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Cosmos are half-hardy annuals known for their ability to flower, produce seeds, and die all in just one growing season but they will usually flower all summer until the first frost and they make a sensational display of colour.
They are very quick to grow especially from seed and you will start to produce flowers in as little as 12 weeks from germination which means you can add a burst of colour to border gardens or containers no matter where you live.
The one thing to be cautious of is the fact that these plants cannot handle the cold so be sure not to put them outdoors until the risk of frost has passed which is usually around May for most parts of the Uk.
How to grow cosmos from seed
If you want to grow Cosmos from seed you will need the seeds, a seed tray, ideally with separate modules which makes them easier to pot on or small pots, and seed compost or quality multi-purpose compost.
Fill the seed tray or small pots with your compost and lightly tamp it down by tapping the trap a couple of times, you could also use the back of your hand being careful not to do it too hard.
If you are using a seed tray which we recommend rather than using single pots, put one seed per cell and if you are using pots, put one seed per pot. They should be placed on the surface of the compost and covered with a very fine layer of compost or vermiculite and then watered thoroughly. A tip when doing this is to use a plastic plant pot and gently sieve the compost or vermiculite through the pots holes over the seeds to get a thin even layer only a few mm thick.
Ideally, keep them in a heated propagator that remains between 18 degrees C (25 degrees Celsius). If you don’t have a propagator, cover the pots with a clear plastic bag but make sure they don’t come into contact with the leaves as they are growing and place on a warm windowsill or in a greenhouse.
Potting on seedling
Once they have sprouted and you see two sets of leaves, you can place them into small individual pots, 7 or 9cm pots are ideal and grow them on for a few weeks until they are well rooted and ready for planting out.
Hardening off young plants
Next, you need to harden them off on your patio putting them outdoors during the day and bring them back indoors if cold weather is forecasted.
Once the risk of frost has passed which is usually sometimes in May and you have hardened them off you can put them directly in the ground or into containers using good quality compost, again multipurpose will be perfect.
During this time you should give them a liquid tomato fertilizer once you notice the first buds appearing and feed approximately once every two weeks.
Planting in beds and borders
When you do this you want to improve the soil with garden compost just to make sure it is nutrient-rich. In order to encourage bushier plants, you should pinch off the growing tip for every stem when you plant them in the ground.
Again make sure you give them an area that has access to full sun and water them well after you plant them. It will help the overall production if you add mulch around the base of the plant to conserve moisture in the roots as they like lots of water but at the same time be careful not to overwater.
Watering and staking
During the growing season make sure to increase the frequency of watering based on the temperature and stake as they get older and need more support as some varieties can get very tell and sometimes need a little support.
Growing cosmos in containers
If you are growing them in containers, there are shorter varieties designed for this purpose including Antiquity and the popular cosmos Sonata. You don’t have to worry as much about the material for your container so long as it’s a suitable size.
Choosing a suitable pot
The materials won’t affect much other than the weight of the plant in which case if you’re getting a very heavy pot and planting multiple Cosmos, you will be better off putting it in its final resting place so that you don’t have to move it across your garden.
You will need to choose a container that is large enough to support the entire plant or plants as you may plant several in one pot and if you purchase the plant from a nursery, make sure the hole is deep enough that you can place the cosmos at a depth that is the same as the container in which they came. Be careful not to plant too deep as this can cause them to rot off.
Choosing the right compost for pots
We we recommend using a good quality potting compost or multipurpose compost and feeding every two weeks once they produce buds. Cosmos grown in pots will need watering more than ones ground in beds and borders and may need watering daily in warmer weather and when established.
Planting seedling in pots
If you are planting seedlings be sure to place them at least 20cm apart and keep them well watered. If you do this early in the season, they will flower by the beginning of July and if you deadhead them regularly you can keep flowers reliably all the way through the month of October when the first frost usually kills them off.
After you have planted your Cosmos, they will flower well until the first frost date if you deadhead spent flowers regularly. Giving them food on a regular basis and deadheading them right back to the first leaf below the flower head rather than just removing the flower head will ensure you can enjoy the beautiful flowers for much longer.
Using as cut flowers
These flowers are so stunning that they make for delightful floral arrangements and you can cut them right off the plant, put them in water right before the buds bloom, and they will remain for 7 to 10 days indoors so long as you store them away from direct sunlight.