Planting
Where to Plant Asters
Asters prefer areas with cool, moist summers and cool nights in sites with full to partial sun. In warmer climates, asters do not like the hot midday sun. Soil should be moist but well-draining and loamy. Wet clay soil will lead to root rot, and dry sandy soil will lead to plant wilt. Mix 2 to 3 inches of compost into the soil before planting.
When to Plant Asters
- The best time to put young aster plants in the ground is in mid- to late spring after the danger of frost has passed. (See local frost dates.)
- Or you can plant mature, potted asters when available at garden centers (typically in the late summer or early fall).
- Asters can be grown from seed, but germination can be uneven. If desired, plant seeds in the fall or start them indoors in the winter.
How to Plant Asters
- When planting young aster plants outside in the spring, space them 1 to 3 feet apart, depending on the type and how large they’re expected to get.
- Fully grown asters, such as those available in late summer or early fall, should be planted about 3 feet apart.
- If planting seeds, sow 1 inch deep in pots or flats and refrigerate them for 4 to 6 weeks to simulate winter dormancy. This cold period will kick-start germination.
- Water well, and spread mulch around the plants to keep the soil cool and prevent weeds.
Types
Even though there are more than 600 aster species, the most common asters available in North America are the New England aster (Symphyotrichum novae-angliae) and the New York aster (S. novi-belgii). Both of these plants are native to North America and are great flowers for pollinators. We recommend planting a native species of aster over a non-native species when possible, so talk with your local Cooperative Extension or garden center about which species are best suited to your area. Look for varieties with disease resistance.
North American Asters
- Aromatic aster (Symphyotrichum oblongifolium): Multi-branched at the base, stiff upright stems, with lavender or pink flowers.
- Blue wood aster (S. cordifolium): Bushy with small, blue-to-white flowers.
- Heath aster (S. ericoides): A low-growing ground cover (similar to creeping phlox) with small, white flowers.
- New England asters (S. novae-angliae): Varieties have a range of flower colors, from magenta to deep purple. They typically grow larger than New York asters, though some varieties are smaller.
- New York asters (S. novi-belgii): There are many varieties of New York asters available. Their flowers range from bright pink to bluish-purple and can be double, semi-double, or single.
- Smooth aster (S. laeve): A tall, upright aster with small, lavender flowers.
European/Eurasian Asters
- Frikart’s aster (Aster x frikartii) ‘Mönch’: Hailing from Switzerland, this mid-sized aster has large, lilac-blue flowers.
- Rhone aster (A. sedifolius) ‘Nanus’: This aster is known for its small, star-shaped, lilac-blue flowers and compact growth.
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Comments
According to the UMass cooperative extension, "Use a 150 to 200 ppm N constant feed program with a well- balanced fertilizer such as 15-16-17. Slow-release fertilizer can be used as top-dressing, but liquid fertilization may be needed as a supplement in case the fertility level drops, Asters are sensitive to high soluble salt levels. Injury is exhibited as root damage, leaf yellowing, and leaf drop."
We stop fertilizing when blooming begins.
It sounds as if you do indeed have an aster disease, probably leaf spot. Here is a helpful guide to diseases and management. You could always get a sample to your local cooperative extension or garden center as a next step in diagnosis.
You can prune asters back spring growth early in the summer (late May or early June). A month later, you can trim the top growth again. This will delay bloom slightly but will cause the plants to bloom more, get less floppy, and grow in a bushier shape.
Try sheering the tops to see if you can get a second round of blooms?
We've never known asters to change color. You can manipulate the colors of hydrangeas by the level of acid in the soil.
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There are many caterpillars that eat aster foliage, including beet armyworms, cabbage loopers, carnation leafrollers, corn earworms, etc. For better or worse, the best control is to handpick the caterpillars off your asters with tweezers. Wear gloves to prevent stings or rashes. Drop the pests into a bucket of soapy water as you pluck. You can apply Bacillus thuringiensis but only when they are very young and you really want to ensure it's a "pest" destroying your plant not a beneficial bug. You could take a sample to your county cooperative extension office.