how to plant, grow & care for cosmos (2024)

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Cosmos seeds can be sown undercover in March or April. The seeds are long and thin, so can be easily handled.

Use seed trays filled with good compost or Jiffy modules. Push two seeds about 3mm deep into each module. Water well and then place in a greenhouse or on a warm windowsill to help germination. If both seeds germinate, remove one of the seedlings a week later and leave the other one to grow on in splendid isolation.

Pot the seedlings on into larger pots when they are big enough to handle. Pinch out the growing tip of each stem when 3 pairs of true leaves have grown to encourage stems to branch and produce more flowers. Then plant out in late May or June once the risk of frosts has passed.

Direct sow your cosmos seeds in May – the risk of frost should have passed by the time they emerge.

Prepare the soil before planting. Rake the patch until you have a fine, crumbly consistency. Space the seed about 5–8cm (2–3in) apart and water well.

About three weeks later, the seedlings should have popped up. At that point, you can thin them out to about 30cm (12in) apart. Pinch out the growing tip of each stem to encourage them to branch and produce more flowers.

Plant out cosmos seedlings after the risk of frost has passed in late May and June. They are happy planted in moderately poor soil.

Plant the cosmos in full sun and water well, then add a mulch to help conserve moisture.Stake and tie plants if necessary during the growing season and don’t forget to water regularly as they grow.

The shorter varieties of cosmos are perfect for pots and seedlings can be planted out from May. You can start by growing them from seed undercover in March-April, or you can buy cosmos seedlings.

Space the seedlings about 30cm (1ft) apart in good compost. Keep them well watered and they'll be in flower by early July.

The taller varieties of cosmos will need staking and you can do this in one of two ways.

You can stake each individual plant with a hazel or bamboo cane when it reaches about 60cm (2ft), using flexi-tie or string to tie it in about 30cm (1ft) off the ground.

Try to tie in every single one of the stems, because otherwise the outer limbs get broken off when it's windy. It may seem a bit harsh, but you won’t notice the cane within a couple of weeks as the leaves will grow over it.

The other way of staking cosmos is by using pea or jute netting. You simply stretch the net horizontally over your young cosmos plants and between bamboo or hazel canes to support it. The cosmos will then grow up and through the grid and are lightly supported. It doesn’t look great at first, but new leaves and growth will soon cover it.

With the taller cosmos, it’s a good idea to add a second layer of netting – the first layer at about 30cm (1ft), and the second at 60cm (2ft) with ultimate height of 90cm-120cm (3–4ft).

Deadhead any spent cosmos flowers and the plant will keep flowering reliably until October or until the first frosts. Better yet, pick your cosmos and enjoy the cut flowers indoors.

To deadhead cosmos, follow the stem down from the flower and cut above the first leaves that you meet, rather than just taking the flower head off. That way, you will promote bud formation encouraging new flowers to emerge.

If you’re picking, you may want a slightly longer stem than that. So go lower down to the next main stem leaf. The lower you go in the plant, the longer it will take for the next flower to emerge.

Most cosmos are annuals and will need to be resown from seed or planted as seedlings each year.

Cosmos atrosanguineus, or chocolate cosmos, is the exception – it is a tender perennial. It’s ideal for container gardens as the pots can be brought undercover over winter. Alternatively, you can protect the plant from the cold with horticultural fleece or straw.

Yes, cosmos can get powdery mildew, which is a fungal disease that shows up as a powdery coating on the foliage. You can remove any affected leaves and try an organic solution such as homemade comfrey tonic to keep mildew at bay.

The best way to avoid powdery mildew in the first place is to ensure the plants are well spaced and not too crowded, and that you water your cosmos plants regularly (but do not allow the soil to get soggy).

Cosmos can suffer with what’s known as fusarium wilt, which is a fungal disease. Fusarium wilt causes discoloration of the stems and foliage, as well as stunted growth.

If your cosmos has fusarium wilt, lift out the plant from the roots and dispose of it. The soil can also be affected, so you can either replace it with fresh soil, or avoid planting in that area for a few years.

Some of the newly bred varieties of cosmos also have a tendency to rust if sown too late. They benefit from March sowing so they have big beefy roots before planting out.

As with powdery mildew, good spacing between plants and regular (but not over) watering can minimise the risk of fungal diseases.

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