Growing Asters: September Flowers In the Garden

You may already know that growing asters in your garden invites numerous pollinators, who flit from the star-like aster to tomato and pepper flowers. In addition to that, did you know there are around 170 species?

These species are mostly relegated to the eastern hemisphere of the world, which encompasses a wide array of climates and conditions. That means there’s definitely an aster you can grow where you live!

They’re a fantastic addition to the garden, especially in summer and early fall. So let’s talk about asters and figure out how to grow them!​

Quick Care Guide

Common Name	Asters, Michaelmas Daisies, September flower, frost flower
Scientific Name	Aster spp.
Family	Asteraceae
Height and Spread	1 to 6 feet tall and 1 to 4 feet wide
Light	Full to partial sun
Soil	Loamy, well-draining
Water	1 inch per week
Pests and Diseases	Lace bugs, aphids, scale, spider mites, thrips, powdery mildew, aster yellows, botrytis, root rot

All About Asters

The gorgeous Aster is a fantastic low-effort addition to your garden. Its name originates from the Greek word for star, but also goes by many other names, with September flower, Michealmas daisy, and frost flower being just a few of them. These flowers originate all over eastern Europe and Asia with some species native to North America, where they bloom in varying vibrant shades.

Asters vary in color, size, and ease of care depending on the variety that you grow. You’ll never find an aster flower with bright orange or yellow petals — they come in blue, purple, white, red, or pink. But no matter the color, it’s easy to identify them by their bright yellow center composed of many flowerets.

In the floral industry, asters are one of the most popular flowers to grow and sell. This is due to their versatility and long life once cut. They’ll last over a week before wilting. They’re also great for gardens that are designed to attract many different species of pollinators.

Asters are called Michaelmas daisies because they bloom in September, or the same month that the celebration of the archangle Michael occurs in the early Christian pantheon. It’s through the aster flowers that Christians see the fire of Michael’s flaming sword.

Many of the asters native to North America have been removed from the Aster genus and split into several genera, including Ampelaster, Symphyotrichum, Ionactis, Eurybia, Seriocarpus, Doellingeria, and Oclemena.

Types of Asters

Asters come in many varieties. When you’re just starting out, it’s hard to tell the difference between them all — but it matters, because many grow much taller than others and have very different flower appearance.

The two most common types of aster are New York Asters and New England Asters. Not surprisingly, they’re also hard to differentiate.

The New York aster has both tall and short varieties, but they tend to clump in the 2-4′ range. Some New York aster reaches upwards of 4′ tall! The stems are thinner than New England aster, and the leaves are usually smoother.

New England asters grow to be around 3-4′ in height, though you can get varieties that grow either shorter or taller. In contrast to New York aster, they have hefty stems with leaves that are covered in hair. The flowers are much denser as well.

Other less-common types of aster include:​

Heath aster is my favorite variety. It grows to 3-4 feet and its leaves are very small and thin. What makes it so special is the multitude of small white flowers that show up as it blooms.

Calico aster and heath aster are difficult to tell apart. The giveaway is that calico only grows to 1-2 feet and the flowers only bloom on one side of the stems instead of all of the way around.

As you might imagine, aromatic aster gets its name from the smell it produces when you touch it. It’s quite a nice smell, so if you get joy out of smelling the flowers in your garden you may want to go with this variety. The flowers are a bit thicker-leaved than either heath or calico aster.

Smooth aster has a unique leaf shape that is more oval or egg-shaped at the end when compared to other varieties. As you might imagine, the leaves are also quite a bit smoother than most other types of aster.

The more common name for this type of aster is heart-leaf, and it’s similar to the blue wood aster. It is strikingly different from the varieties we have covered already, due to the thin stems and darker centers.

Planting Asters

If you’re planting aster seeds, ensure the soil you’re planting them in is at least 70°F. In most regions, starting aster seeds indoors is required. You can start them indoors 6 weeks before the last frost. Use a heat mat and starter trays, like our Epic Cells to maintain minimal root disturbance of young aster plants in the transplant process.

No matter which propagation method you used for your asters, spacing them correctly is important. Plant asters too close and you run the risk of competition for water and nutrition.

If you’re growing a miniature aster cultivar, you can get away with half a foot between each plant. If you’re growing one of the taller varieties, you’ll need to increase spacing to at least a foot (and up to two feet on the tallest varieties).

Give your young aster plants full sun, loamy soil, and plant asters with level with the soil.

How To Care for Asters

​Asters are quite easy to take care of, with little maintenance required to grow a healthy, beautiful bunch of aster flowers. But, with a little extra effort your plants will thrive and produce a much more vibrant set of flowers.

Light and Temperature

Asters require at least partial sun, but prefer full sun for optimal growth. In areas with hot summers, grow asters in partial shade, with access to morning sun and afternoon shade. Asters prefer areas with cooler summers, but there are cultivars that can survive hot summers.

Because they have a hardiness range of zones 3 to 8, they can also withstand some cold. The perennial asters may die back in very cold winters, but they have no problem with a quick cold snap. In areas with colder winters, protect your plants with a thick layer of mulch to keep the roots alive so they can return in spring.

Water and Humidity

If you live in a drought-ridden area and get less than an inch of rain per week (4 inches per month), then you will need to water your plants during the hot summer months. Be careful not to over-water — aster is very sensitive to too little or too much water and will be adversely affected by over-watering.

You’ll know you over-watered if the lower leaves and stems of your plants are falling off after a heavy watering. Drip irrigation or watering with soaker hoses is best, as asters are prone to mildew. However, they enjoy humid climates.

Soil for Asters

Asters will do well in “OK” soil, but they prefer loamy soil that drains very well as they’re moisture-sensitive. If your soil doesn’t drain well, add in sand, vermiculite, or perlite. Every spring, amend your soil with a very thin layer of organic compost​. Add 1-2″ of high quality mulch on top for protection.

Potted asters need a larger container (at least 5 gallons) with a good sized drainage hole. Prepare a soil that is rich and well-draining. A basic potting soil will do.

Fertilizing Asters

While asters don’t need fertilizer, they may benefit from the application of a 5-10-10 balanced fertilizer. This type of fertilizer will promote more blooms. An annual top-dressing of compost will definitely help your asters as well. Especially if you’re growing potted asters, you’ll want to provide fertilizers in at least one or two applications ahead of the blooming season.

Pruning and Training Asters

If you are growing one of the taller varieties of aster, you’ll need to hand stake each plans to they don’t collapse under their own weight.To encourage your aster to grow in a more bushy shape, pinch off the younger growth. After they’ve bloomed for the season, cut them back in the winter to control them a bit.

Deadheading spent aster flowers will promote more aster blooms, too. As they fade, cut the dead flowers off.

Propagating Asters

Although you can grow aster from seed, many gardeners opt to use the division method to propagate their plants. Every few years you can divide your aster patch to effectively ‘reset’ them and restore their vigor and quality.​

If you already have asters in the garden, or know a friend who is growing them, you can simply divide their plants. By digging out over half of the aster plants and transplanting them into your own garden, you can skip the germination and seedling phases and get straight to the beautiful flowers!

If you are growing from seed, it’s best to start the seeds indoors where you can control the environment more. You can also sow them earlier indoors.​ Lay down seedling mix into a 10×20 propagation trays, or container of your choice.  Sow 2-3 seeds in each hole, about 1/8″ deep.

Once they sprout, thin each cluster of seeds by selecting only the strongest seedling in each clump. This way you ensure the healthiest, most vigorous aster seedlings possible. After 4-6 weeks, your aster seedlings will be ready for transplanting out into the garden.

Troubleshooting Asters

You may encounter some issues when you’re growing asters, as they have plenty of pests and diseases to contend with. However, they’re pretty hardy plants, so any problems that arise can be handled with ease. Let’s discuss those now that we’ve covered care for asters.

Growing Problems

Most of the problems asters face involve over or underwatering. Too much water can put the aster’s roots at risk of contracting disease. Too little may cause yellowing and browning of leaves, and leaf drop. In this regard, poor drainage can produce the same issues as overwatering.

A lack of water can prevent blooms from forming, but so can overcrowding, and a lack of sun. Remember to plant them in proper conditions with proper spacing to prevent this. Divide your asters regularly to keep them blooming year after year.

Pests

Asters are quite pest resistant in general, however there are a few that they struggle with.

The tiny, grayish-white lace bug will show up around the middle of the summer, just in time for summer blooms. They prey on the underside of aster leaves and suck nutrients out of the leaves.

Look for yellowing leaves. If you catch the infestation before these leaves turn brown, you can use organic sprays or insecticidal soaps to beat the bugs back. Be sure to cover the entire underside of your aster plants if you spot any lace bugs, as leaving even a few untreated will just prolong the problem.

Common garden pests, like aphids, thrips, spider mites, and scale may also affect asters. All of these can be blasted off the aster plant with a strong stream of water. Spider mites and scale insects can be hand removed with a damp cloth, and a q-tip soaked in alcohol, respectively.

While it’s possible to treat your asters with insecticidal soap or neem oil, because they attract so many pollinators, it’s not recommended. Both can harm pollinators.

Diseases

Just as asters are resistant to most pests, they also fare well against most plant diseases…save for a few and one of the peskiest of all: powdery mildew.

Powdery mildew plagues gardeners around the world, whether they’re growing flowers or edible plants. The best way to prevent this mildew is to keep air circulation high and remove any leaves or plants that you spot the white, dusty powder on immediately as it spreads fast. Also trim your aster stems and rake around your plants to increase air circulation and remove organic matter that powdery mildew is prone to attack.

Botrytis cinerea and root rot are common in water-stressed asters. Both are fungal diseases that cause sporangia to form on parts of the plant. You may have an easy time treating them by witholding water until the soil dried. In more severe cases you’ll need to trim away brown and mushy plant parts, and replant in fresh soil elsewhere.

The most damaging of the diseases that asters face is the aptly-named aster yellows. This disease is spread by leaf hoppers, which carry it, causing deformed flowers and yellowing foliage. Control leaf hoppers to prevent the disease altogether. You can blast them with water, or hand pick them if they don’t escape your grasp.

Frequently Asked Questions

Q: Do asters come back every year?

A: Most often they do! Since they’re perennial and have a wide hardiness range, they may die back in winter, but should return in spring in adequate conditions.

Q: Where is the best place to plant asters?

A: Plant them in full sun to partial shade, in loamy, well-draining soil. A region with cool summers and nights is great for asters as well.

Q: Are asters easy to grow?

A: They are! They’re fairly hardy, so caring for them and dealing with any problems they may have is not hard.

Q: Do asters multiply?

A: They do spread fairly easily. This is why it’s important to divide them regularly.

Q: Should asters be cut back in the fall?

A: While you should remove any diseased or dead foliage in fall, you should instead do your annual pruning for shape in summer. You can leave the stems above the soil to provide insulation to the plant’s roots in winter.

Q: How do you keep asters from getting leggy?

A: Prune them back to about 18 inches in summer to promote bushier growth.

Q: Should I deadhead asters?

A: You don’t have to, but deadheading will help your asters bloom again in fall.

Q: Are asters low maintenance?

A: When it comes to perennial plants as a whole, asters are some of the easiest.