Plant guide: Cosmos
Latin name:
Cosmos Bipinnatus
The cheerful cosmos is, for us, a must-have plant in the garden. An easy-to-grow annual, with round flowerheads dancing on thin stems above fine, feathery foliage, cosmos is a cottage garden favourite that never seems to go out of fashion. It’s a great introductory plant to try if you’re new to growing your own flowers; a single pack of seed can reliably produce lots of tall impressive plants that give pops of colour in a late Summer border, or for cutting and filling the house with vases full of blooms from July all the way through to Autumn.
Plant type:
A half hardy annual – you can easily grow cosmos from seed in the Springtime, and it will flower later in the same year. However, this does also mean you need to regrow / replant it each year as well, as it will die completely once the first frosts of Winter arrive.
Growing conditions:
Cosmos like to grow in full sun, so this is one for a sunny bed or pots in bright sunlight. (West or South facing beds would be ideal.) It’s happy in most soils, as long as they’re not waterlogged.
Cosmos can reach heights of more than 1.5m, and grow to up 0.5m wide, so allow them plenty of space. This can sometimes be hard to believe, as you plant out little seedlings in the late Spring, but these are quick growers and they put on a lot of height and bulk in the space of the Summer months, so plan this out at the very beginning! As more cosmos varieties are being introduced, you can increasingly find smaller “dwarf” types, so if space is an issue then look out for these – they’re especially good for growing in pots.
How to grow:
This is one of the simplest (and most cost effective) plants to grow for maximum effect. It’s really easy to grow from seed, or you can buy plug plants in the Spring from most nurseries and garden centres. Growing from seed gives you the best choice of varieties: you can get cosmos in a range of flower colours from white to pink, purple and yellow.
To get bigger well-established plants more quickly, sow seeds indoors in March. Sprinkle seeds thinly over a seed tray filled with moist compost, or place individually in a modular seed cell tray. Cover with a fine layer of compost or vermiculite. The ideal temperature for germination is 21°C: so keep the trays inside the house or in a heated greenhouse.
See our guide to growing seeds for more detailed information on seed sowing. Germination usually takes 1-2 weeks. Once the seedlings have germinated, ensure they have plenty of light and good air circulation. Water them to keep the compost moist, but not dripping wet.
When the seedlings have two pairs of leaves, prick them out and grow in individual small pots, keeping them inside until around May time, once the last frosts have passed.
After the last frosts, harden them off, and plant in the garden, or large containers outside, in their final position. Allow about 30cm of space between each plant in a border. You can space a bit closer together in a container, where the plants will not grow quite as tall.
When you plant them out, pinch off the very tip of the plant. This encourages them to grow bushier and wider – rather than very tall and thin – and also leads to more flowers. If you don’t have the time, or space, to grow seeds inside first, they can be sown directly where they are going to grow in the border or in pots once the last frost have passed, usually around April or May time. These will be slower to establish and probably later to flower than seeds sown inside, but they can keep flowering right up until the Autumn months, so this can still be a great option.
Finally, you can buy plugs from nurseries or garden centres in Spring time, which will be well-established bushy plants. Plant these straight into their final position, again waiting until the last frosts have passed. Dig a hole as deep as the pot is tall and a bit wider, remove the plant from the pot and place it inside, and back fill the hole. Gently firm into position and water well.
Cosmos prefer less rich soils, so there is no need to add any organic matter. Soils that are high in fertility (those with plenty of nutrients) can lead to plants that grow very bushy and green at the expense of producing flowers.
