15 Common Problems With Flowering Cosmos Plants

Cosmos species, particularly C. bipinnatus and C. sulphureus, are charming, whimsical flowers also known as Mexican asters. Cosmos flowers belong to the Asteraceae family, which includes dandelions, black-eyed susans, and sunflowers.

Flowering cosmos plants are easy to grow from seed and can be prolific bloomers. They come into their own in the Fall here in the Midwest. A patch of cosmos is simply magical when in full bloom!

Although cosmos are easy to grow, there are a few problems that can occur when growing these Mexican natives. In this article, we’ll dive into the 15 most common cosmos problems and how you can help reduce or prevent these problems in your very own garden.

Seedlings Fail to Germinate

One of the earliest cosmos problems you may have (and many other flowers) is either a failure to germinate or a seedling disease called damping off. Cosmos seeds are long and skinny. While they do easily sprout on the surface of the soil, I find that I have the best germination rates if I bury the seeds shallowly–at about ⅛ to ¼ inch deep.

If you are direct sowing, you can broadcast seeds and then lightly rake the soil to get good seed-to-soil contact. Good seed-to-soil contact ensures the seeds have a more even temperature and moisture. Gently press seeds into your germination mix if you’re starting them inside, and then cover them with a light dusting of vermiculite.

Damping off is a disease caused by a few different types of fungi. You’ll notice your seedlings germinate and look healthy, and then all of the sudden they’ll begin to die off for no apparent reason.

Oftentimes a large section of a tray or area in the garden may die off due to damping off. Sometimes seedlings may fail to emerge from the soil. The cotyledons (the first “leaves”) will become mushy and water-soaked in appearance. The stems will become water-soaked and thread-like. If you pluck the seedling out of the soil, roots can be nearly absent.

The pathogens responsible for damping off thrive in cool, wet conditions. They also survive well in soil and debris. If you are starting your cosmos inside, make sure that you have cleaned and sterilized your pots and trays well.

Use only soil media (like germination mixes or potting mixes) instead of outside soil. Do not reuse old potting mix. Use a heat mat under seedling trays to keep the temperatures evenly warm. To water, use clean, warm water with a temperature between 68 to 77 °F.

If you are sowing your cosmos outside, you can prevent damping off by making sure you are not planting during cool and excessively wet conditions. Damping off occurs mostly in soil temperatures below 68°F. Try planting later in the spring or early summer to avoid favorable damping-off conditions.

Plant is Not Fully Mature Before Blooming

While this isn’t always a problem, cosmos can sometimes begin to bloom before the plant has begun to put on enough foliage or has really established itself. In other words, your cosmos is still basically a seedling when it decides to put out its first bud.

It’s important to remember that when seedlings are stressed, they may put out a bud prematurely. This is because the plant knows it’s nearing the end of its life, essentially, so it throws all of its energy into reproductive growth versus vegetative growth.

But a major reason cosmos may bloom prematurely is that it is a short-day plant. What does this mean?

Some plants will bloom based on the patterns of light and day. Others disregard the amount of light they receive and instead bloom whenever they’ve grown the appropriate amount. Long nights (i.e. short days) will signal “short-day plants” that it’s time to bloom.

If you happen to be planting your cosmos later in the growing season (like mid-summer), you may see them begin to bloom as immature plants. No need to worry, though. Often you can pinch off the bloom to delay flowering and promote branching. This will mean more blooms in the long run!

Plant Does Not Have Many Flowers or Branches

This goes along with blooming before fully mature (sort of). Some cosmos cultivars naturally branch really well, whereas others I’ve noticed tend to focus on one main central stem with maybe a few side stems.

It is a good practice to pinch your cosmos when they are about 4 to 8 inches tall. Pinch by snipping off the tip of the plant just above a set of leaves. It doesn’t take much if your seedlings are still small. You can pinch when the plants are taller, too.

Pinching promotes branching, so you’ll have more blooms in the long run, and it helps the plant stay upright. Cosmos, depending on the variety, can get pretty tall. Pinching your plant creates a bushier plant, instead of a tall, narrow plant.

Deadheading the plant will also help promote a continuous display of blooms throughout the season. Deadheading is clipping off the flower when it begins to fade and set seed. Make sure to keep your cosmos deadheaded to prolong blooming.

If you cut deep within the plant, you’ll notice the new stems will be long and strong. So, don’t be afraid to remove more than just the top few inches of the flower stem.

Flowers Falling Over

Again, cosmos plants can become quite large! Some varieties can grow as tall as 7 feet! This can be awesome for your garden, but the detriment is that if not supported well, they can flop over.

Falling over is more likely to occur with heavy wind or rainfall. Also, if you heavily fertilize cosmos, there is a likelihood that this overfertilization can cause rampant growth that results in weak stems.

Plant cosmos in an area that is protected if you live in an area of high winds. You may also try staking your plants or decreasing spacing between plants so that they support each other as they grow. It’s likely that you will only need to fertilize cosmos once during the season, if at all.

Flowers Won’t Fully Open

Most of the time this is a problem on an individual flower basis. There are times that I have tried to cut cosmos flowers for a flower arrangement and noticed half of the petals of the bloom have unfolded, while the other half haven’t.

You can help the petals unfold by gently squeezing the side of the bud that isn’t open. This can happen because the sepals don’t break open. The sepals are the green casing around the unopened bud that splits open so the petals can unfurl.

There’s also a chance that the petals are damaged due to feeding by insects like thrips. When thrips feed on the unopened flower bud, they can cause damage that results in only one side opening or dwarfed petals on one side of the flower head. Thrips are tiny (only 1/16 inch long or less), so you often won’t notice them until the damage is already present. We’ll talk more about thrips later in this article.

Droopy Stems

Droopy stems, for many plants, indicate that the plant isn’t fully hydrated. This could be because there hasn’t been adequate rainfall in the garden or because of an underlying disease that may have clogged the vascular system of the plant.

If you water the plant and the stems perk up, then you’ll know that low moisture was the issue. If you water the plant and it does not revive, or the top of the plant begins to die back, then it’s likely there is some sort of disease in the stem or roots.

Some varieties of cosmos have naturally droopy stems due to the weight of the flower head. This often occurs in double-petaled varieties like ‘Double Click’. I personally love double-petaled cosmos, but it can be frustrating when the flower head is always drooping down! Unfortunately, there’s not much you can do for cosmos flowers in this case except enjoy their whimsical nature.

Deformed Flowers With Green Leafy Growth

Aster yellows is a disease that can plague cosmos. It is caused by a phytoplasma that is spread by leafhoppers. This disease affects many members of the Asteraceae family. You may notice the foliage turning yellow and new flowers having a very strange appearance.

The flowers will have a greenish cast to them and oftentimes there will be leaf-like growth inside the flower. The plant overall will not look healthy. Eventually, the plant will die.

It is best to remove any plants you suspect may be infected with aster yellows. Burn or bury the infected plant. Do not put any infected plant parts into the compost pile. Aster yellows cannot live without a vector, such as a host plant, or an insect vector, like leafhoppers.

Remove any common weeds from the area that are of the Asteraceae family (like dandelions) that could possibly harbor this disease. Aster yellows is most common in cool, wet summers.

Holes in Flower Petals

Cucumber beetles are small beetles (¼ inch long) that can be either yellow with black spots (spotted cucumber beetle) or yellow with three even black stripes (striped cucumber beetle). They flit around cosmos flowers and are hard to catch and pick off. They love to munch on foliage and petals, especially petals of the cosmos flower.

Unfortunately, there is no real effective control for cucumber beetles. Any damage to mature plants is not usually detrimental. However, small plants may suffer. You can try covering your seedlings with a protective cloth until they are large enough to withstand damage if these beetles are an issue in your garden.

A great way to reduce cucumber beetle damage is by planting a trap crop nearby. In my cut flower garden, I’ve noticed that the cucumber beetles are attracted to my amaranth foliage.

Since I use only the flowers of my amaranth and strip the leaves for flower arrangements, it’s a win-win for me. The beetles are attracted to the amaranth leaves and usually stay away from the cosmos.

Lots of Ants on Stems

While ants themselves do not cause damage to cosmos flowers, they are often a sign of a bigger problem. Aphids.

Aphids are small insects (1/16 to ⅛ inch long) that can be any shade from brown to gray to green. They have piercing-sucking mouthparts that not only do damage to the plant but also transmit viruses. Aphids can reproduce very quickly. It’s said that if you see one aphid then there are probably many more you don’t see.

Ants and aphids have a symbiotic relationship. Aphids feed on plants and secrete a sugary waste called honeydew. The ants feed on honeydew. In exchange, the ants will herd aphids around to particularly delicious parts of the plant and they will also protect them. They will even bring aphids to their nest during the winter!

If your cosmos are mature enough, it’s likely that aphids won’t cause too much damage. However, aphid feeding can cause twisted and curled leaves, yellow foliage, stunting, and overall poor plant growth. Viruses may be transmitted as a result of aphid feeding.

The best way to get rid of aphids is to use practices that promote beneficial insects that feed on aphids (lady beetles, lacewings, parasitic wasps) or to spray any heavily infested plants with a forceful stream of water. If I notice one plant is particularly infested, I’ll often remove the plant from the garden entirely.

Branches Are Breaking Off

Sometimes your cosmos plant will grow so vigorously that it can’t support itself any longer. The branches will begin to break off easily after small winds or simply brushing up against the plant.

You can help prevent breakage by staking up the plant or decreasing spacing between individual plants when you are planning out your garden so that the plants can support each other as they grow.

It is also a good idea to reduce the amount you are fertilizing your cosmos, especially if you already have rich soil. Cosmos are a great candidate for those poor soils in the garden or landscape because they actually become too vigorous in areas of high fertility.

Petals Have Unsightly Streaks

Streaking on the petals is often an indication of thrip damage. Thrips are extremely tiny (1/16 inch long or less) cigar-shaped insects that feed on flowers and foliage. They’re so tiny that you often can’t see them unless you shake them onto a white paper.

Thrip damage can result in distorted flowers that don’t develop fully. Thrips can also transmit viruses. Oftentimes thrip damage is more noticeable in darker colored flowers because the streaking on the petals is more prominent.

If you suspect your plants are damaged due to thrips, you can try spraying them with a forceful stream of water to knock off any thrips. Also, you can use yellow or blue sticky traps placed near the plants to catch thrips, but this is more for a greenhouse or indoor situation.

Little to No Blooms on Mature Plants

If you plant your cosmos and notice as they grow that the foliage is a lovely shade of green and looks healthy, but you spend the entire growing season with no sign of flowers, then it is likely you’ve over-fertilized. If you haven’t fertilized at all, it’s likely the soil is too high in fertility for your cosmos. Sounds strange, right?

High fertility (especially, high nitrogen) can cause plants to promote vegetative growth instead of reproductive growth. Vegetative growth (basically, leaf and stem growth) is important, but it is likely that you are wanting to see those pretty flowers, not a bunch of bushy foliage!

Back off on fertilizing if you suspect that’s the problem. If you haven’t been fertilizing at all but you’re not seeing flowers, don’t worry. Cosmos flower according to day lengths naturally, so by the fall you should have some flowers to show for it–if you’re patient enough!

It can take about 65 days for cosmos to flower from seeds on average. Sometimes they’ll flower sooner because of changes in daylight. Next year, try reserving your poorest soil for your cosmos or reduce fertilization altogether.

Stunted Growth With Wilting Leaves

Fusarium wilt is a fungal disease that lives in the soil. When a plant is infected, the pathogen will clog the vascular system of the plant, causing wilting, yellowing of the leaves, and eventually death of the plant.

An early sign may also be stunting of the plant with wilted leaves. You might notice reddish staining in the vessels inside the stem and rotting of the root system.

If Fusarium wilt has been a problem in the past, try rotating where you are planting your cosmos each year. In some cases, you may need to “rest the soil” from any affected plants for at least 4 to 5 years.

It’s also important to remove any infected plants from the garden immediately. Make sure you are buying your seed from a reputable seed company to ensure disease-free seed.

White Spots or Pustules on Leaves

If you notice powdery white splotches or raised white pustules on cosmos leaves, you are likely seeing one of two diseases–powdery mildew or white smut. Both are fungal diseases that can cause problems for your cosmos, and both come on due to humid conditions.

Powdery mildew symptoms include powdery spots or patches on leaves and stems. They’ll first appear on the upper surface of the leaves. Hot, humid conditions will often bring on powdery mildew in cosmos. Eventually, if the disease continues long enough, the leaves will turn brown and shrivel.

White smut starts out as yellow flecks or spots on leaves and stems. Eventually, these spots become white pustules that burst open to reveal powdery, white spores. The leaves will turn brown and shrivel after the pustules have opened.

In both cases, make sure the plant has a reduced time that the foliage is wet throughout the day. Ways to help with this problem for your cosmos are by increasing spacing between each plant to promote airflow, watering in the mornings only, and watering at the root zone instead of overhead. You may also try to pick off any infected leaves to prevent spread (especially for white smut).

Papery Brown Flowers

Botrytis blight is a fungal disease that can affect cosmos flowers. The flowers will turn papery brown and then develop a grayish fuzzy mold. This is most common in flowers that have already bloomed and are beginning to die back.

Make sure to remove any infected flowers from the garden completely to prevent the spread of botrytis. In some cases, leaves may have brown to tan spots with a target appearance. Cloudy, humid weather is where this disease thrives. Cleaning up debris is especially important in controlling this disease.

Along with cleaning up debris, some other practices you can implement to reduce botrytis blight is reducing fertilization, avoiding overhead watering, and increasing the spacing between plants to promote good air circulation.