Ornamental · Asteraceae
Aster
Symphyotrichum
Give it full sun and well-drained soil, and the aster will carry your garden's color gently into fall.
Full sun
USDA 3–8
1–5 ft · 0.3–1.5 m
Overview
Asters arrive with a generous wave of daisy-like blooms right as the rest of the garden begins to settle down, so they are a lovely plant to lean on. They sit in the same family as sunflowers and daisies, and the hardy garden kinds grow as clump-forming perennials that return a little stronger each year. Whether you want a low mound at the front of a bed or a tall stand toward the back, there is an aster to suit nearly any sunny corner you have.
Color is really the heart of it. The flowerheads open in blue-violet, pink, and white, often so thickly that you can barely see the leaves beneath them. Bloom carries from late summer into fall, commonly August through October, which means asters become one of the last good nectar stops for pollinators before frost, and that feels rewarding to offer.
Best of all, they ask very little of you. Full sun, ground that drains, and one midsummer pinch to firm up the stems are about the whole of it, and a stumble here is easy to recover from. Crowd them into damp shade and you may swap blooms for mildew and floppy growth, so a sunny spot with a little elbow room sets you up to succeed.
Care
The short version: asters want full sun and well-drained soil of almost any texture, with steady moisture through the growing season. Pinch the stems back in June for fuller plants, and divide crowded clumps every few years to keep the flowers coming.
Light
Asters give their best flowers in full sun, and they will accept a touch of partial shade if that is what you can offer. Tuck them into real shade, though, and you tend to get tall, floppy stems and only a few scattered blooms.
Water
Offer them average, steady moisture through the growing season, since dry spells trim the flower count. Established plants will see out a brief drought just fine, but the soil really does need to drain well, because constant wet invites wilt.
Soil
Asters are forgiving and grow happily in loam, sandy loam, or clay so long as it drains well and holds a little organic matter. They are easy about pH too, taking a wide band from 5.5 to 8.0, so most garden ground will suit them nicely.
Humidity
Outdoor humidity is simply whatever the season brings, and asters take it in stride. Air movement matters far more than any moisture reading, since crowded, still air is how powdery mildew gets its foothold.
Feeding
A light spring helping of balanced general-purpose fertilizer carries them comfortably through the season. Keep a gentle hand with the nitrogen, or you may end up with soft, floppy stems that need staking and bloom less.
Pruning
Pinch the stems back by a few inches from early to late June to coax out a bushier, sturdier plant that stands up on its own. Stop pinching after early July so you don't accidentally remove the forming flower buds, and trim the spent foliage to the ground once the show is over.
Size & spacing
Asters can run anywhere from 1 to 5 feet (30 to 150 cm) tall depending on the species, and spread 2 to 4 feet (60 to 120 cm) wide. Allow each clump that full room so air can move through it and mildew stays at bay.
When to plant
Asters lean into the cool ends of the year. Start seed indoors in late winter or sow it outside in spring, set young plants out after the last frost once the soil has warmed, and then you wait for the lovely 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 troubles trace back to crowding, poor drainage, or too little sun, and that is reassuring because each is fixable. Catch the early signs on the leaves and stems and a simple change of spot or spacing usually sets things right. Find what matches below, then step in before it works its way through the clump.
Stems going yellow and folding, usually along one side
This one calls for a steadier hand, since it usually means verticillium or aster wilt, a soil fungus that takes hold in heavy, wet ground and cannot be cured once it arrives. You will see the plant droop even when the soil is still nicely damp. Lift out and dispose of the sickest plants, do what you can to help the ground drain, and choose a fresh spot for asters next time so you can start clean.
Fix:
- Improve soil drainage
- Remove and destroy severely affected plants
- Avoid replanting susceptible species in the same spot
A healthy plant that just isn't giving you many blooms
When the flowers come up short, the cause is nearly always too little light or too little water, and both are easy to put right. A plant sitting in shade or left to dry out over summer will simply bloom less than one in full sun with steady moisture. Move it somewhere brighter and keep the soil from drying through, especially in the heat, and you should see it reward you.
Fix:
- Relocate or site plants in full sun
- Maintain consistent soil moisture during the growing season
A pale, floury film settling over the upper leaves
Take heart, because this is powdery mildew, and it is one of the easier troubles to turn around. The fungus moves in when plants sit too close together in still air, usually toward late summer. Open the clump up by thinning and spacing it, keep the leaves dry when you water, and pick off the worst ones. If it presses on, a fungicide every 10 to 14 days will help you get the upper hand.
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 twisting under a crowd of sticky little bugs
Don't worry, those small soft-bodied bugs gathered on the shoot tips are only aphids, and the sticky shine they leave is honeydew. A firm blast of water knocks most of them right off, and you only need insecticidal soap if their numbers really build. Going gentle on the nitrogen helps too, since the soft new growth it pushes is just what tempts them.
Fix:
- Dislodge with a strong spray of water
- Apply insecticidal soap if populations are high
Toxicity
Here is some welcome reassurance 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. The worst a nibble is likely to bring is the mild stomach upset that almost any plant can cause.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
You can relax with a cat in the house, because garden asters are safe around them and the ASPCA lists the plant as non-toxic. Should a cat chew a leaf, the most you might see is a mildly upset stomach, the same as with almost any plant.
Dogs None
A dog is welcome to share the garden with no worry on your part, since asters hold no poison. The worst a curious chewer faces is a little drooling or a passing tummy upset.
Horses None
Horses are perfectly fine around asters, which stay non-toxic and harmless in or beside a pasture. A mouthful of the foliage brings nothing worse than the occasional mild digestive grumble.
Rabbits Safe
Rabbits can nibble asters freely, with nothing about the plant to fear. As with any fresh greens, just keep the helpings modest so a small gut stays settled.
Livestock Safe
Cattle, goats, and sheep are safe to graze around asters and come to no harm. There is no toxin in the plant to trouble a grazing animal.
Birds Safe
Asters are harmless to birds, and as a happy bonus the late seedheads bring in finches and other songbirds. No part of the plant poses any hazard to them.
People Safe
You needn't fret over a curious child near asters, as the plant is not poisonous to people. They simply aren't grown for eating, so there's no real reason to taste them anyway.
Varieties
There are hundreds of asters out there, but a handful of species show up in gardens again and again. These are the ones worth getting to know by name.
Symphyotrichum novi-belgii (New York aster)
A dependable choice for the front of a border, this one grows 3 to 4 feet and opens blue-violet, pink, or white flowerheads from August to October, staying hardy down to zone 4.
Symphyotrichum dumosum (Bushy aster)
A neat, bushy American aster covered in small button-like flowerheads, a friendly fit for the lower edge of a border.
Symphyotrichum oblongifolium (Aromatic aster)
An easygoing, drought-tolerant aster with scented foliage that carries its bloom very late into fall and stays hardy across zones 3 to 8.
How to propagate
Asters are wonderfully easy to multiply, whether you split a settled clump or begin fresh from seed. Division keeps named varieties true to type, and seed is the gentler-on-the-budget route for the straight species.
DivisionEasy
In October as flowering deteriorates, or in early spring · One growing season
Division is the kindest, most reliable way to keep asters vigorous, and it is well within reach for a beginner. Every few years, lift a crowded clump and gently split it, either in October as the blooms fade or in early spring, then settle the pieces into well-drained soil. Give each one a single growing season and it will root down and fill out on its own.
SeedEasy
Sow indoors in late winter or outdoors in spring · Germinates in about 15 days indoors or 20 to 30 days outdoors
Starting asters from seed is just as approachable, whether you sow indoors in late winter or outdoors in spring, with little sprouts appearing in about 15 days inside or 20 to 30 days in the ground. Just know that named cultivars do better divided, since seed-grown plants often won't match the parent, and that is perfectly normal.
Myths
Asters are poisonous to dogs and cats.
You can set this worry aside, as the ASPCA lists garden asters, China aster included, as non-toxic to dogs, cats, and horses. A pet that nibbles one is in no real danger, though eating any plant can leave a pet with a mild, passing stomach upset.
Asters need rich, constantly moist soil to thrive.
Good news for a first-time grower: asters are happy in average, well-drained ground, take a range of textures and pH in stride, and established plants ride out a short dry spell. Soggy, poorly drained soil is the real problem, since it opens the door to the wilt and mildew that bring these plants down.