How to deadhead cosmos for seemingly endless flowers all summer long

Cosmos are avid bloomers, producing masses of flowers in a single season. To keep them blooming all summer long, though, you’ll need to know how to deadhead cosmos.

Learning how to grow cosmos is relatively straightforward, and you’ll get real bang for your buck if they flower properly. To make that happen, you’ll need to get your hands on a pair of snips and remove the spent flowers. It’s a busy task if you’ve got a prolific bloomer, but learning how to deadhead cosmos is one of the best things you can do for your plants at this time of the year.

Here’s how to get it right – because it’s not as simple as removing the flower alone…

Figuring out how to deadhead cosmos is pretty simple, but there's a thing or two you’ll need to know before you get started.

Deadheading involves pinching off spent flowers by hand or using a pair of snips to trim them away. I'm a huge fan of the Niwaki Garden Snips, £15.40, which you can buy from Amazon, for comfort and precision. Just make sure you know how to clean your garden tools properly between deadheading sessions – using unclean snips is one of the worst deadheading mistakes you can make!

It isn’t just the flower you need to think about when it comes to deadheading cosmos. ‘As well as removing the faded flowerhead, you should remove the top of the stem down to the next set of leaves,’ says Richard Barker, horticulture expert at LBS Horticulture.

So, what counts as the top of the stem? And where should we be making the cut?

‘Make a cut from the clump of leaves where the flower stem emerges,’ advises Richard. ‘You can find this by looking at the flower head, and then following the stem back to the first clump of leaves and cutting here.’

According to Richard, it can be tricky to tell the difference between a cosmos flower bud and a faded flower head, but buds usually have a more swollen and colourful appearance. Faded flowerheads, on the other hand, will look more washed out, and they’ll sometimes have a spiky stamen protruding from the top.

Just don’t cut the wrong part of the stem. ‘Take care and be gentle to avoid damaging the plant's stems, which could do more harm than good!’ warns Elise Harlock, flower expert at Prestige Flowers.

If you get it right, cosmos are just one of many plants you should deadhead now for a second bloom.

If you’ve got a few cosmos plants, or you’re overwhelmed with too many flowers on one plant, removing each flower and stem can be time-consuming. Luckily, there’s an easier way to deadhead cosmos in larger numbers.

‘If you are trying to deadhead an area with a large amount of flowers, you can use a pair of shears to cut away faded flowers and trim back the tops of plants at the same time,’ suggests Richard. The McGregor Garden Hand Shears from Argos are just £10, and highly rated.

This can actually promote better flowering, too. ‘Cutting back the foliage as well as removing the flowers can make your cosmos grow bushier and encourage the production of more flowers,’ Richard explains.

How often should you deadhead cosmos?

We’ve established that learning how to deadhead cosmos is key to keeping your plants flowering for as long as possible, but how often should we be doing it?

‘There’s no need to overdo this; however, it does need to be consistent, so try to repeat the process every few days,’ says Elise from Prestige Flowers.

Although they’re undemanding plants in other ways (they're one of the best flowering annuals that thrive on neglect), cosmos can flower prolifically, meaning you’ll have a constant task on your hands if you want to keep the displays up. It’s worth it if you’re hoping to get the most out of this summer’s blooms, though.

‘If you’re wanting to ensure that your garden looks fresh and that your cosmos are blooming fruitfully all season, then deadheading is absolutely something you should consider doing,’ agrees Elise.

And that's it! Learning how to deadhead cosmos is simple – just make sure you're removing the top of the stems, back to the next set of leaves, too.