Garden Asters

Few things are as depressing to the avid gardener as the end of the growing season, and September tends to remind us of that annual inevitability. However, September is the time of year when some ornamental plants are just coming into their full glory and garden aster is one of those plants. Their petite, daisy-like flowers are one to two inches in diameter and come in a wide array of vibrant colors, including some of the truest blues to be found anywhere in the plant world. At a time when many gardeners are making one last trip to the local nursery for hardy chrysanthemums, garden asters should be considered also. They share many of the same cultural requirements with mum, making them an ideal companion plant in the garden.

Aster is actually a genus of plants containing 250 species. Garden aster, along with the likes of zinnia, marigold, dahlia, chrysanthemum and many, many other useful garden species, is a member of the Asteraceae (formerly Compositae) family. The word Aster is derived from a Greek word meaning "star" and refers to the star-like shape of the flower. Garden aster (as well as all members of the Asteraceae family) bears a compound flower known as a head consisting of disc florets that comprise the center or eye, and ray florets that radiate from the eye to form what most people erroneously refer to as the petals. Most garden asters belong to one of two different species: Aster novibelgii (New York Aster) or A. novae-angliae (New England Aster); both are native to North America and hardy to zone 4. Important related species include A. tongolensis (East Indies Aster) and A. lateriflorus ‘Horizontalis’ (Calico Aster). Plant breeders have succeeded in improving the wild aster when selecting for compact plant habit and different flower colors. Garden aster often goes by the common name of Michaelmas Daisy; the latter taken from the fact aster blooms around Michaelmas Day (or the feast of St. Michael the Archangel) which is observed on September 29th.

As previously mentioned, the culture of aster is very similar to that of chrysanthemum. Like chrysanthemum, aster is a short-day plant meaning that it requires a long, uninterrupted period of darkness in order to bloom. The long days of spring and early summer promote vegetative growth in aster; the shorter days of late summer trigger flowering to occur. Aster enjoys a full-sun exposure in a well-drained soil of average fertility. The addition of welldecomposed organic matter can help to loosen tight soils. Garden asters purchased in bloom growing in containers need only to be watered; those established in a perennial garden should be given only modest amounts of fertilizer during the growing season since excess fertility leads to tall, "floppy" plants. Garden asters are relatively insect-free but do suffer from several troublesome diseases including aster wilt (yellows) and powdery mildew. Several new cultivars have been developed that are more tolerant of mildew than some of the older, more familiar cultivars.

Unlike chrysanthemums, asters have a fairly upright growth habit and range in height from three to five feet in nature. Although plant breeders have succeeding in developing shorter cultivars (often through intragenic hybridization), most aster cultivars that have been overwintered as established plants in the garden need to be pinched as they grow in order to keep them compact and attractive. After emerging in the spring and achieving a height of about six to eight inches, plants should be pinched every two to three weeks until about July 25, the date on which flowering is triggered in most cultivars. Pinching also promotes axillary buds to develop which results in a fuller plant with a more attractive floral display.

The following represent some popular cultivars of garden aster available commercially:

• ‘Alma Potschke’ Bright, rose-pink flowers on vigorous, 36-inch plants. Requires staking.
• ‘Celeste’ Striking, dark lavender-blue flowers with yellow centers. Medium vigor, 24 inches tall.
• ‘Frida Ballard’ Medium-sized, deep raspberry flowers. Medium vigor, 24 inches tall.
• ‘Patrica Ballard’ Large, double lavender-pink flowers. Medium vigor, 36 inches tall.
• ‘Prof. Kippenberg’ Large, purple-blue flowers. Compact growth; 15 inches tall.
• ‘Purple Dome’ Purple flowers profusely borne. Forms 18 inch purple mound in bloom.
• ‘Winston Churchill’ Large, bright raspberry flowers with yellow centers. Medium vigor, 24 inches tall.

David Trinklein
Associate Professor Plant Sciences