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How to plant, grow & care for cosmos

How to plant, grow & care for Cosmos

Even if you only have a tiny garden – or just some pots – you have to grow at least one cosmos. They are the lowest maintenance, floweriest plants in the world. For me, cosmos is the classic cut flower and a supremely lovely garden plant. In reward for very little effort, they give you buckets of cut flowers – and they have a very good vase life too. I love to grow lots of different varieties putting together different heights and colours in pots and borders. I hope you enjoy exploring our wide selection of cosmos seeds and seedlings.

Written by: Sarah Raven

Last updated: 4th Mar 2025

DetailsWhere & when to plantHow to sowHow to plantCare guidePests, diseases & common issuesFrequently-asked-questions
Details
Calendar
Jan J
Feb F
Mar M
Apr A
May M
Jun J
Jul J
Aug A
Sep S
Oct O
Nov N
Dec D
sow / Seed
Planting
Flowering
Seasonal checklist
Spring
  • Sow cosmos seeds undercover in early spring.
  • Sow directly outdoors in late spring.
Summer
  • Plant out cosmos seedlings in the ground or in pots.
  • Pick cosmos flowers through the summer.
Autumn
  • Continue to deadhead and pick your cosmos, which will help them flower reliably until October.
Winter
  • Plan which cosmos varieties you’d like to grow next year.

Where & when to plant

Where to plant

Soil type

Cosmos need good, well-drained soil to thrive.

Aspect & position

Cosmos need a position in full sun to flower productively

When to plant

sow cosmos seeds undercover in March-April, or direct sow in May. Plant out seedlings once the frosts have passed in late May or June.

How to sow

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. 

Photography by 
Jonathan Buckley
How to plant

For the best results, we recommend potting these on into 9cm pots (or similar), before planting in their final position outside.

Step 1

Choose a good quality, multi-purpose, peat-free compost and mix with a small amount of horticultural grit to ensure good drainage.

Step 2

Fill a clean 9cm pot with the mix and in the middle of the pot make a hole slightly deeper and wider than the seedling.

Step 3

Position the seedling in the hole, ensuring the roots are fully covered and that the seedling is firmly placed, tapping to settle the soil. Top tip – always handle seedlings by the leaf, not the stem so you don’t damage the lifeline of the plant.

Step 4

Water well, and leave somewhere bright but sheltered during the day

Step 5

Cover with horticultural fleece overnight or bring undercover if you have a greenhouse, cold frame, or conservatory.

Step 6

Allow your seedlings to grow on for a few weeks. Frost-hardy plants (i.e., hardy annuals, biennials, and perennials) will be ready to go into the garden once the roots have filled the pot (check by looking at the bottom). Frost-tender plants (i.e., half-hardy annuals and tender perennials), should be planted outside after the last frosts, usually around mid to late May.

Photography by 
Jonathan Buckley
Care guide

Watering

Water cosmos regularlarly, particularly in a drought.

Fertilising

Feed cosmos in pots with a liquid fertiliser every couple of weeks throughout the summer. There’s no need to feed cosmos growing in borders.

Staking

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).

Deadheading

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.

Propagating

At the end of the season, collect seed from flowering cosmos and save it to sow the following spring. The seedlings won’t necessarily be true to the parent plant.

Overwintering

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.

Pests, diseases & common issues

Earwigs

Earwigs can eat cosmos flowers, but they are also useful in keeping numbers of fruit aphids down, so tolerate them if you can rather than resort to chemical control.

Slugs & snails

Very young cosmos plants and seedlings are very attractive to slugs and snails, so think about protecting the plants early in the season with slug barriers.

Why are my cosmos not flowering?

Cosmos are sun-loving plants, so any cosmos sown or grown in shade will have trouble flowering. Plants that have been overfed may also produce abundant foliage at the expense of flowers.

Why are my cosmos seedlings leggy?

The germinated seeds need enough light to grow strong and bushy. A seedling straining for light may grow leggy. When transplanting seedlings (either potting on or planting outdoors in pots and borders), I advise pinching out the growing tip of each stem to encourage stems to branch and produce more cosmos flowers.

Can cosmos get mildew?

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).

Why are my cosmos discoloured and dying?

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.

Photography by 
Jonathan Buckley

Frequently Asked Questions

  • Are cosmos perennials?

    Most cosmos varieties are annuals, which means they won’t return year after year. You can collect the seed and sow new cosmos each spring. 

    Cosmos atrosanguineus, or chocolate cosmos, is a tender perennial and will return each year if it is given protection from the winter cold. Bring pots of chocolate cosmos undercover over winter. Alternatively, protect the plant from the cold with horticultural fleece or straw.

    If you are growing chocolate cosmos, you can also lift, divide and replant the tubers to propagate.

  • When do cosmos flower?

    Cosmos will flower from July until October or the first frost if you ensure they are in a sunny spot, regularly deadheaded or picked, and fed and watered well.

  • How tall do cosmos grow?

    Cosmos can grow over 1m (3ft) tall, but there are compact varieties that reach around 60cm (2ft).

  • Do cosmos need full sun?

    Yes, cosmos need full sun to flower productively.

  • Can you sow cosmos in autumn?

    No, cosmos is a half hardy annual so should be sown in spring.

  • Do cosmos self seed?

    Yes, cosmos may self-sow, but these seeds will germinate very late in the season and may only produce very short-lived plants. You do sometimes get seeds that have overwintered in the ground germinating the following year, but they are unlikely to be true to the parent plant.

  • Do cosmos need staking?

    Yes, it’s important to stake cosmos, particularly if you are growing a tall variety, and there are a couple of simple ways to do this.

How to cut and arrange Cosmos

Cosmos make fantastic cut flowers that last 7-10 days in a vase, with no conditioning needed. Pick fresh and place straight into water. Just remember not to pick in the heat of the day.

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