Growing fall asters
Asters are hardy perennials that bloom in fall in various colors and heights. They provide color for us and a late-season source of pollen for bees and other pollinators.
The image many have of New England in the fall includes purple asters and yellow goldenrod. Although we may take these for granted, the English and Europeans have not. In fact, they collected asters extensively earlier in this century and took them home to breed many new selections.
Many of these selections are back in this country now, along with many other recent cultivars (cultivated varieties) by American growers. These generally range in bloom time from early September to late October, with a particular species or cultivar usually blooming for two or three weeks.
The main types of asters are the New York (Aster novi-belgii) and the New England (Aster novae-angliae). The New York ones generally tend to be shorter (a foot or so) than the New England ones (three feet or more). Both come in a range of colors from red to purple, bluish to white, and provide a nice complement to the colors of fall mums.
Other cultivars have been developed from other species. While most of these are short, some such as heath asters (ericoides) may reach two feet and be covered with hundreds of tiny white flowers. Calico aster (Aster lateriflorus horizontalis), named from the appearance of its many tiny pink and white flowers, is a species reaching one to two feet. Unlike most asters, stems of this species are arranged in horizontal layers giving rise to another common name, horizontal aster. This one also can be found growing wild locally, or as a cultivar with dark leaves called Lady in Black. Quite popular are the Frikart’s asters, named after the Swiss nurseryman who developed them in the 1920’s. These hybrids are marginally hardy (to USDA zone 5) in some parts of our region or need some winter protection.
Purple Dome is one of the more recent introductions, and is an introduction from this country of our New England asters. It is covered with purple flowers through a long period in the fall, and unlike many in this species only gets two feet tall or less.
Treat asters as you would other perennials. Plant in good loamy soil as most don’t like wet feet or may get frost-heaved and dry out in sandy soils. Plant at least one foot apart for the shorter cultivars, three feet apart for the taller ones. Some light fertilizer such as one-fourth to one-half cup of an organic fertilizer early in the season will help. Some of the tall New England asters may need staking. Alternatively, cut them back by one third in early summer to promote shorter, bushy growth.
Since most are grown and sold in pots, they can be planted any time during the season. If plants require moving or dividing, do it in May as the new shoots emerge. If growing well, asters may need division every two to three years.
The main plant disease is powdery mildew–a whitish growth that may appear on leaves from late June or July onwards, mainly on the New York varieties. Research at the University of Vermont (UVM) and other institutions has shown that applying sprays beginning in late June, according to label directions for horticultural oils (as used for insects), will help prevent powdery mildew. Sprays closely related to baking soda also can be used.
Sprays must be applied before diseases become established and must be continued throughout the season. Such diseases, although unsightly, often cause no lasting harm to the plants and vary in severity depending on the weather and even the cultivar.
The main destructive insect pest in our area seems to be the lacebug, a small grayish insect that appears in midsummer and sucks the plant juices from the undersides of leaves, primarily of the New York and related types. Leaves turn yellowish and eventually brown and fall off. Organic or synthetic insect sprays can be used for control. Read and follow all label directions for best control, and safety for you and the environment.
Dr. Leonard Perry is Extension Professor, Department of Plant and Soil Science at the University of Vermont. Visit his website at http://www.uvm.edu/~pass/perry/index.html