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.
Alternatively, 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. See our guide to planting out container grown plants for more information.
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.
Care:
As the plants are getting established, check and water them regularly. If you’re growing cosmos in pots, expect to water them throughout the whole year. Pots, especially terracotta pots, will dry out much more quickly than the beds, so be prepared to water at least weekly unless there has been a really extended rainy spell. In the height of summer, you might need to water the pots once a day to make sure they don’t dry out. If you’re growing cosmos in beds, they should be okay for most of the Spring time (assuming there has been some rain) but will need watering in the Summer months as the soil dries out.
Once they start to produce flowers, make sure to deadhead the plants regularly. This will encourage more flowers to grow, and will also help to make bushier, sturdier plants. You can also cut flowers to bring inside. The more you cut, the more flowers the plant will produce!
Check for pests and diseases and treat as required. Cosmos can be prone to aphid infestations at the start of the growing season, though these tend to clear up on their own. If the infestation is causing a young plant to have too many distorted tips and stopping it growing properly, you can either spray with a horticultural soap to kill the aphids, or encourage predators such as ladybirds or lacewings to feast on the problem. Powdery mildew can also be a problem: a fungal disease that shows on the plants leaves as a grey, powdery substance. This can happen if cosmos is growing in too much shade, has been allowed to dry out too much, or doesn’t have adequate air flow. Make sure the plant is well watered and is receiving enough sunlight.
Annual cosmos (Cosmos bipinnatus) will die back at the very end of the growing season, around the time of the first frosts. At this point, cut back the plant and remove it completely – you can grow more next year…
Uses:
Great for cut flowers, cosmos will last about 7-10 days in water. Cut them just as the bud is about to bloom to maximise flowering time in the vase. They look great as a mass of one single variety together, or mixed with different colours. Mixing cosmos together with other cottage garden plants such as scabious, Ammi majus, echinacea and perhaps a few grass heads makes a really spectacular display.
Grow with:
Cosmos fits in with a huge range of other planting styles. You can grow it on its own in pots, either of one variety, or mixing a few together for different colours of flowers. It works well with other annuals, and has great impact grown en masse with plants like California poppy, nicotiana and coreopsis. It also works well filling any holes in a permanent border, especially among other cottage garden style plants.
Our favourite varieties:
Cosmos bipinnatus ‘Purity’ is just a must have! We’ve found it the most reliable of the white flowering cosmos. ‘Daydream’ is a stunning two-toned flower, with shades of deep pink in the centre, fading out to white at the edges. It looks wonderful grown on its own or mixed with other white and pink varieties. It was the very first flower we grew – filling buckets and buckets for a friend’s summer wedding, and starting our obsession with growing flowers for cutting! ‘Xanthos’ is a really unusual pale cream yellow coloured flower, which grows to a smaller height (around 60cm) so is great for growing in containers.
