Grow Cosmos from Seeds – My Growing Tips

Cosmos (Cosmos bipinnatus) has become one of my absolute favorites for cut flower growing here at Aspegren’s Garden in Ostrobothnia, Finland. It is beautiful, easy to grow from seeds, and blooms abundantly – and the best part is that it fits perfectly in cut flower bouquets. With its airy shape, elegant flowers, and generous blooming, cosmos is a must-have in my garden every summer. Read my guide to get wonderful cosmos bouquets and succeed in growing cosmos from seeds!

1. Sow the seeds in seed compost in March.
2. Place them in a dark and warm spot, like a bathroom with underfloor heating.
3. Once germinated, move them to a cooler location under grow lights.
4. Cosmos seeds germinate quickly, so be careful not to let them become leggy.
5. Prick out into larger pots when the plants are overcrowded.
6. Pinch the plants when they have 2-3 sets of leaves.
7. Plant them outside in the garden after the last frost, around June.
8. Support with plant netting.
9. Pinch off faded flowers for a fuller bloom.
10. Harvest for bouquets!

You can definitely sow cosmos directly in the ground in May when the soil is warm, or cold-sow cosmos in pots or plastic boxes during late winter (February–March). However, for those growing in the Nordics who want a long cutting season, the best option is to start cosmos indoors.

I usually start sowing cosmos in March. You can use seed trays with seed compost, but I prefer to use soil blockers with my own soil mix of seed compost, vermiculite, and coconut fiber. Then, I lightly cover the seeds with vermiculite and place them in the bathroom with underfloor heating in the dark. Underfloor heating works great instead of a heat mat! When I sow in soil blockers, they take up very little space, so I sow them in sushi boxes with a perfect lid that keeps the heat in and the air moist. Just remember to cut a few holes in the lid! I keep them moist and spray them daily. The seeds germinate quickly, within 1-3 days, so keep an eye on them! As soon as they start sprouting, move them to light and a slightly cooler place, around 10-16 degrees, depending on the conditions at home. Also, use grow lights, otherwise, there’s a big risk they’ll grow too tall and weak before they make it outside to the garden.

For those who like cold sowing, cosmos works excellently! Cold sowing is a convenient option if you want to avoid starting seeds indoors. You can cold sow cosmos in plastic boxes outdoors in February, and when spring arrives, they will germinate on their own. It’s an easy way to prepare the planting early – especially if you want cosmos in beds, pots, or raised garden beds without filling up your windowsills indoors.

When the plants have their first pair of leaves, I transplant them into larger pots. I still use seed compost, but I start giving them some plant food for seedlings. I plant them deeper than they were before to provide stability. At the beginning of May, I start hardening them off outdoors, and once the risk of frost is over, they can be moved to my plot at Aspegren’s garden. I plant them 30–36 cm apart, depending on the height of the variety.

To get strong plants, I pinch them early – after 2–3 leaf pairs. This makes them bushy and increases flowering. Cosmos thrives in poor to normal soil. Too much nutrients will only result in large foliage and fewer flowers.

Cosmos prefers a sunny location and well-drained soil, ideally slightly sandy. It works great in garden beds, raised beds, and larger pots. Cosmos in pots requires a bit more attention to watering and choosing a variety that doesn’t grow too tall – but it works perfectly.

When the cosmos flowers are in the garden bed, I make sure to water them, as the spot in Aspegren’s garden is very sunny and can get quite dry. I fertilize the soil mainly with lime and chicken manure before planting the cosmos. I regularly remove faded flowers and cut them for bouquets – this encourages new buds and extends the blooming period.

Cosmos flowers can get quite tall, and they definitely need support to avoid falling over in rain and wind. I usually use a plant net for support. Some cosmos varieties can grow up to 120 cm tall. The first year, when early summer was warm and sunny and I watered them well, the plants grew really tall and looked absolutely beautiful!

Cosmos is a classic “cut and come again” flower. The more you pick, the more it blooms. I prefer to harvest early in the morning or late in the evening, using everything from buds to fully bloomed flowers. The foliage is also beautiful in bouquets. Cosmos in the flower bed gives you hundreds of opportunities to fill vases with ease and style.

Cosmos is beautiful in bouquets, and I particularly like the different varieties of the white ones, as well as the pastel and apricot-colored ones. Here are my favorites:

• ‘Apricotta’ – softly apricot-colored, perfect for pastel bouquets.
• ‘Double Dutch Rose’ – fluffy light pink with long-lasting blooms.
• ‘Kiiro’ – creamy yellow, fantastic with marigolds and zinnias.
• ‘Psyche White’ – airy, wild, beautiful in natural bouquets.
• ‘Double Dutch White’ – incredibly beautiful, but difficult to get to sprout.
• ‘Purity’ – a simple white classic that I grow every year.

Cosmos is also an excellent plant for pollinators. Both bees, bumblebees, and butterflies are attracted to the open flowers. I often photograph and film the butterflies that enjoy my cosmos flowers!

I always let a few plants go to seed at the end of the season. They dry on the stem, and the seeds are easy to collect. They look like small, prickly needles. I have collected seeds from cosmos from previous years and have been able to make them bloom again the following year from my own seeds, just as easily as buying new ones!

Here are some of my bouquets with cosmos – often used as filler flowers in my colorful bouquets! They are so useful, durable, and beautiful!