Why Aren’t My Cosmos Blooming?
Cosmos flowers are easy-to-grow, bright, and cheerful annual flowers, but they need a few requirements to keep blooming all season long. Join organic farmer Jenna Rich for issues that might be causing your cosmos to fail and how to avoid them this season.
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Cosmos have always been a farm favorite for us. I dare you to look at them and try not to smile. Their consistently bright and cheerful pinks and bright whites visually delight us and keep our local pollinators happy.
These annual flowers are relatively easy to grow and perfect for beginners. Their range of white, pink, and magenta will complement all garden styles, and they don’t mind much about soil type or fertility. Plant them along borders, in containers, or around your vegetables for simple beauty, height, and airiness.
A lack of blooms in the garden can be frustrating when you planned and hoped for a flower-filled summer! Let’s get into some possible issues that may be causing your cosmos not to bloom and how to avoid each one.
The Short Answer
Cosmos is one of the first seeds I directly sow in my garden each year and the last annual plant flowering when the first fall frost arrives. However, less-than-ideal conditions may cause failure to bloom. Keep reading if you’re wondering what’s happening with your cosmos plants and how to help them get their groove back.
The Detailed Answer
Cosmos are easy to grow and low-maintenance, but they have their needs. Here are a few issues that might be causing your cosmos not to bloom and remedies to fix them.
Not Enough Sunlight
Cosmos thrive in full sun. Plant them in an area that receives six to eight hours of direct sunlight daily. That said, if you live in a southern region where the sun is constant and harsh, your cosmos will appreciate some dappled sun or shade. Use shade cloth or plant them under the light shade of other plants to offer them added protection.
Transplant Shock
Cosmos are simple to start indoors from seed. Seeds are large and easy to work with, and germination rates are typically very high. However, you must ensure the plants don’t become rootbound so don’t sow seeds earlier than three to four weeks before transplanting them.
Check the weather forecast and ensure garden space is ready and available for them. If they sit in cell trays too long, they may become stressed, which can have effects that last all season, including double blooms reverting to single blooms. You may also notice them prematurely producing flowers when they’re short and immature.
Harden them off properly for several days to a week before transplanting so they acclimate to outside weather conditions. If there are no negative symptoms after this period, proceed with transplanting.
Transplant cosmos in the early part of a warm but mostly cloudy day to avoid harsh sun and heat. Give seedlings a good drink of water and then gently coax them out of their cells. Create a small hole using a dibbler, a trowel, or your hand. Drop one seedling in each hole so the root ball is completely protected and the top is level with the soil surface. Fill in the hole with native soil, tamp it down, and water it in.
Over-Fertilization
If you’ve ever spent all season tending to a plant only to receive lush foliage with no blooms, you may be over-fertilizing. Feed cosmos at the time of planting. Start with the low end of the recommended dose and add fertilizer into the planting medium when sowing seeds. Look for fertilizers low in nitrogen to promote a healthy, continuous blooming season.
Over-fertilizing can lead to extremely tall plants with few to no blooms that may topple over!
Under-Fertilization
While cosmos don’t require highly nutritious soil, they need some nutrients. Test your soil If you notice lots of lush foliage but no blooms. This will give you an idea of a baseline and recommendations for general treatments to balance it. A good place to start is bone meal, which is high in phosphorus and calcium.
Cosmos grown in containers, like most plants and flowers, will require more nutrients as they have a fixed amount of soil to root around in. Feed container cosmos at planting and every three to four weeks or as recommended on the fertilizer packaging. Cut back your fertilizing if you notice the flower production slowing down so the plant can work through what’s in the soil. Take note of the timing for next season and adjust as needed.
Overly Rich Soil
This may sound odd as most plants are particular about their soil type and levels of nutrients, but cosmos perform well in moderately poor to poor soil. This is great news if you’re starting a new garden plot in poor soil but still want the joys of a low-maintenance, beautiful garden.
Clay or Heavy Soil
Cosmos prefer soil that is somewhat loose in consistency. Add compost to assist with drainage and practice broadforking. Incorporating worm castings may enrich your soil and adjust the tilth of your soil.
Overwatered
Water direct sown cosmos regularly until germination and as seedlings grow and establish. Pull back once they start to flower. Add drip lines to control the flow and direct the water to the roots. Water them regularly about an inch per week or as needed.
Underwatered
Cosmos are fairly drought-tolerant once they’re established. However, do not allow them to completely dry out. To prepare seedlings for transplant, limit water during the hardening-off period to get them used to receiving less water.
Stems, branches, and blooms will weaken and droop if they need water. There is a point of no return, so keep your eye on them during extended periods of dry and drought-like conditions.
Weed Competition
Prepare a stale seedbed before direct sowing cosmos so they don’t have competition during the emergence phase. Cultivate shortly after they germinate and every few weeks to keep the weed pressure down. The towering blooms will begin to shade out tiny weeds that germinate, but helping them out in the early days is critical for their success.
No Support
Cosmos make an amazing border flower because they’re tall, whimsical, and full of gorgeous blooms. When planted in a standalone garden, they may topple over and cease to bloom. Tall cosmos varieties will perform best when planted in the back of an annual garden against a fence line or shrub line. This will help them stand up when strong winds and rain occur.
Pinch your seedlings at four to eight inches to encourage bushing out. This will also create a stronger base, help them stand up better, and produce more foliage and buds.
Lack of Deadheading
Deadheading is a sure way to keep your cosmos plants producing buds and flowers all season. Simply snip off spent flowers regularly to encourage new, fresh blooms. Do this by harvesting them to use in bouquets or clipping them back and discarding them into the compost.
Cosmo plants can be neglected and bounce back just fine. If there is a large amount of drooped, dried, or spent flowers, simply chop them down. While typically used on perennials, cosmos may benefit from a total refresh by performing the Chelsea chop.
Disease
If you’ve confirmed your plants are adequately watered, did not experience transplant shock, and have been properly cared for in general, but you notice issues like deformities, lack of blooms, or yellowing, you may suspect a disease is present.
Aster yellows, blight, and powdery mildew are likely suspects. Look for yellowing or browning flowers, deformities, and white fuzz on the petals, stems, and foliage. Keep a sanitary garden, avoid overhead watering, and ensure proper spacing, circulation, and airflow to decrease humidity between plants and create an environment where these diseases can thrive.
Pests
Aphids, Japanese beetles, and cucumber beetles are the most common pests of cosmos plants, and they can do quite a number on them!
Aphids
If you notice ants on your plants, you may have aphids. They’re attracted to the sweet honeydew substance aphids produce and leave behind. In return, the ants offer the aphids protection.
Japanese beetles
I finally caved two seasons ago and installed several Japanese beetle traps around our farm, and it has drastically helped decrease damage on multiple crops.
Cucumber beetles
These tiny spotted or striped devils work quickly, reproduce often, and eat more leaves than you think possible. They can turn a plot of cucumbers, cosmos, or greens into a skeleton patch overnight. Use insect netting to keep them off your plants. Avoid killing them on your plants to reduce the risk of spreading bacterial wilt. They carry this disease in their gut and it’s easily spread.
Final Thoughts
With a little TLC and research, cosmos are a charming, prolific annual flower to grow. They readily self-seed, can be started indoors or directly sown in your garden, and complement most garden styles well. I hope you learned something from this article that will help you keep your cosmos blooming and your gardens thriving.
