When to plant cosmos seeds – experts share their timing tips

Whether you're sowing these summer blooms indoors or outdoors, correct timing is key to success

pink cosmos flowers
(Image credit: pailoolom / iStock / Getty Images Plus / Getty Images)

Knowing when to plant cosmos seeds is important when growing these dazzling, daisy-like blooms. Being half-hardy, they can't handle cold temperatures, so you will need to wait until spring.

Growing cosmos is really straightforward, whether you're sowing these flowers indoors or directly into your flowerbeds. However, the optimal timing for each approach does differ slightly, as the following expert advice explains.

sowing cosmos seeds into tray

Cosmos are easy to grow from seed

(Image credit: Deborah Vernon / Alamy Stock Photo)

When to plant cosmos seeds, according to the experts

Cosmos are one of my favorite annual blooms. In my opinion, they're must-have flowers for cutting gardens, and I can't wait for mine to put on their summer show.

Cosmos are not just beautiful, but they're easy to look after, too. Juan Palacio, CEO of BloomsyBox, points out how they are drought-tolerant plants once established, making them a perfect addition to a low-maintenance garden.

Juan Palacio
Juan Palacio

Juan Palacio is the proud founder of BloomsyBox, a Miami-based floral subscription company. BloomsyBox has made it onto the prestigious Inc. 5000 list of the fastest-growing companies in the US for two consecutive years. To ensure the sustainability of his business, Juan has returned to his roots in South America to cultivate relationships with sustainable flower farms.

When to plant cosmos seeds indoors

cosmos seedlings in trays

Start seeds under cover in spring for early blooms

(Image credit: Richard Newton / Alamy Stock Photo)

Meredith Bishop, a cut-flower grower of Bloom & Bounty, says that many people prefer to sow their cosmos indoors in order to harvest blooms earlier.

'For indoor sowing, it's preferable to sow between March and April,' she says. 'This timing will allow the seedlings to grow to an appropriate size with adequate root development to withstand transplanting outdoors.' Transplanting usually takes place around six weeks after germination, she says.

Sowing at this time also ensures that the risk of frost has passed by the time the seedlings are transplanted out, she adds.

Juan says, 'Make sure to use a light, well-draining soil mix and keep the seeds moist until germination, which typically occurs within 7–10 days.'

Top tip: Remember to harden off your seedlings before moving them into their final outdoor planting spot.

smiling headshot of Meredith Bishop wearing a dress
Meredith Bishop

Meredith Bishop is an heirloom cut-flower grower in urban Nashville, TN. She operates a design studio called Bloom & Bounty with her organically-grown flowers, and enjoys writing, speaking, and leading all manner of gardening workshops.

I'm currently tending to a tray of 'Lemonade' cosmos seeds (available from Burpee) on my windowsill, chosen for their pale yellow blooms. I planted them three weeks ago, and they are just starting to establish their first sets of leaves. This was my first batch and on the earlier side of things. I'll be planting another round once I've potted these ones on.

When to plant cosmos seeds outdoors

cut cosmos flowers in woven basket

Cosmos can carry on flowering until the first frosts

(Image credit: botamochi / iStock / Getty Images Plus / Getty Images)

'Direct sowing outdoors, once conditions allow, encourages strong root development right where the plants will grow,' says Juan.

'Outdoor sowing is very easy, but timing is critical, as seeds will not germinate if temperatures are too low,' warns Meredith. She recommends waiting until May or June, before sowing the seeds into prepared soil. 'This will allow the plants time to grow and bloom well before the first fall frosts arrive.'

You may be able to plant them earlier in warmer climates, such as in Miami, notes Juan.


When should you plant potted cosmos plants outdoors?

Cosmos can often be bought as small potted plants during late spring and early summer. As long as frosts have finished, you can plant them out into your flowerbeds straight away.

What happens if you sow cosmos later in the season?

Cosmos take around three months to germinate, grow, and flower, and will die when the frosts begin. Because of this, sowing them later in the season generally results in a shorter blooming period.

Cosmos are just one of many types of flowers to sow in March – I've got my sweet pea seeds underway, too.

Holly Crossley
Contributing Editor

The garden was always a big part of Holly's life growing up, as was the surrounding New Forest where she lived. Her appreciation for the great outdoors has only grown since then; over the years, she's been an allotment keeper, a professional gardener, and a botanical illustrator. Having worked for Gardeningetc.com for two years, Holly now regularly writes about plants and outdoor living for Homes & Gardens.