How to grow cosmos (2024)

Published: Thursday, 3 June 2021 at 8:00 am

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Find out all you need to know about growing cosmos, from sowing to planting in this easy Grow Guide.

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Cosmos are sun-loving plants with a long flowering season. They fill borders with a profusion of flowers, ranging in colour from white through to pink, red and orange, in contrast with masses of feathery foliage. Shorter varieties also look good in containers and all cosmos make fantastic cut flowers. The simple open flowers are guaranteed to brighten up any garden well into autumn, and pollinators like them just as much as gardeners do.

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How to grow cosmos

Grow cosmos in moist but well-drained soil in full sun. Deadhead to prolong flowering and lift in autumn, after the first frosts. Save seed from spent blooms to sow the following season.

More on growing cosmos:

Where to grow cosmos

How to grow cosmos (1)

Cosmos are native to Southern and Central America and therefore need to be planted in a warm, sunny spot. Soil should be free-draining.

How to plant cosmos

How to grow cosmos (2)

Sow seed in early spring directly into the soil where you want your cosmos to grow, or into small pots or modules filled with free-draining seed compost. If growing in pots, pot on seedlings when they are large enough to handle. Plant out in late April/May after the danger of frost has passed.

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Watch Monty Don plant out cosmos for a late summer display:

How to care for cosmos

How to grow cosmos (3)

Once your seedlings have formed 2-3 pairs of leaves, you can pinch out the growing tips to produce bushier plants with more flowers.

Taller varieties may need staking. Otherwise all varieties will flower for a long period if deadheaded regularly and fed with a liquid fertiliser. When deadheading, cut the stem back to the first leaf beneath the flowerhead.

The perennial chocolate cosmos varieties will need winter protection. Place them in pots until they've finished flowering, then shelter over winter in a frost-free place until spring.

Here, Monty Don explains how deadheading spent flowers will keep them blooming for as long as possible:

How to propagate cosmos

How to grow cosmos (4)

You can collect seed from flowering cosmos and save for next year. Or if you're growing chocolate cosmos, you can divide the tubers when you lift the plants for storage over the winter months.

Growing cosmos: problem solving

How to grow cosmos (5)

Cosmos are pretty much trouble-free to grow, apart from the threat of slugs and snails when seedlings are young and tender.

Great cosmos varieties to grow

How to grow cosmos (6)

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Frequently asked questions

My cosmos are very tall and sturdy, so why do they have no flowers?

If your cosmos are tall, strong and bushy but without blooms, they may have been overfed with nitrogen-based fertiliser or grown in soil that is too fertile. This can cause cosmos to grow foliage at the expense of flowers. Cosmos flower best when planted in full sun and well-drained, average garden soil. Plants in shade will also struggle to flower. Pinching out cosmos plants should lead to more blooms.

When do I pinch out my cosmos?

The best time to pinch out the tops of cosmos seedlings is when they have grown three sets of true leaves (fern-like leaves). This will encourage plants to branch out and become bushier. It should also lead to more flower production.

Help! My cosmos are leggy. How do I prick them out?

Don’t sow cosmos too early and make sure they get plenty of light to avoid leggy growth. But if your cosmos seedlings have already become leggy, don’t worry. Just prick out and pot on into a slightly larger container, planting the seedlings a little deeper than they were. If you haven’t already pinched out the tops, do that now to encourage bushier growth.

How to grow cosmos (2024)
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