How to Plant, Grow, and Care for Cosmos Flowers
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The soil should be well-draining, but other than that, cosmos don’t need any special soil preparation. In fact, they like soil that is not too rich, as rich soil will encourage foliage at the expense of blooms. Cosmos are also tolerant of most soil pH levels but grow best in neutral to alkaline soils (pH of 7.0-7.5).
Cosmos can tolerate warm, dry weather very well. They are even drought-tolerant.
When to Plant Cosmos
- Direct-sow seeds outdoors once the danger of frost has passed.
- Alternatively, sow seeds indoors 4 to 6 weeks before your last spring frost date in trays or pots with a good seed-starting mixture. Move them into 5-inch pots as soon as they’re 3 or 4 inches tall.
- Young plants (transplants) can be planted outdoors once the danger of frost has passed.
How to Plant Cosmos
- Just sow seeds lightly—no more than 1/4-inch deep.
- Thin to 12–18 inches apart when seedlings are a few inches tall. Depending on the variety, cosmos can grow anywhere from 18 to 60 inches tall, so plan accordingly.
- If you are growing cosmos from seeds, be mindful that it takes about 7 weeks to first bloom. After that, your flowers should continue to bloom until the first fall frost.
- If you let the spiky-brown seed heads blow away during the fall, cosmos are likely to self-sow throughout your garden.
- To prolong flowering, you should deadhead the plants (prune off dead/faded flowers). This speeds up flower development and aids branching.
- Because some of these plants can grow really tall, staking may be necessary. Offer them protection from strong winds, encourage balanced branching by pinching out (aka pruning) central shoots or stem tips, or stake them. Growing them against a fence can also provide the necessary support.
- Water regularly until plants are established or if it is unusually dry. Make sure you don’t over-water cosmos; over-watering and over-fertilization can lead to plants with fewer flowers. Cosmos can tolerate dry soil, even in a hot, arid, sunbaked spot.
- Cosmos beds may become weedy due to the fact that they self-seed, so remember to remove flowers before they go to seed or to thin seedlings in the spring.
Harvesting Seeds and Cut Flowers
- To harvest more seeds, remember to leave a few flowers on the plant because they will self-seed.
- You can cut the flowers off anytime after blooming, but it’s best to pick some right when the petals have opened.
- If you cut the blossoms when they first open, they’ll last more than a week in water. Strip off the lower leaves and put them in a vase.
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Do Cosmos need to be cut back for winter? We live in Kentucky and have moderate winter weather.
Most cosmos are annuals, so after all the blooms are spent this fall you can cut them back/pull them from the ground. Cosmos self-seed so if you do not want them to grow in the same area, make sure you remove spent flower before they go to seed. If you want them to remain in that area of your yard or garden, you can allow the flowers to remain on the stalks so that the seeds fall to the ground and then remove. You can also remove the flowers and save the seeds to plant in the spring, either indoors 4 to 6 weeks before your last frost date or directly into the ground once the threat of frost has passed.
I planted cosmos from seeds and they are 4-6' tall, look healthy, but no flowers or buds. At all. Some did get chomped by an animal but still there are plenty that didn't. Any ideas how come they wouldn't produce flowers? So bummed!
When flower dies do you just cut the flower off and leave the stem
An important part of maintaining your cosmos is to deadhead. When you deadhead spent cosmos flowers, it forces the plant to put its energy into creating more flowers instead of making seeds. When the blooms are spent, cut back the stem to above the next set of leaves and soon your plants will produce a new set of buds and before you know it a new set of flowers will be brightening up your garden!
Beware of Cosmos, a beautiful orange flower, named “Diablo” the dried seed heads will attack, at least according to my wife who gets attacked by everything in the garden. The seeds are about 1/2” long and very sharp when dry, no fun in your shoes or sandals.
I planted 2 Cosmos plants I bought at HD. Something… decided to make my Cosmos plants their evening meal overnight, named down to the nubs? Animal or insect? That critter or critters totally destroyed my delicate little flowers 🫤🤔
Rabbits. Love Cosmos….
Hi Gayelle,
It is unfortunate that your beautiful flowers turned into someone’s late night snack. There must be some incredibly hungry animals living around your home. One culprit to cross off the potential list is deer because cosmos are a flower tend to avoid. Hopefully they did not eat too much of the plant so that it can continue to grow and more flowers will bloom this year.
I'm not sure how unusual this is, but our cosmos plant in the backyard measures exactly 12 feet high! I wish I could post a picture of it!
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