How to Plant, Grow, and Care for Cosmos Flowers
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 tolerant of most soil pH levels, too, 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, though, 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.
- In order 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. Simply strip off the lower leaves and put them in a vase.
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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!
I live part of the year in Toronto, Ontario in Canada, and the other part of the year 200 kilometres north of Toronto, where cosmos are definitely annuals. But in Toronto itself, they seem to be perennials, coming back year after year near sidewalks, in sunny spots, in ground that is less than fertile, as happy as the proverbial clams. Not too many clams in Toronto, though. Wherever they grow, cosmos are very satisfactory, happy-making plants.
Is there a rule of thumb on how deep to plant leggy cosmos?
'Diablo' Cosmos is a strikingly gorgeous one with shades of dark orange to deep rusty orange.
Very complete post about Cosmos, thanks.
I did grow them from seed and here is a photo post of the steps and results. There was one error I committed. Curious?
https://www.dearplants.com/growing-cosmos-from-seed-step-by-step/
Happy gardening!
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