April is the perfect month to turn your attention to cosmos. In summer, they will reward you with the most romantic and timeless display, but now that the soil is warming and the days are getting longer, it’s time to sow.
Growing cosmos is straightforward, and whether you have already started your seeds off in trays or sow them directly, minimal intervention is required to stimulate strong growth, but there are some tips that will make all the difference.
It’s a rewarding spring job, and if you follow the correct steps, you’ll have bounteous summer growth and long-lasting colour throughout the season.
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April is a month of unpredictability in the weather, one day bright sunshine, the next lashing rain. If you want to get ahead with sowing cosmos but without risking the cold, you can sow them indoors.
Use small pots or seed trays, like the WY8 seed tray starter kit from Amazon, and fill them with a light compost that is free-draining, such as the John Innes seed sowing compost from Amazon. Sow the seeds thinly on the surface and cover them with a light layer of soil.
They should have sunlight and be protected from frost, in either a greenhouse or on a windowsill. I like the Haws indoor watering can to keep them gently watered and the compost moist.
Within a couple of weeks, it’s a cheering sight to see the seedlings appear from the soil, and once temperatures are more consistent, you can take these stronger seedlings and move them outside.
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If seedlings are deprived of light, they will become leggy and weak as they try to reach a light source. A sunny windowsill or greenhouse can help them see the most daylight, and you can turn containers to ensure no seedling misses out. Remove weaker seedlings to ensure they are not overcrowded, as that can also affect how much sunlight is received.
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When your seedlings reach between 15-20cm and have a few sets of leaves, you can pinch them out using your fingers, scissors or plant snips to remove the growing tip above a leaf node. What this does is encourage the plant to branch out from the sides rather than growing a tall stem, which means you’ll have more abundant flowering in the summer (very helpful for those who are using cosmos for cut flowers).
Pinching out can also help you to prevent future floppiness, too, as multi-stemmed plants are typically better at supporting themselves.
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If conditions outside are mild, hardening off can begin in late April. Hardening off is where you allow the cosmos to become accustomed to the cooler temperatures outdoors by popping them outside in a sheltered spot for a few hours each day, but then bring them back in at night.
The time they spend outdoors can increase over two weeks, so they are exposed to more sun each day, but if it’s pouring down with rain or the winds are very strong, keep them indoors for the day, as this will cause damage to growth.
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If the risk of frost has passed, you can sow cosmos directly into the ground. Choose a sunny spot and remove weeds from the soil using a Burgon & Ball weeding fork available from Amazon, breaking the soil up so it’s easier to sow. Scatter the seeds and cover them lightly with soil, and then finish by giving them a light watering. Monitor the forecast, and if it’s going to be dry, make sure to continue watering to help them germinate.
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Cosmos are not huge fans of overly rich soil, so avoid improving the soil with compost or fertiliser. Light soil will help their roots to spread. Too much fertile soil can result in lots of leafy growth, rather than the beautiful flowers you’re aiming for. Grit or sand from Westland can be added to heavy soil to help improve its drainage.
Your April efforts will pay dividends in the summer, when these elegant flowers emerge to dazzle. If you’re wondering which varieties to sow, my favourites are the deep pink Cosmos bipinnatus ‘Antiquity’, richly red Cosmos bipinnatus ‘Rubenza’ and the bright white Cosmos bipinnatus ‘Purity’, all available from Crocus.
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