Last updated on May 12th, 2022
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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. They will usually flower all summer long until the first frosts, and they make a sensational display of colour.
They are very quick and easy to grow, especially from seed, and you will start to produce flowers in as little as 12 weeks from germination. This 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. For most parts of the UK, this is usually around May.
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 because this makes them easier to pot on) or small pots, and seed compost or quality multi-purpose compost.
See our top 5 recommended multi-purpose composts in this review
Step 1
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.
Step 2
If you are using a seed tray (which we would personally 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 pot’s holes over the seeds to create a thin and even layer that is only a few millimeters thick.
Step 3
Ideally, keep them in a heated propagator that remains between 16°C and 25°C. If you don’t have a propagator, cover the pots with a clear plastic bag making sure they don’t come into contact with the leaves as they are growing and place on a warm windowsill or in a greenhouse.
Check out our reviews of the best propagators in this article we have written
Potting on Seedlings
Once they have sprouted and you see two sets of leaves, you can place them into small individual pots, 7cm 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 multi-purpose will be perfect.
Feeding
Once you have noticed the first buds appearing you should give them a liquid tomato fertiliser and during this time, feed time approximately once every two weeks.
Planting Cosmos in beds and borders
Before you plant your Cosmos, 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 you can add mulch around the base of the plant as this will conserve moisture in the roots because Cosmos like lots of water, but at the same time you need to be careful not to overwater them.
Watering and staking
During the growing season make sure to increase the frequency of watering based on the temperature. You may need to stake them as they get older to give them more support because some varieties can get very tell and sometimes need a little extra support.
Growing Cosmos in containers
If you are growing Cosmos in pots and containers, there are shorter varieties cultivated specifically for this purpose, including ‘Antiquity’ and the very popular Cosmos ‘Sonata’. You don’t need to worry as much about the material for your container as long as it’s a suitable size.
Choosing a suitable pot
The materials the pots are made from won’t affect them much, other than the weight of the plant. If this is the case and you’re using a very heavy pot and planting multiple Cosmos, you will be better off putting it in its permanent position beforehand so that you don’t have to move it across your garden once fully planted.
You will need to choose a container that is large enough to support the entire plant (or plants). If you are planting several in one pot you will need to make sure there is enough space to accomodate them all.
If you have purchased Cosmos from a nursery or garden centre, you will need to make sure the hole you have created 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 them too deeply because this can cause them to rot off.
Choosing the right compost for pots
We recommend using a good quality potting or multi-purpose compost and feeding them every two weeks once they produce buds. Cosmos that is being grown in pots will need watering more than Cosmos planted in the ground in beds and borders. They may need watering daily in warmer weather and when fully established.
Planting seedlings 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 going all the way through to the month of October when the first frosts usually kills them off.
Regular Care
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 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, as long as you store them away from direct sunlight.
