Ornamental · Asteraceae
Aster
Symphyotrichum
Give it full sun and well-drained soil, and the aster keeps your garden's color going right into fall.
Full sun
USDA 3–8
1–5 ft · 0.3–1.5 m
Overview
Asters show up with a big wave of daisy-like blooms right when the rest of the garden is calling it a season. They're in the same family as sunflowers and daisies, and the hardy garden types grow as clump-forming perennials, so they come back a little stronger every year. Whether you want a low mound up front or a tall, leggy patch in back, there's pretty much an aster for any sunny spot you've got.
Honestly, the color is the whole reason to grow them. The flowerheads open up in blue-violet, pink, and white, and they often pile on so thick you can barely see the leaves underneath. They bloom from late summer into fall, usually August through October, which makes them one of the last good nectar stops for pollinators before the frost hits.
And here's the best part: they don't ask for much. Full sun, soil that drains, and one midsummer pinch to keep the stems sturdy is about it. Crowd them into damp shade, though, and you'll trade the blooms for mildew and floppy growth, so a sunny spot with a little breathing room really pays off.
Care
The short version: asters want full sun and well-drained soil of just about any texture, plus steady moisture through the growing season. Pinch the stems back in June for fuller plants, and split crowded clumps every few years to keep the flowers coming.
Light
Asters flower their best in full sun, and sure, they'll put up with a bit of partial shade if they have to. But push them into real shade and you'll end up with tall, floppy stems and just a few scattered blooms.
Water
Give them average, steady moisture through the growing season, because dry spells really do cut the flower count. Established plants can ride out a short drought, but the soil's got to drain well, since sitting wet just invites wilt.
Soil
Asters aren't picky at all and grow in loam, sandy loam, or clay, as long as it drains well and has some organic matter in it. They handle a wide pH band too, from 5.5 to 8.0, so most garden ground suits them just fine.
Humidity
Outdoor humidity is whatever the season gives you, and asters really don't mind. What matters way more is air movement, because crowded, still air is exactly how powdery mildew gets going.
Feeding
A light spring dose of balanced general-purpose fertilizer is plenty to carry them all season. Go easy on the nitrogen, though, or you'll get soft, floppy stems that need staking and don't bloom as well.
Pruning
Pinch the stems back by a few inches from early to late June to get a bushier, sturdier plant that holds itself up. Then stop pinching after early July so you don't snip off the flower buds that are forming, and cut the spent foliage to the ground once the blooms are done.
Size & spacing
Asters can run anywhere from 1 to 5 feet (30 to 150 cm) tall depending on the species, and they spread 2 to 4 feet (60 to 120 cm) across. Give each clump that full width so air can move through and mildew stays away.
When to plant
Asters are all about the cool 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's warmed up, and then you just wait for the late show, 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 not enough sun, which is good news since all three are fixable. Spot the early signs on the leaves and stems and a quick change of spot or spacing usually turns it around. Find what matches below, then jump on it before it spreads through the clump.
Stems yellowing and flopping, mostly on one side
This is the tougher one. It usually means verticillium or aster wilt, a soil fungus that gets comfortable in heavy, soggy ground, and there's no cure once it's in. The giveaway is a plant that wilts even though the soil's still nice and damp. Dig out and toss the worst plants, sort out the drainage, and don't replant susceptible asters in that same spot.
Fix:
- Improve soil drainage
- Remove and destroy severely affected plants
- Avoid replanting susceptible species in the same spot
Plant looks fine but barely any flowers
Wondering where all the blooms went? It's almost always too little light or too little water. A plant sitting in the shade, or one that's been left to dry out all summer, just won't flower like one in full sun with steady moisture. So move it somewhere sunnier and keep the soil from drying out, especially when it's hot.
Fix:
- Relocate or site plants in full sun
- Maintain consistent soil moisture during the growing season
Leaves looking like someone dusted them with flour
That powdery film? It's powdery mildew, a fungus that loves crowded plants and stale air, and it usually shows up by late summer. The fix is mostly about airflow, so thin and space the plants out, keep the hose off the leaves, and pull anything that's badly coated. If it just won't quit, a fungicide every 10 to 14 days will back you up.
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
New shoots curling up with sticky little bugs all over them
Those tiny soft-bodied bugs piling up on the shoot tips are just aphids, and the sticky shine they leave behind is honeydew. Good news: a strong blast of water knocks most of them off, and you only need insecticidal soap if their numbers really take off. Ease up on the nitrogen too, since all that soft new growth is exactly what brings them around.
Fix:
- Dislodge with a strong spray of water
- Apply insecticidal soap if populations are high
Toxicity
Here's some good news if you've got pets or kids around: asters are safe all around. The ASPCA lists them as non-toxic to cats, dogs, and horses, and they're no real danger to other animals or to people either. The worst a nibble usually brings is the mild stomach upset that just about any plant can cause.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Got a cat? No problem here, because garden asters are safe around them and the ASPCA calls the plant non-toxic. If a cat chews a leaf, the worst that's likely is a mildly upset stomach, which honestly goes for just about any plant.
Dogs None
Dogs can hang out in the garden with no worries, since there's no poison in asters. About the worst a curious chewer gets from a stem is a bit of drool or a quick tummy upset.
Horses None
Horses are fine around asters, in the pasture or right next to it, because the plant's non-toxic. Munching the foliage won't cause anything more than the odd mild digestive grumble.
Rabbits Safe
Rabbits can nibble away at asters with nothing to fear from the plant itself. Like any fresh greens, though, it's smart to keep the helpings small so their tummies stay happy.
Livestock Safe
Cattle, goats, and sheep can graze around asters and they'll be just fine. There's simply no toxin in the plant to bother a grazing animal.
Birds Safe
Asters are harmless to birds, and here's a bonus: those late seedheads pull in finches and other songbirds. No part of the plant is going to cause them any trouble.
People Safe
No need to panic if a curious kid gets near asters, since they're not poisonous to people. They're not really a thing you'd eat anyway, so there's no reason to make a snack of them.
Varieties
There are hundreds of asters out there, but only a handful of species really turn up in gardens over and over. These are the ones worth knowing by name.
Symphyotrichum novi-belgii (New York aster)
It's a dependable front-of-border pick that grows 3 to 4 feet and opens blue-violet, pink, or white flowerheads from August to October, and it's hardy down to zone 4.
Symphyotrichum dumosum (Bushy aster)
This one's a compact, bushy American aster that's covered in little button-like flowerheads, and it's a great fit for a low border.
Symphyotrichum oblongifolium (Aromatic aster)
It's a drought-tolerant aster with scented foliage that keeps blooming very late into fall, and it stays hardy through zones 3 to 8.
How to propagate
Asters are super 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 way to go for the straight species.
DivisionEasy
In October as flowering deteriorates, or in early spring · One growing season
Want the surest way to keep asters going strong? Divide them. Every few years, lift a crowded clump and split it, either in October as the blooms are fading or in early spring, then pop the pieces back into well-drained soil. Each one settles in and fills out over a single growing season, no fuss.
SeedEasy
Sow indoors in late winter or outdoors in spring · Germinates in about 15 days indoors or 20 to 30 days outdoors
You can also start asters from seed, sowing indoors in late winter or outdoors in spring, and you'll see sprouts in about 15 days inside or 20 to 30 days out in the garden. Just keep in mind that named cultivars are better off divided, since seed-grown plants often don't come true to the parent.
Myths
Asters are poisonous to dogs and cats.
You can let that one go, because the ASPCA lists garden asters, China aster and all, 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 stomach upset that passes on its own.
Asters need rich, constantly moist soil to thrive.
Not really, since asters do great in average, well-drained ground, they shrug off all kinds of textures and pH, and once they're established they handle a short dry spell just fine. Soggy, poorly drained soil is the actual problem, because that's what invites the wilt and mildew that take these plants down.