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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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Next to wild white Daisies with the yellow eye, Cosmos are a favorite. I too have a sandwich bag of Cosmo seeds. I am excited to get winter over so i can get the seeds growing. Thank you for the information. My question is," can i mix wild daisies in the same garden with the Cosmos?" Thanks again.
This advice is too late for you, but hope it helps others. We’d sow our cosmos seeds in March or April. Harden off the seedlings before planting out in a sunny spot. Many flowers mix well with cosmos. We haven’t tried daisies but I’m sure they’d grow. It depends on the color of the cosmos. Against a white daisy, we probably would not pick a white cosmos; it would be better to have a deep magenta cosmos. However, it’s a personal preference!
i have a sandwich bag full cosmo seeds. plz sumone tell me how many seeds i need to plant per plant.
Hi John,
Plant Cosmo seeds 2 feet apart to give them adequate space. This distance also gives them the benefit of leaning on each other when they are mature.
Should the Cosmos be cut back? pulled out? left long/leggy? or what? Thanks. BTW - mine grew to at least 8' tall - Monarch, Painted Lady, and Skipper butterflies, and bees LOVE the flowers!!
Cosmos is an annual, so it will not continue to grow noticably (and will die) this growing season. However, it sometimes self seeds, so you may get more of the same…which sound spectacular! Kudos on the cosmos, Pat!
My grandmother is 90 and has dementia. I help take care of her and while going thru some old junk drawers I came across a package of cosmos seeds. I don't know how old they are, when she bought them, or if they are still any good or not but in the article it says they can be "self-sowing" so if this next spring I were to just take them out and scatter them in a bare place in the yard and water it occasionally, when I do the rose bushes say, do you think they might grow? Also she is wanting to plant something in her hanging baskets on the back porch is there anything we can plant now, in the mid-summer in zone 6 without going to the garden center and spending a lot on already half grown flowers?
Thanks for any help!
Cosmos seeds may last between three and four years in a cool, dry environment, so your grandmother’s seeds may still be viable. It’s worth giving them a try, in any case! Maybe buy a supplemental packet, too, if you can.
As for your hanging baskets: It’s a tad late to be starting most flowers from seed, unfortunately, however you do have a couple options. Pansies are probably your best bet if you want to start from seed. You may be able to find a few cheap impatiens, portulaca, and petunia transplants on sale at this time, too.
I live in southern NH and was wondering if I scattered Cosmo seeds in my wildflower field in September would they give me flowers next spring? Any special variety? TX
Hi, Kitty. Cosmos are best planted in the spring and need 7 weeks to grow, and will not survive the first fall frost.
