Cosmos are popular garden flowers – for good reason. They're not only nice to look at but also affordable, easy to grow and a boon to pollinators like bees and butterflies thanks to their wide-open flowerheads.
If you're not already growing these pretty blooms, now is the ideal time to get going. The increasingly warm weather in April and May provides the perfect conditions for sowing seeds outdoors, where they germinate quickly, filling beds and borders by summer.
"Cosmos are really popular plants in the UK, largely because they’re so easy to grow and give a lot back for very little effort," says Chris Bonnet, plant expert and founder of online garden centre Gardening Express. "You can grow them from seed, they germinate quickly and they don't need much fuss or special care."
Cosmos are perennials, so if you plant them right, you get to enjoy vibrant, humming borders as they come back year after year. We've consulted some of our favourite garden experts to help you achieve just that.
Keep reading for everything you need to know about growing cosmos, from sowing the seeds to cutting the flowers to encourage regrowth.
1. Sowing the seeds
Cosmos seeds can be sown as early as late March under cover in modules or seed trays. From mid-April, when the last frost has passed and days are getting warmer, they can be planted directly in the ground.
Pick a sunny spot (these native South American blooms don't enjoy shade) and make sure the soil is well-draining. "If the soil is too rich, they tend to produce lots of leafy growth but fewer flowers – it's worth keeping that in mind," says Chris.
Julian Palphramand, head of plants at British Garden Centres, advises sowing them thinly at around 1cm deep and watering gently. From there, they will germinate quickly, typically within 7 to 10 days.
2. Nurturing the seeds and seedlings
Fully grown cosmos are reasonably drought-tolerant and don't need much water. As long as they're still establishing, however, it's recommended to keep the soil slightly moist.
When the seedlings begin to emerge, it's time to give them room to grow. Chris suggests thinning them, so they're spaced out about 30 to 45cm apart, which prevents overcrowding, improves air circulation and helps protect against issues like powdery mildew.
3. Encouraging better growth
To help your cosmos become the best version of themselves, Chris recommends removing weaker plants to give stronger ones an even better chance to grow dense and bushy.
Do this when the seedlings are a few centimetres tall and have two to three sets of leaves. Snip off leggy seedlings at soil level with scissors (don't pull them out as this can disturb roots of neighbouring ones), keeping only the sturdy, upright ones.
"It can feel a bit harsh at first, but thinning really does pay off – the remaining plants grow bigger, bushier and produce far more flowers," Chris says.
Then, when the seedlings reach 20 to 25cm tall, Julian advises pinching out the growing tips, using your thumb and index finger. "It feels counterintuitive, but this single step transforms the plant, so instead of racing upward on one spindly stem, it branches out and produces far more flowers," he explains.
Limit pinching to once or twice per plant and remove no more than 2 to 5cm; otherwise, you'll delay flowering.
4. Caring for comsos
"Cosmos are meadow plants at heart, and they thrive on a little neglect," says Julian. This means that, once they're fully grown, these vibrant blooms are extremely low-maintenance.
During dry spells or heatwaves, resist the temptation to water frequently and give them one good soak once a week. "The key is less often, but more deeply," says Chris. If you're growing your cosmos in containers, water every few days in hot weather.
"Just remember that cosmos actually prefer it slightly on the dry side, so if you’re unsure, it’s better to underwater than overwater," he adds.
5. Cutting and deadheading
"Cosmos are classic cut-and-come-again plants, and the more you harvest or use as cutting flowers, the more they bloom, right through to autumn," Julian tells us. The same goes for deadheading – simply snip off spent blooms and watch them regrow all season long.
To make sure they come back year after year with little to no effort, allow a few late stems to run to seed and drop naturally. This will help them self-sow freely, giving you another season of colour the following year. Yes, it really is that easy.
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Wanda Sachs is the Multiplatform Writer for Country Living and House Beautiful, exploring the latest in gardening, wildlife and sustainable living alongside interiors and property. She is particularly interested in human-interest stories, the intersection of design and pop culture, and the evolving relationship between urban and rural life. Previously, she served as Associate Editor at The Berliner in Berlin, where she reported on culture and fashion. Wanda studied English and German at the Goethe University Frankfurt and Exeter University.
