.
The tallest asters become top-heavy when they come into bloom, requiring staking to prevent them from flopping. You can avoid having to stake asters by pinching them back in the same way you pinch back chrysanthemums. Pinch back once when the plants are about 6 inches high, and again about a month later. Don’t pinch plants after mid-summer (around the 4th of July), otherwise they will not have time to put on enough growth to bloom before frost.
Fall-blooming asters typically come in cool shades of blue and violet, making a excellent contrast with the warm colors of autumn chrysanthemums. Asters are very useful as cut flowers, serving to fill out around taller, single-stemmed flowers. They also make an excellent addition to any butterfly garden, since they are are a favorite nectar source for the monarch, painted lady, and other types of butterflies.