When Do Asters Flower: What To Do If Aster Plants Don’t Bloom

By: Kristi Waterworth

Asters brighten the garden with their bright, happy blooms. But what can you do when there just aren’t any fireworks anymore? Read on to learn all about getting your asters back on track, and how to deal with an aster with no flowers.

Why Won’t My Asters Bloom?

A blooming aster is a welcome sight. No matter the season, the cheery daisy-like flowers always seem to light up the garden. Although generally pretty hardy, asters can seriously disappoint when they refuse to bud or flower. If your aster plants don’t bloom, don’t worry, fixing an aster with no flowers isn’t that difficult once you find the reason behind it.

Growing asters is usually a pretty simple undertaking, until it isn’t. When asters hit problems, such as non-blooming, it can be tricky to know just what to do. Several common and easy to treat problems can be the reason for asters not flowering, including:

Timing. If you asked a hundred people “when do asters flower,” you’d get a lot of different answers. This is because there are over 200 cultivars currently available in the garden trade. While you may be referring to a “Snow Flurry” aster, someone else may think you mean Alpine asters. Before you panic about your asters not blooming, check your variety’s specific bloom period.

Maturity. Many perennial asters need time to really grow up before they’ll explode with brilliantly colored flowers. If your plant is reasonably young or was very small when you bought it, it might just need more time. When everything else seems right and your asters are still refusing to even set flower buds, maturity is often the problem. Give it another year and you’ll reap the benefits of your patience.

Overfeeding. Although cultivated asters can look very delicate, the truth is that they haven’t come far from their wild roots. These plants are tough like weeds and, like weeds, don’t need a lot of fertilizer to really thrive. When your aster is adding a lot of foliage rapidly, even growing really tall in a relatively short span of time and skipping its flowers, this is a good sign that you’re overfeeding with nitrogen fertilizer. Decrease your feedings to just once or twice per season with a diluted fertilizer and watch your big, bushy asters burst into flower.

Powdery mildew. Powdery mildew on asters is usually pretty obvious, as it’ll look like white or grey powder was spilled on leaves and flower buds. Rarely deadly when it comes to asters, powdery mildew can be very annoying since it has a reputation for causing buds to fail. Treating powdery mildew is a two part process.

First, you must increase the airflow around your plant, whether that means thinning the inside branches or removing plants that are crowding it. Improved circulation makes it hard for powdery mildew to really set in. Secondly, you can treat powdery mildew infections themselves with neem oil; just make sure to test a few days ahead of treatment and then apply it liberally. Keeping plant debris away from the asters will also help to prevent reinfection, since spores can winter over this way.

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It's true that feeding your plants with a high-nitrogen fertilizer will promote lots of green, leafy growth, but all this growth is usually at the expense of flowers. It's also true that plants need phosphorous to set flower buds. But before fertilizing, you need to look at the whole picture.

If your plants look otherwise healthy, check for other problems first. If they show signs of malaise or stress, check all the growing conditions. Is the plant getting plenty of sun and water? Are there any signs of disease or pests? How do the other nearby plants look? Have you had your soil tested recently? If your soil pH isn't in a good range, it won't matter how much fertilizer you add your plants won't be able to access it. All these factors work in combination.

Thinning young asters in spring encourages larger blooms throughout the season. To thin asters, remove the weakest shoots on each plant, cutting the shoots down to the ground. Leave six to eight healthy shoots to grow and develop on each plant. Because asters are prone to mildew, thinning promotes air circulation around the shoots, resulting in a healthier plant that's less likely to succumb to disease.

Deadheading refers to the removal of wilted blooms. If wilted flowers remain on the plant and allowed to go to seed, the aster no longer has reason to create new flowers. Plants that are regularly deadheaded continue to bloom until the end of the blooming season. Deadheading also prevents the plant from self-seeding and helps keep the plant confined to its designated area. Deadheading asters involves pinching or snipping the wilted bloom, along with the stem down to the next leaf, stem or bloom. If you want the plant to self-seed, leave a few wilted blooms on the plant in fall.

Cosmos, Common Cosmos, Mexican Aster 'Mixed Hybrids, Noids'


Water Requirements:

Drought-tolerant suitable for xeriscaping

Average Water Needs Water regularly do not overwater

Sun Exposure:


Foliage Color:




Where to Grow:


Parts of plant are poisonous if ingested

Bloom Color:

Bloom Characteristics:

This plant is attractive to bees, butterflies and/or birds

Bloom Size:

Bloom Time:

Other details:

Soil pH requirements:

Patent Information:

Propagation Methods:

From seed sow indoors before last frost

From seed direct sow after last frost

Self-sows freely deadhead if you do not want volunteer seedlings next season

Seed Collecting:

Allow seedheads to dry on plants remove and collect seeds

Properly cleaned, seed can be successfully stored


This plant is said to grow outdoors in the following regions:

North Billerica, Massachusetts

Munsonville, New Hampshire

Gardeners' Notes:

On Sep 7, 2012, Dosetaker from Mason, NH (Zone 5b) wrote:

Love their tendency to self-seed without it becoming invasive.

On Oct 5, 2010, e_e_e from Atlantic City, NJ wrote:

ive grown cosmos the past 3 summers.
i originally had them in a pack of assorted wildflowers. i didnt know what they were at first but they each grew to 8-9 ft and were beautiful so i saved the seeds from the flowers for the next spring.
i had to look them up for info but contrary to what i read about their size, my cosmos have varied greatly in height.
i have some now that are around 5-6 ft high and another 2 around 9 ft. and i have one plant right now thats over 10 ft and today is the first day it bloomed a flower. (oct. 5) ive been waiting all summer for it as my other cosmos have been flowering all summer. dont know why this big one has taken so long.
i grow them in full sun and they get water every day along with an occasional drink of miracle grow.
/> so far ive had 2 colors, pink and white.
they are beautiful willowy plants and im constantly finding more and more places to grow them.

On Jun 9, 2007, Opoetree from Oak View, CA wrote:

I have grown cosmos for years. the flower is my husband's and son's birthday month flower. I also notice them all over my little town in the Ojai Valley. For some reason, they are blooming earlier this year. Generally they bloom in October (when they are supposed to, because they are the flower for the month of October!). Easy to grow, I even have some popping up from seed in my cactus pots. Seems like the purple variety is the most durable and prolific. Like the foliage as well. feathery. Easy to grow, friendly to have around. not demanding in any way.

On Jul 26, 2006, lemmons75 from Rock Hill, SC wrote:

This plant is unlike anything I have ever come across.It was giving to me by a neighbor in a seed pack (with other seeds).It first came up it was 2 ft tall but no flowers.I transplanted it twice and now it sets alone in a 5 gal. bucket with awsome pink flowers.I'm 6ft tall and it is a bit taller than me.I have ended up tieing it up to keep it from falling over.This is one plant that I will plant next year and give some seeds away.

On May 22, 2006, Gabrielle from (Zone 5a) wrote:

They are very pretty flowers, but need pinching back or staking early on to prevent flopping.

On May 2, 2006, Suze_ from (Zone 7b) wrote:

What can I say, but that I just love cosmos. C. bip regularly gets to 4-6 ft or more in my garden, though.

On Aug 26, 2005, tiffanya from Sumner, WA (Zone 8a) wrote:

Cosmos are very easy to grow. I sow along the fenceline each spring. In 2004, they grew very tall (most measured at 9 feet high), mostly due to a combination of steer manure in the garden and the neighbor's active sprinklers sharing water along the fenceline. It was literally a jungle of cosmos. I kept dead-heading them, and they bloomed pretty much from July into October.

I especially love the star-shape pistil/stamen area as the pollen is released. very pretty. The dragonflies also appear to be attracted to the color.

On Jun 24, 2005, PurplePansies from Deal, NJ (Zone 7a) wrote:

I believe cosmos to be a "must have" for every garden. They are a beautiful flower. now with so many fun varieties . and so easy to grow! Simply scatter seeds in the garden and they will grow like mad! That being said they are also not harmfully invasive. Birds also love the seeds. Many of its colors blend nicely with other colors in the garden. Attractive divided foliage. A winner all around. Try it you will not be dissapointed! :)

Such great performance makes it a favorite in my SW garden.

On Jan 16, 2005, LilyLover_UT from Ogden, UT (Zone 5b) wrote:

Cosmos are easy to grow from seed sown directly in the garden in mid to late spring. They also self-sow vigorously, and they attract beneficial insects.

On Jul 29, 2002, lupinelover from Grove City, OH (Zone 6a) wrote:

Best grown in poor soil rich feeding leads to very big branches with poor flowering which tend to be floppy. Deadheading prolongs flowering.

On Jan 4, 2001, lantana from (Zone 7a) wrote:

On Nov 7, 2000, jody from MD &, VA (Zone 7b) wrote:

This species of cosmos is an annual, grows 5' to 6' high. Flowers in summer and autumn in various colors of pink, red, white and purple. May need staking. Best cultivated in sun, with average soil. Not very frost hardy.

Watch the video: How to Grow and Care Aster Plant. Beautiful Flowering Plant - Aster

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