With few exceptions, cosmos are a low-maintenance plant that should be included in any flower garden. Not only are they low maintenance, but they are also prolific bloomers during the growing season.
Cosmos will bloom all summer long, starting about 12 weeks from when the seeds were planted. Starting seeds indoors and transplanting them out will extend the blooming season. Methods such as succession sowing and deadheading will also ensure a continuous harvest of blooms.
What is the flowering season for cosmos?
The flowering season for cosmos is the summertime until fall. Cosmos are planted in the spring, blooming about 10-13 weeks after the seeds have been sown, depending on the variety. Once they bloom, cosmos will produce flowers for weeks, if not months. Late fall frost will kill the plants, ending their flowering season.
Cosmos must be planted in the spring after all risk of frost has passed because they are known as a tender annual. This means they will not survive frost and must be grown in warm weather.
Most gardeners in the US will be past any risk of frost by April or May, depending on which zone they are in. The last threat of spring frost is called the last average frost date.
You can find out the date for your area by using your zip code. Just plug it in here on Dave’s Garden, and you’ll know when it should be safe to plant your cosmos in the garden.
When you look up your zip code, you’ll also see the first average frost date. That date is when you can expect the first frost in the fall, which is essential because it will mean the end of your cosmos plants.
Until that date hits, you can expect your cosmos to flower, especially if you follow a couple of extra steps to encourage the most blooms possible.
Three ways to ensure your cosmos bloom all summer
You can use a few tricks to maximize the summer blooming season of cosmos and ensure you have flowers for as many months as possible.
1. Choose varieties with different days to maturity
Most cosmos varieties will reach maturity in 70-95 days from sowing, but a few varieties bloom earlier and later than this general time frame. Planting multiple varieties with different maturities can increase the time you’ll have cosmos in bloom.
An easy-to-implement strategy would be to choose one variety from each time frame: an early, middle, and a late bloomer. Combine that with the succession planting recommended next, and you’ll probably have more cosmos than you know what to do with!
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2. Make multiple sowings of cosmos
To take full advantage of the flowering season of cosmos, you can sow seeds indoors to get a jump start on the days to maturity. Follow that up with one or two sowings in the garden you will enjoy cosmos flowers all summer long.
This process is called succession planting. It takes some practice to get right, but once you do, it can mean doubling your garden’s productivity for the season.
To get early seedlings to transplant in the garden, start cosmos seeds indoors 4-6 weeks before your last frost. That’s the date you looked up to make sure it will be warm enough for your cosmos.
If you aren’t sure how to start seeds indoors, check out this informative video by flower grower Danielle of Northlawn Flower Farm. She’ll walk you through the whole process.
Or, you can jump over to this article for all the steps: Step-By-Step Guide To Starting Seeds Indoors (Plus a sample setup).
In addition to early transplants, you can direct sow your cosmos seeds into the garden. This will give you staggered plating times, so you always have vigorous plants starting to produce fresh blooms.
So, for example, on the same day you transplant your cosmo seedling out in the garden, sow some seeds as well. The seeds will be 4-6 weeks behind the transplants and be ready to go with fresh new blooms when the transplants have already been in production for weeks.
Follow that up with another sowing one month later, and you’ll have a domino effect of bloom times.
Even if you only do one planting or sowing of cosmos, there’s still another method to ensure you have blooms all summer long.
3. Deadhead your cosmos regularly
Deadheading is one of the primary methods to encourage cosmos to bloom. Deadheading means removing the flowers once they are done blooming.
Deadheading is a straightforward process. Once the flower petals start to wilt, fade, or furl, you’ll know the flower is past its peak. Use scissors or garden clippers to snip the flower from the stem, preferably just above a set of leaves. The result will be renewed growth from the plant as it sends out a new branch and bud to replace the dead flower.
Check out these two if you’re in the market for some new garden clippers that can keep up with you all season.
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.
If you were to leave the dead flowers on the plant, it would slow the production of new flowers. Since cosmos are annuals, they complete all their growth in one growing season.
The plant’s job is to make flowers that will turn into seeds. Once those seeds are formed, the plant considers its job done and will stop flowering.
By removing the spent bloom, you trick the plant into making more flowers since its “job” wasn’t yet successful.
Since cosmos are such prolific bloomers, even if you’re cutting blooms regularly for fresh bouquets, chances are there will still be some leftover flowers. Make sure to snip those next time you harvest to avoid leaving any old flowers on the plant.
If you miss any spent blooms that do turn to seed, you can still snip those off and toss them on the ground. Cosmos are great at reseeding themselves, so chances are good you will see little cosmo seedlings at the foot of your established plants.
Bonus cosmos flowers without any work? Yes, please!