Cosmos are generally an easy flower to grow that will produce many blooms throughout the summer. The plants grow quickly and easily from seed, and in just two to three months, you should see your first flowers.
But what if you don’t? If your cosmos plants aren’t flowering, there are a few conditions that you can investigate to figure out why.
Common reasons that cosmos aren’t flowering include too much nitrogen or too much water in the soil, too little sunlight reaching the plants, or the plants not yet reaching maturity. To promote blooms, cosmos should be grown in average soil and full sunlight and be watered sparingly once established.
Read on to see if one of these factors could be why you don’t see any flowers on your cosmos plants.
1. Too much fertilizer
The number one reason that cosmos don’t bloom is that they are getting too much fertilizer.
Cosmos are sensitive to soil amendments, and a fertilizer heavy in nitrogen, in particular, will promote foliage growth at the expense of flowers. Cosmos perform best when planted in average soil without extra amendments. They will even tolerate poor soil as long as they get full sunlight.
It’s a habit to amend garden soil at the beginning of the season and when planting the garden. You want to create rich soil that promotes growth and healthy plants, so adding compost and fertilizer seems like the right choice. For many garden flowers and vegetables, that is the right approach.
Cosmos won’t benefit from that extra care, though. If you have a patch of soil or a neglected garden bed with so-so soil in it, that’s a much better spot for your cosmos flowers. Save the compost-rich beds for heavy feeders, instead.
Fixing this is going to take time. You can’t successfully transplant full-sized cosmos plants, and there’s no easy way to remove fertilizer from the soil. You can try planting another batch of seeds in a different area or just wait until next year and try again in less rich soil.
2. Too much water
Just like fertilizer, cosmos don’t need too much water, even in the heat of summer.
Giving too much water to your cosmos will promote excessive leaf growth and few flowers. If the soil is consistently saturated or boggy, the plant roots may even rot due to a lack of oxygen, which will kill the plant. To avoid overwatering, irrigate your cosmos once per week with one inch of water.
Cosmos prefer to grow in dry conditions, relying most heavily on regular irrigation only when they’re young seedlings. As native plants to Mexico and the Southwestern US, cosmos have adapted to be drought-tolerant plants.
Once the plants have reached their mature height, only water once per week to avoid overwatering. If you have a wet spring or summer rainstorm, you might not have to irrigate that week at all.
Check the soil before watering to make sure you don’t overwater. Scratch the soil’s surface to expose the dirt a few inches down. If it’s moist, the plants don’t need any water. If it’s dry, go ahead and give them their weekly one inch of irrigation.
3. Too little sunight
Cosmos are heat and sun-loving plants, and without both, they’ll struggle to flower as productively as cosmos are known to do. If your plants are absent of blooms, or there are fewer blooms than you’d expect, then take a look at where you planted them.
If your plants are located in an area of partial sun or shade and aren’t flowering, try to remove anything blocking the light, such as tree branches or taller plants. Or, set out a new batch of seeds where they will get a full day of sunlight through the summer.
Make sure to grow your cosmos on the southern side of anything taller than them so they don’t spend hours in the shadows cast by the taller object. This could be a wall, mammoth sunflowers, your garden shed…take a look before you plant.
If you have a slightly shady yard, try growing your cosmos in a pot or other container. That way, you have more flexibility for moving it where the sunniest spot is in your yard.
If you need a reminder of how the mature height of your cosmos, check out this article for all the details: How Tall Do Cosmos Grow? (It depends on the variety).
4. Too little time
If you started seeds early before the weather got consistently warm, the cosmos’ days to maturity (in other words, bloom) might be delayed. Cold soil and cosmos seeds do not mix, as the seeds require warmth to germinate.
Cosmos also need enough time to reach maturity, so if you’re chomping at the bit for flowers, make sure the plants are actually within the date range for developing flowers. If you planted seeds when the weather was cold, then you might need to tack on an extra week or two to the bloom time to make up for that slow start.
Here are a few popular varieties with their days to maturity. There is a huge range from 56-110 days, so if you have different varieties growing in your garden and some plants aren’t flowering, reference this table or your seed catalogs to review the days to maturity.
|Variety||Days to maturity|
|Double Click Mix||75-90|
How can I make my cosmos bloom?
To help promote cosmos blooms, plant seeds or transplants in a location that receives full sun for at least 8 hours per day. Make sure the soil is well-draining and don’t overwater them. Avoid adding any fertilizer high in nitrogen since that will promote leaf growth instead of flowers.
Keep in mind that cosmos are short-day plants, so if they are young plants, they will bloom best once days are under 14 hours long. They will eventually bloom, even if the days are long. To get the strongest flush of flowers, try starting them indoors 4-6 weeks before planting them out.
This jump start will help the cosmos reach maturity while the days are still shorter than they will be during the peak of summer.
You can then start a new round of seeds in late spring and even a third in early summer, which will take longer to bloom. As your early crop starts to wind down, the second (and third) plantings will just be hitting their stride for summer and fall blooms.
Now that you’ve got your cosmos sorted out, find out how long you can expect them to keep it up in this post, Do Comos Bloom All Summer? Tips To Keep Them Flowering.
Hint: You’ll need to keep them deadheaded by snipping off old blooms.
My favorite garden shears
I constantly misplace my garden shears and clippers, so I’ve tested a lot of pairs. Good thing these ones have red handles to help me keep track of them!
- Corona Leaf & Stem Micro Snips: Perfect for cutting small stems, deadheading spent blooms, or keeping the mint plant from taking over my garden.
- FELCO Classic Manual Hand Pruners: Better for heavier-duty pruning, such as dead sunflower stalks and tomato vines and cutting old zip-ties off the trellises.