Ornamental · Asteraceae
Aster
Symphyotrichum
Give it full sun and well-drained soil, and the aster carries your garden's color straight into fall.
Full sun
USDA 3–8
1–5 ft · 0.3–1.5 m
Overview
Asters bring a wave of daisy-like blooms just as most of the garden winds down. They belong to the same family as sunflowers and daisies, and the hardy garden kinds grow as clump-forming perennials that come back stronger each year. From a low front-of-border mound to a tall, leggy stand, there's an aster to fit almost any sunny spot.
Color is the whole point. The flowerheads open in blue-violet, pink, and white, often in such numbers that the foliage all but disappears beneath them. Bloom runs from late summer into fall, commonly August through October, which makes asters one of the last big nectar sources for pollinators before frost.
They ask for very little. Full sun, ground that drains well, and a midsummer pinch to keep the stems sturdy are about all it takes. Crowd them in damp shade and you trade those blooms for mildew and floppy growth, so a little room and a sunny site go a long way.
Care
The short version: full sun and well-drained soil of almost any texture, with steady moisture through the growing season. Pinch the stems back in June for bushier plants, and divide crowded clumps every few years to keep them blooming.
Light
Asters flower best in full sun, and they'll take a bit of partial shade if they must. Push them into real shade and you'll get tall, floppy stems and only a scattering of blooms.
Water
Give them average, steady moisture through the growing season, since dry spells cut the flower count. Established plants ride out a brief drought, but the soil has to drain well, as constant wet invites wilt.
Soil
Asters aren't fussy and grow in loam, sandy loam, or clay as long as it drains well and holds some organic matter. They take a wide pH band from 5.5 to 8.0, so most garden ground suits them fine.
Humidity
Outdoor humidity is whatever the season hands you, and asters don't mind. Air movement matters far more than any moisture number, since crowded, still air is how powdery mildew gets started.
Feeding
A light spring dose of balanced general-purpose fertilizer is plenty for the whole season. Go easy on the nitrogen, or you'll grow soft, floppy stems that need staking and bloom less.
Pruning
Pinch the stems back by a few inches from early to late June to build a bushier, sturdier plant that holds itself up. Stop pinching after early July so you don't snip off the forming flower buds, and cut the spent foliage to the ground once the blooms fade.
Size & spacing
Asters range from 1 to 5 feet (30 to 150 cm) tall depending on the species, spreading 2 to 4 feet (60 to 120 cm) across. Give each clump that full width so air moves freely and mildew stays away.
When to plant
Asters are creatures of the cool seasons at both ends of the year. Start seed indoors in late winter or sow it outside in spring, set plants out after the last frost once the soil warms, and the show arrives late, with flowers from August into October.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Late winter to early spring; seed germinates in about 15 days indoors
- Plant: After last frost in spring once soil has warmed
- Harvest: Summer into late fall (commonly August to October)
Hardiness
USDA hardiness zones: 3–8
Temperature: down to -40°C
Troubleshooting
Most aster trouble comes down to crowding, poor drainage, or too little sun. Catch the early signs on the leaves and stems and you can usually turn things around with a change of site or spacing. Match what you see below, then act before it spreads through the clump.
Stems yellowing and collapsing, often down one side
This points to verticillium or aster wilt, a soil fungus that takes hold in heavy, soggy ground and gives no cure once it sets in. The plant wilts even when the soil is plenty moist. Dig out and destroy the worst plants, fix the drainage, and don't replant susceptible asters in that spot.
Fix:
- Improve soil drainage
- Remove and destroy severely affected plants
- Avoid replanting susceptible species in the same spot
Few flowers on an otherwise healthy plant
Too little light or too little water is almost always behind a thin show of blooms. A plant tucked in shade or left to go dry through summer simply makes fewer flowers than one in full sun with steady moisture. Move it into the sun and keep the soil from drying out, especially through a hot spell.
Fix:
- Relocate or site plants in full sun
- Maintain consistent soil moisture during the growing season
White, powdery dusting across the upper leaves
This is powdery mildew, a fungus that settles in when plants are crowded and air sits still, usually by late summer. Thin and space the plants so air can move, keep water off the foliage, and pull any badly coated leaves. A fungicide on a 10 to 14 day schedule helps if it keeps spreading.
Fix:
- Improve air circulation by thinning and spacing plants
- Avoid overhead watering
- Remove and discard heavily infected foliage
- Apply an appropriate fungicide on a 10 to 14 day schedule if needed
Twisted new shoots crawling with sticky insects
Those tiny soft-bodied insects on the shoot tips are aphids, and the shine they leave behind is honeydew. Knock them off with a hard jet of water, and reach for insecticidal soap only when the numbers climb. Easy on the nitrogen, since lush soft growth is exactly what draws them in.
Fix:
- Dislodge with a strong spray of water
- Apply insecticidal soap if populations are high
Toxicity
Good news for pet owners and parents: asters are safe all around. The ASPCA lists them as non-toxic to cats, dogs, and horses, and they pose no real danger to other animals or to people. About the worst a nibble brings is the mild stomach upset that almost any plant can cause.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Garden asters are safe around cats, and the ASPCA lists them as non-toxic. A cat that nibbles a leaf might get a mildly upset stomach, which is true of almost any plant.
Dogs None
Dogs can share the garden without worry, since asters carry no poison. The most a curious dog risks from chewing a stem is a little drooling or a brief tummy upset.
Horses None
Asters are non-toxic to horses and pose no danger in or near a pasture. Grazing on the foliage causes nothing beyond the occasional mild digestive grumble.
Rabbits Safe
Rabbits can browse asters freely with nothing to fear from the plant. As with any fresh greenery, keep portions modest so the gut stays settled.
Livestock Safe
Cattle, goats, and sheep can graze around asters without harm. The plant holds no toxin that would trouble a grazing animal.
Birds Safe
Asters are harmless to birds, and the late seedheads draw finches and other songbirds. There is no part of the plant that poses a hazard.
People Safe
Asters are not poisonous to people and present no risk to a curious child. They aren't grown to be eaten, so there's no reason to make a meal of them either.
Varieties
Hundreds of asters exist, but a few species turn up again and again in gardens. These are the ones worth knowing by name.
Symphyotrichum novi-belgii (New York aster)
A reliable front-of-border perennial that reaches 3 to 4 feet and carries blue-violet, pink, or white flowerheads from August to October, hardy to zone 4.
Symphyotrichum dumosum (Bushy aster)
A compact, bushy American aster smothered in small button-like flowerheads, well suited to a low border.
Symphyotrichum oblongifolium (Aromatic aster)
A drought-tolerant aster with aromatic foliage that blooms very late into fall and stays hardy through zones 3 to 8.
How to propagate
Asters are easy to multiply, whether you split an established clump or start fresh from seed. Division keeps named varieties true to type, while seed is the cheaper route for the straight species.
DivisionEasy
In October as flowering deteriorates, or in early spring · One growing season
Division is the surest way to keep asters vigorous. Lift and split crowded clumps every few years, either in October as the blooms fade or in early spring, and replant the divisions in well-drained soil. Each one settles in and fills out over a single growing season.
SeedEasy
Sow indoors in late winter or outdoors in spring · Germinates in about 15 days indoors or 20 to 30 days outdoors
You can raise asters from seed by sowing indoors in late winter or outdoors in spring, with sprouts showing in about 15 days inside or 20 to 30 days in the ground. Named cultivars are better divided than seeded, since seed-grown plants often won't come true to the parent.
Myths
Asters are poisonous to dogs and cats.
The ASPCA lists garden asters, including China aster, as non-toxic to dogs, cats, and horses. A pet that nibbles one is in no real danger, though eating any plant can bring on a mild, passing stomach upset.
Asters need rich, constantly moist soil to thrive.
Asters do well in average, well-drained ground and shrug off a range of textures and pH, and established plants ride out a brief dry spell. Soggy, poorly drained soil is the real problem, since it invites the very wilt and mildew that take these plants down.