With their delicate, daisy-like blooms, cosmos bring a relaxed, romantic feel to summer borders. They can flower for months, but once individual blooms begin to fade, a little attention can help keep the display going.
Whether you want to encourage another flush of flowers, tidy up leggy growth, or collect seeds for next year, these are the tasks worth doing after they have flowered.
1. Deadhead spent flowers
Encourage your cosmos to keep flowering by reguarly deadheading spent blooms (unless you want them to self-seed).
‘Regular deadheading can encourage it to produce new flowers,’ says Richard Barker, horticultural expert and commercial director of LBS Horticulture. ‘Any dead or fading flowerheads should be cut back to the next leaf joint or sideshoot that has a bud.’
This stops the plant from going to seed so it uses its energy to create new flowers instead. Doing this regularly prevents it from putting its energy into seed production, and instead encourages it to produce more blooms.
2. Cut back leggy plants
If your cosmos has become tall, sparse, or leggy after flowering, a harder cutback can help encourage a fresh flush of growth.
'If your cosmos looks leggy, cut the whole plant back by a third,' says Richard. 'This will briefly stop it from flowering, but it will then produce a flush of new growth and flowers.'
Rather than removing it completely, cut back the stems by around one-third, making your cuts just above a healthy set of leaves. Although you may have fewer flowers immediately after cutting back, the plant should soon produce fresh shoots.
3. Let cosmos go to seed
Deadheading isn't essential if you want to collect cosmos seeds or encourage the plant to self-seed.
'Alternatively, you can allow the plant to go to seed,' explains Richard. 'Leave the final flush of flowers in autumn, and allow the flowerheads to dry out on the stem.'
Once the flowerheads have dried completely, you can remove the seeds and store them somewhere cool and dry before sowing them the following year. If you'd rather let nature do the work, simply leave the dried seedheads on the plant and allow them to drop naturally.
4. Know what to do with chocolate cosmos
While many cosmos can be treated as annuals, chocolate cosmos need a little more care if you want to keep them from one year to the next. 'If you are growing chocolate cosmos, these will need to be lifted from the ground once their growing season ends,' says Richard.
Chocolate cosmos are more tender than common cosmos and their tubers can be damaged by frost, so it's important to protect them as temperatures fall. 'Container-grown plants can be moved inside to a garage, shed or greenhouse where temperatures remain above freezing,' explains Richard.
For plants growing directly in the ground, carefully dig around them and lift the tubers, taking care not to damage them with your garden fork or spade. Then, allow the tubers to dry thoroughly before storing them in trays of dry compost.
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