Cosmos have big beautiful daisy-like flowers. They are easy to grow and they are a great addition to any garden. Grow them in your borders, in a cutting patch and even in pots. There’s a Cosmos suitable for most places!

How to Grow Cosmos

Sowing Cosmos Seeds Indoors

Cosmos seeds are large, easy to handle and quick to germinate. Sow them in modules in March and April with nice fresh compost. They must be sown indoors at this time of year so they are protected from cold and wet weather. Make sure the seeds are covered with 2mm of compost then water your module tray from below. Allow excess water to drain away then place the seed tray in a warm spot to encourage germination.

Cosmos seeds ideally need a temperature of 16-21°C. They can take up to 30 days to germinate but I find that placing them in my dark warm boiler room speeds the process up and the seedlings are sprouting within a week. Once germination has occurred seedlings can cope with much lower temperatures. Move the module tray to a nice light place such as a greenhouse, poly tunnel or cold frame to allow the seedlings to grow on in the sheltered environment for a few weeks.

Cosmos in the Garden

Cosmos seeds can be sown directly in their flowering positions when the soil has warmed up from May onwards. Choose a sunny planting spot then prepare the ground by adding organic material such as garden compost or well rotted manure. Work the soil into a seed bed by breaking down any large clumps with a rake. Sow the seed thinly in rows taking care to label each line and variety. Sowing in rows means you’ll easily be able to spot and remove any weed seedlings. Thin your Cosmos plants out when the seedlings are well developed. Seedlings transplant well so you could do what I do and transfer some to large pots (3 to a pot) to fill any gaps in your borders later on. You can do this even with the tall varieties too… they are great for placing in a gap at the back of a bed.

Cosmos plants are really easy to grow but watch out for slugs and snails, particularly when your plants are young and tender. They love the juicy young foliage and can quickly demolish your seedlings so be vigilant and remove slugs and snails when you see them. A really easy way to gather up these pests is to provide hiding places for them. I use wilted foliage from other plants to attract them away from my Cosmos and halved and squeezed oranges or grapefruit for them to hide under. It’s easy to collect and remove them each evening or morning.

May is also the time to plant out Cosmos plants raised indoors. They need plenty of space to grow and develop so allow 45cm/18 inches between the taller varieties and 30cm/12 inches between the shorter varieties. Water them well and feed each week to promote growth. When the young plants reach 20cm/8 inches tall pinch out the growing tips to encourage the stems to send out side shoots.

Tall Cosmos varieties such as Purity and Sensation will need support of some kind. If you are growing them in blocks then pea and bean netting stretched horizontally will provide invisible support.

Shorter varieties such as Sonata and Antiquity don’t need support. The pictures on the left show Cosmos Sonata.
They will support each other and are brilliant for mass bedding and container planting.

Flower Production

From a spring sowing Cosmos take approximately 14 weeks from seed to flower. In their native Mexico Cosmos are perennial plants and will carry on growing all year round. Here in the UK we treat Cosmos as Half Hardy Annuals. They don’t like the cold early in the year but Cosmos plants will flower from June until the first frosts in October as long as you remember to water, feed and deadhead them each week. If you allow them to set seed or go hungry and thirsty then they will stop producing flowers.

Cut Flowers

Cosmos flowers are brilliant for cutting to add a splash of colour to your home and make lovely gifts for friends. Cut your Cosmos flowers when it’s cool… either early in the morning or in the evening and pop them straight into a tall clean bucket of fresh water. Cosmos will last 7 -10 days in a vase and combine beautifully with other tall plants such as Ammi, Malva, Nicotiana and Cornflowers.

I hope this information has helped you to understand how easy it can be to grow Cosmos flowers from seed.
I wouldn’t be without them in my garden. I hope you enjoy them too!   Gillian 🙂