Monty Don's top tips for growing cosmos – and the one mistake to avoid

Find out how to grow this beautiful bloom which is perfect for cut-flowers

Monty Don’s top tips for growing cosmos
(Image credit: Unsplash Mathew Schwartz)

Cosmos is a true garden favorite. Coming in a wide range of colors, they add instant wow-factor to the border and pack a punch in a pot.

As if their beauty wasn't enough to make you fall in love with these beautiful blooms, their pollen-rich flowers are a favorite with wildlife.  Bees especially like cosmos making it a great addition to the garden, as it supports this vital pollinator.

Cosmos is best grown from seed and can be sowed any time throughout spring. 'The beauty of growing cosmos from seed, is that you can get so many' says Monty Don in a recent Gardener's World video. This makes it a really easy and affordable way to fill your borders with a stunning floral display come summer. 

Monty Don’s top tips for growing cosmos

(Image credit: Getty Images)

How to grow cosmos

When it comes to growing cosmos from seed you have two options. You can grow them in modules in a greenhouse or plant them directly out in the garden. There are benefits to both. 

Growing cosmos in modules gives you more control over your blooms. Keeping them off the ground and undercover will also protect the delicate seedlings from slugs and harsh weather. Plus, since you are starting your plants off indoors (whether that's in a greenhouse or on a windowsill), you are able to start planting earlier (in early spring) so will have a well established, flowering plant by early summer. 

See: Monty Don’s tips on getting the perfect lawn – and the mistakes you're making

If you are starting your cosmos in the greenhouse, then it is essential to harden them off. Thankfully, hardening off is very important and very easy. Simply put your plants in a sheltered place for a week or two to adjust to the temperature variation in your garden. It 'means much faster growing and longer-lasting flowers' says Monty Don on his popular blog. He also recommends doing the same for young cosmos that are bought in the garden centre too. 

Direct planting is a low-maintenance choice that creates a beautiful cottage garden aesthetic. 'I place them in groups so they make drifts and clumps rather than straight lines.' continues Monty.

Simply rake the ground and sprinkle the seeds directly in the bed. You can then thin these out once the seedlings have appeared. However, be sure to wait until all threat of frost is over and ensure that your soil has sufficiently warmed up. 

Monty Don’s top tips for growing cosmos

(Image credit: Future/Leigh Clapp)

Do cosmos like full sun?

However you start off your plants, cosmos like being planted out in full sun and in moist but well-drained soil. You can also encourage bushier plants by pinching out the tips at 15-20cm, and by staking the plants once they are tall enough. 

Bee on orange cosmos getty images 146143087

(Image credit: Getty Images)

How to keep your cosmos flowering? 

If your cosmos just aren't flowering, you could be making one crucial mistake. In order to keep your cosmos flowering, it is important to deadhead the blooms. This stops the plant putting its energy into creating seeds and instead puts its efforts into creating more flowers. 

'What I like to do, is to deadhead really early on as the flower starts to fade and keep them as cut flowers for the house' says Monty Don in this Gardeners' World video. 'If you manage it right, the flowers will keep going week after week'.

'Cut between the main stem and a leaf' advises Sarah Raven in this video from her Perch Hill Gardens 'The lower you go in the plant, the more delay between the flower you've just picked and the next flower.'

Just before the first frost, allow a few blooms to go to seed, so that you can harvest and save the seeds for planting next year.

cosmos and dahlia floral display in vase

(Image credit: Pippa Blenkinsop)

Do cosmos come back every year?

Cosmos are annuals meaning they do not come back every year. In order to have blooms every year, you will need to resow your seeds the following spring.

The only difference, however, is Chocolate cosmos (also known as cosmos atrosanguineus) which is grown from like a dahlia from a tuber. Chocolate cosmos is loved for its delicious vanillary-chocolate scent and velvety brown flowers, and since it is a perennial, will come back year after year. 

chocolate cosmos gettyImages 629380757

(Image credit: Getty Images)

Growing cosmos as a cut flower

'Cosmos is one of our absolute favorite plants here' says Sarah Raven in a video from her Perch Hill Gardens. 'They give you the highest productivity of any other cut flower and have a very good vase life'. 

To get the most from your floral arrangement, be sure to strip off all leaves and sear the stem ends in boiling water for 10 seconds. You can expect your cosmos to last between six and ten days in a vase. 

cosmos and dahlia floral display in vase

(Image credit: Pippa Blenkinsop)