When to Plant Asters: Spring Planting, Fall Seeding, Pinching and Dividing New England Asters

The asters most gardeners grow in North America are New England aster (Symphyotrichum novae-angliae) and New York aster (S. novi-belgii) (Almanac), plus the shorter aromatic aster (S. oblongifolium). Illinois Extension explains that DNA studies moved most North American asters out of the genus Aster, and the old names are still common on tags. New England aster grows 3 to 6 feet tall (NC State) and blooms from late summer into fall (Clemson); aromatic aster forms a mound of about 1 to 3 feet and blooms from late summer to frost (NC State).

When to plant asters

Use our planting calendar to see your frost dates. On seed, sources differ on depth: the Wildflower Center gives 5/8 inch for fall sowing outdoors, the Almanac 1 inch in chilled pots, and Outsidepride only a light covering. Our reading is that shallow is the safer default for small seed indoors. The Almanac warns that aster seed germinates unevenly and that self-sown seedlings may not match the parent's color, so buy named plants if you want a particular cultivar.

How to plant asters

Wisconsin and Illinois both tie powdery mildew to crowded, dense foliage, so wide spacing and thinning are part of disease control. Outsidepride and the Almanac mix 2 to 3 inches of compost into the soil before planting, and say wet clay can rot roots while dry sand leads to wilting.

Watering, pinching and staking

• Water until established. Clemson keeps new plants watered; the Almanac waters when summer rain is under an inch a week, and mulches to keep roots cool.
• Pinch for shorter plants. Clemson pinches stem tips in spring and stops in July; NC State says spring pinching makes a bushier plant and delays bloom. The Almanac cuts back by a third once or twice in early summer. Illinois shears New England aster, which can reach eye level and flop.
• Stake tall types in late summer if they lean (Wisconsin, Clemson, NC State).
• Feed lightly. Clemson adds 2 inches of compost each year; the Almanac uses a thin layer of compost or balanced fertilizer each spring.
• Divide every 2 to 3 years (Wisconsin, Almanac); Illinois divides New England aster every other year to thin crowded stems, and Clemson every 3 years or more. Iowa State keeps several shoots on each division and replants right away.

Asters in containers

The Almanac says asters can be grown in containers and suggests shorter varieties for pots. NC State lists 'Purple Dome' at about 2 feet, and aromatic aster at 1 to 3 feet is suggested by NC State as a substitute for fall mums. Our reading is to choose one of those short forms, use a large pot and keep it evenly moist, since dry stress costs asters their lower leaves (Almanac) (GardenPal reasoning).

Fall and winter

• Leave stems standing. Wisconsin cuts dead stems in late winter, and NC State suggests cutting aromatic aster stems to 12 to 24 inches so stem-nesting bees can use them. Prairie Moon notes bees overwinter in the hollow stems.
• Or cut back after frost. The Almanac cuts asters back once foliage dies, or leaves them for winter interest and seed for birds.
• Deadhead to limit seedlings. Clemson, Illinois and Prairie Moon say New England aster self-seeds freely; remove spent flower heads before seed sets if you don't want volunteers.

Powdery mildew and other problems

Powdery mildew is the problem nearly every source names (Clemson, NC State, Wisconsin, Illinois, Almanac), and good spacing, thinning and full sun are the main defenses. NC State also lists aster wilt and stem rot, and pests such as spider mites, aphids and leaf miners; the Almanac adds aster yellows, rust, leaf spot, slugs and snails. Deer resistance varies by source: Clemson says New England aster is moderately deer resistant and NC State calls aromatic aster deer and rabbit resistant, while Prairie Moon says deer and rabbits love asters and suggests protecting young transplants. For cultivars, NC State and Wisconsin name compact 'Purple Dome' and 'Alma Pötschke', and NC State lists aromatic aster 'October Skies'.

Asters around the vegetable garden

Asters bloom when most of the garden is finished, which Illinois Extension calls a valuable late food source for pollinators, including migrating monarchs. Clemson and the Wildflower Center say New England aster is a monarch nectar plant and a host for pearl crescent caterpillars. Plant asters at the back of a border near the vegetable plot, with goldenrod, which NC State suggests pairing with New England aster. Good neighbors include coneflowers, black-eyed Susans, liatris and phlox. See also flowers that attract beneficial insects.

Frequently asked questions

When is the best time to plant asters?

Spring, according to Wisconsin Extension. Potted asters sold in late summer or early fall can also go in then (Almanac).

Can I plant asters in the fall?

Potted plants sold in late summer or early fall, yes (Almanac). Fall is also the time to sow seed outdoors, letting winter supply the cold (Clemson, Wildflower Center).

Do aster seeds need cold stratification?

Perennial asters do. Outsidepride chills seed for 30 days, and Prairie Moon codes New England aster C(60), 60 days of cold, moist stratification. Fall sowing lets winter do it.

Should I pinch back asters?

Yes, for shorter, bushier plants. Clemson pinches tips in spring and stops in July; the Almanac cuts back by a third once or twice in early summer.

When should I divide asters?

In early spring as new growth appears (Iowa State), every 2 to 3 years (Wisconsin).

Why are the lower leaves on my aster dying?

On New England aster, lower leaves that dry up in late summer are normal (Prairie Moon, Clemson). The Almanac adds that too much or too little water makes asters drop lower foliage.