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.
More Like This
If excessive height is the problem, then you should pinch back the young shoots to encourage the plants to become bushier. If they are simply too big all-around, you can cut them back by about half up until the beginning of July (the 4th of July, essentially). Don't wait any longer than that, though, as it could result in delayed flowering.
Asters are pretty forgiving plants. Are you sure you don't have a biennial variety? Asters bloom best in full sun but tolerate part shade. They do not like to sit in water, so well draining soil is important, and good quality—loamy or composted—soil helps. Next year, add a shovelful of compost at the base in spring.
You can grow asters of varying sizes; choose what you want. You could plant it in a pot and put the pot into the soil to control it. If the plant is cramped in the pot, it may become cramped (leggy or some other manifestation of stunted growth) but you should go to or contact a plant vendor and talk to them with the goal of purchasing a plant that will perform in the way you want it to.
It sounds like your asters have a rust disease. You should remove all of the leaves that are infected and the entire plant if all of the leaves have rust.
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