How to deadhead cosmos

Gardeners' World: Monty Don tidies up his Writing Garden

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Deadheading is standard practice for gardeners everywhere, playing an important role in the health of your garden. Focusing on spent blossoms to refocus the plant’s energy on its healthy blooms will keep it looking vibrant for longer, ensuring you get the most out of your plants. Deadheading the delicate cosmos flower is no different to most other summer-blooms, but what is the best way to do it?

Cosmos come in both annual and perennial varieties and are most commonly known as a relative of daisies and sunflowers as part of the upright-growing aster family of plants.

The two varieties can be characterised by their colour and their ability to grow back year-after-year.

Annuals have more feathery, fern-like leaves and can grow pink, red, orange, white and yellow flowers with a light floral scent.

The perennial cosmos is characterised by its chocolatey-scented dark red flowers.

Why should you deadhead cosmos flowers?

Deadheading cosmos is key to maintaining strong recurrent growth in perennial varieties and to get the most out of the short flowering of the annual variety.

The cosmos’ tall-genes means it is also important to deadhead in order to sustain a strong, straight stem which can grow up to six feet tall in some plants.

Avoiding ‘leggy’ growth of these evergreen stems and keeping spent blossoms to a minimum is the key to growing a healthy, vibrant pastel display of cosmos.

When should you deadhead?

Tending to your garden throughout the year will ensure that all of your plants receive the correct pruning at the right time, but when do cosmos need that TLC?

With annual varieties which flower from summer through to autumn, deadheading early on is key to get the most out of the short flowering period so you can really enjoy the beauty of these delicate stemmed flowers.

Deadhead your cosmos from the first flower through to the end of autumn to keep your plant flowering week after week until October.

Whilst the rich-red perennial cosmos will return the following year, make use of your annual varieties by taking cuttings and collecting seedlings to plant for another round of blooms the following summer.

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Cosmos seed when they stop flowering, so look out for wilted and faded petals on your cosmos as well as half-closed buds with loose pollen on as a tell-tale sign of spent blooms.

Use a pair of scissors to cut away the old flowers to promote new growth and preserve energy for healthy buds to guarantee a continual full-bloom.

Cut between the main stem and a leaf when deadheading spent flowers – being careful not to go too deep into the plant as this will delay the regrowth period.

To avoid being mistaken when deadheading your cosmos, use this number one rule as the clue to identifying spent blossoms:

A seeded, wilted bud means its time is up and should be cut – look for that loose yellow pollen here.
A closed compact coloured bud is ready to flower in the coming months and should be left to blossom – avoid deadheading unnecessarily.

Re-purpose spent blossoms

In his Gardener’s World video – ‘How to deadhead to prolong flowering’, Monty Don shares his tip for utilising the deadheaded flowers of the scented cosmos.

He recommends taking cut stems and placing in vases inside to make the most out of your deadheaded blooms, to boost your home with the unmissable chocolatey and floral scent of the cosmos.

Cutting slightly spent blooms early on means they are still in-tact for an added touch to your home, while stimulating new growth on the outdoor plant for weeks to come.

Save your annual cosmos seeds when deadheading pollinated buds and sow them the following spring. You can sow them in modules or in flower beds to brighten up your borders.

Cut the stem of fallen petals and hang it upside down to dry out, waiting for the seeds to turn black.

Remove the stem once the seeds have blackened and seal in a container or envelope for the spring – Cosmos seeds will grow in as little as 12 weeks so they’ll be ready for the summer.

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