Sowing, growing and caring for Cosmos plants

Cosmos is a versatile and easy-to-grow flower that can add colour and vibrancy to any garden. Cosmos has become one of my favourite flowers to grow from seed. Its bright and colourful flowers bring much-needed interest to the garden during late summer and autumn. Since I love Cosmos so much, I wanted to create a quick guide on how to sow, grow and care for them so you can have stunning flowers as the days start to draw in.

In this post, I’ll cover the following:

  • Why Cosmos is a great flower for the garden
  • How to sow Cosmos seeds
  • How to care for Cosmos seedlings
  • Common problems with Cosmos seedlings, and
  • Where to plant Cosmos flowers in the garden.

Why Cosmos is a Great Flower for the Garden

Cosmos is a low-maintenance flower that can thrive in various growing conditions, making it a great choice for gardeners of all experience levels.

Cosmos has a wide range of varieties; some are annuals, others perennial. They have feathery leaves which can help fill out flower borders during the summer months and flower prolifically. Cosmos come in many colours, including pink, white, red, and orange, and can grow up to six feet tall. Cosmos also attracts pollinators like bees and butterflies to the garden. Plus, its long blooming season makes it a reliable source of colour throughout the summer and autumn.

How to Sow Cosmos Seeds

Sowing Cosmos seeds is relatively easy and can be done indoors or outdoors.

Hardey annual varieties can be sown outside in spring, March to April, or autumn, September to October, due to being frost resistant. If you sow your seeds in autumn, they will flower earlier the following year. Tender annual varieties do better when sown indoors in spring and planted after the last frost.

Cosmos seeds are easily sourced from reputable online seed suppliers and garden centres. Sow Cosmos seeds in small pots or trays filled with peat-free potting compost. Keep the soil moist but not waterlogged, and place the pots in a warm, sunny location.

If you’re sowing cosmos seeds directly into the ground, wait until after the last frost date in your area. Prepare the soil by removing any weeds or debris, and rake until it’s loose and fine. Then, scatter the cosmos seeds over the soil and lightly cover them with a thin layer of soil or compost.

Cosmos Seedlings

How to Care for Cosmos Seedlings

Once your Cosmos seedlings have germinated, providing them with proper care is important to ensure they grow strong and healthy.

Water the Cosmos seedlings regularly, but be careful not to overwater them, as this can cause the seedlings to rot. If the soil is dry to the touch, it’s time to water.

As the seedlings grow, they will benefit from plant feed every two to three weeks. I have found that half-strength organic plant feed is sufficient to help the seedlings grow. Alternatively, a slow-release fertiliser added to compost when potting on seedlings can give them an extra boost.

Pinch back the tips of the seedlings once they reach a height of 4-6 inches to encourage bushy growth.

Common Problems with Cosmos Seedlings

One common problem with Cosmos seedlings is that they can become leggy and grow tall and thin with few leaves. This can happen if the seedlings don’t get enough light or are crowded together.

To prevent leggy seedlings, provide them with plenty of light by placing them in a sunny window or using grow lights. If the seedlings are crowded, thin them out by removing the weaker ones.

Another problem that can affect Cosmos seedlings is damping off, a fungal disease that causes the seedlings to wilt and die.

To prevent damping off, use clean, sterile potting compost and avoid overwatering your seedlings. Water your Cosmos seedlings at soil level, avoiding wetting the foliage.

How to Pot on Cosmos Seedlings

If you started your Cosmos seeds in small pots or trays, you will need to pot them on into larger containers before planting them outdoors. This is especially important if the seedlings are growing too large for their current container or if the weather outside is not warm enough to be planted.

To pot on your Cosmos seedlings:

  • Prepare a new larger pot with fresh peat-free compost
  • Use a trowel, dibber or your hands to make a hole in the new compost
  • Gently lift them out of their current container, but be careful not to damage their delicate roots
  • Place the seedling in the hole, ensuring the top of the root ball is level with the soil’s surface
  • Fill in the gaps with more compost, and
  • Firm it down gently with your fingers.

Water the newly potted seedlings well, and place them in a warm, sunny location. Continue to care for them as you would any other Cosmos seedlings, watering them regularly and fertilising them every few weeks.

Potting on Cosmos seedlings will give them more space to grow and develop strong roots, which will help them thrive once they’re planted outdoors. Just wait until after the last frost date in your area before transplanting your cosmos seedlings outside.

Where to Plant Cosmos Flowers in the Garden

Cosmos can be planted out after the last frost has passed; in the UK, this is usually around May and June and can be planted out in relatively poor soil in a flower bed or into pots.

Cosmos flowers prefer full sun and ideally be planted in a location with at least six hours of direct sunlight daily. They can be planted in almost any soil type, as long as it’s well-draining. Cosmos can be an excellent filler plant and look great in mass plantings.

Comos Flowers

Ongoing Care of Cosmos Plants

Once your cosmos plants are established and blooming, they will require ongoing care to keep them healthy and produce flowers throughout the growing season. Here are a few tips for caring for adult Cosmos plants:

  1. Watering: Cosmos plants prefer to be kept consistently moist but not waterlogged. Water them deeply once a week or more often during hot, dry weather.
  2. Feeding: Cosmos plants benefit from regular feeding with a balanced fertiliser every two to three weeks if they are in pots. This will help to promote healthy growth and abundant flowering. Cosmos flowers in borders do not need additional feed, but a feed high in potassium, such as tomato feed, can help increase the number of flowers a Cosmos plant will produce.
  3. Deadheading: To keep your Cosmos plants flowering throughout the season, it’s important to deadhead them regularly. This involves removing spent flowers as soon as they fade or wilt. Use sharp scissors or garden shears to cut off the flower stem just above the first set of leaves.
  4. Supporting: If your Cosmos plants become tall, you may need to support them with stakes or a trellis to keep them from falling over. You can also pinch back the tips of the stems to encourage bushier growth and more branching.


Sowing Cosmos Seeds

  • Sow hardy, semi-hardy and tender Cosmos varieties in March & April undercover.
  • Sow directly late spring after.

Care of Seedlings

  • Pot seedlings on into larger pots when their true leaves show
  • Keep seedlings watered but prevent them from becoming too damp
  • Provide plenty of light to prevent seedlings from becoming leggy
  • Feed seedlings with half-strength organic plant feed every two to three weeks.

Planting Cosmos

  • Plant out into flower borders or pots after the last frost
  • Plant in sunny locations.

Ongoing care of Cosmos

  • Water regularly, especially during drought
  • Feed every two to three weeks if planted in a pot
  • Deadhead to keep flowers coming
  • Provide support if the plant grows tall.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.