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Planting, Growing, and Caring for Cosmos
Planting
The soil should be well-draining, but other than that, cosmos don鈥檛 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鈥檙e 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鈥攏o more than 1/4-inch deep.
- Thin to 12鈥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.
Growing
- 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. According to the , 鈥淏e cautious not to overwater your plants. That and over-fertilizing can lead to the plant producing fewer flowers.鈥
- 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
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鈥檚 best to pick some right when the petals have opened.
- If you cut the blossoms when they first open, they鈥檒l last more than a week in water. Strip off the lower leaves and put them in a vase.
Catherine Boeckmann
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We're not sure where you live, but probably not. As written above, cosmos are best planted in the spring and need 7 weeks to grow, and will not survive the first fall frost.
Hi, I grow my Cosmos in planters and was wondering with winter coming how I should care for them so that they may grow back in the spring. It gets quite cold here in Indiana so I am concerned. Thanks!!
Hi Jane,
Cosmos are annuals and will not survive the winter. It's best to start with fresh seed next spring. You can collect the seeds from your cosmos at the end of summer. Make sure the seeds are dry and save them in an envelope or jar.
I love cosmos and they always do well. Do only the orange ones have the seed heads that can be pulled off? I've gotten other colors and they don't seem to have the dried seeds to pull off like the orange ones, wondering how to know which varieties has the dried seeds heads. I can't seem to find the answer anywhere online. Thank you
Most cosmos varieties develop seedheads if you leave the faded flowers on the plants. The seedheads may look a little different but you'll be able to see and collect the seeds after they have dried.
the stem of a Cosmos plant broke, I buried the entire stem, will the stem grow new roots ?
My plant broke in middle at bottom from rest of stem. What can I do to save it? Also some of my plants do not flower or even bud. Is there something I can do?
I have about 10 cosmos' in a large raised bed, they were grown from seed in a greenhouse and planted out mid June, it's now mid August and I don't have any buds or flowers. They are nearly 4ft high and still growing. Do i need to do something to help them bud. They are in full sun. Thanks
Mine also took until the beginning of Sept. To bloom, but I started them a little late from seeds. This is great info, thank you
Hi, Vicki, It sounds like the soil that your cosmos is in may be too rich, possiblyl especially heavy with nitrogen. For example, if you fertillize with a 20-20-20 (nitrogen, phosphorus, and potassium) the balance may be too much of a good thing. Nitrogen promotes leave growth. In future, you might try tomato fertilizer, a blend that promotes fruit (don't expect tomatoes to form on your cosmos; it's just that the blend of ingredients provides less nitrogen than the 20-20-20.)
They like sun, and it sounds like you're giving them plenty.
We hope this helps!
