Cosmos are half-hardy annuals that grow, flower, set seed and die all in one year, but unlike hardy annuals, they can’t withstand low temperatures.
They’re quick and easy to grow from seed, flowering in as little as 12 weeks, and can be added to borders and pots for a burst of showy colour.
To give your cosmos as long a flowering season as possible, sow the seeds early, indoors, in March or April. Once they’ve sprouted and have two pairs of leaves, transplant into individual 7cm pots and grow on under cover.
In May, harden them off for a week by placing in an open cold frame or on a patio, before planting them out where they are to flower. Feed fortnightly with a liquid tomato fertiliser when the first buds appear.
Discover 10 more half-hardy annuals to grow.
Find out how to sow cosmos seeds, below.
You Will Need
- Cosmos 'Sensation'
- Seed tray or small pots
- Multi-purpose, peat-free compost
Fill a seed tray or small pots with sieve, good-quality seed or multi-purpose compost. Lightly firm down the compost with the back of your hand or a small block of wood.
Sow the seed on the surface of the compost, preferably just one seed per cell.
Sow a small pinch of seeds on the surface of each cell, then cover with a 3mm layer of sieved compost or vermiculite. Place in a tray of water to moisten the compost, drain, then label.
Place in a heated propagator at 18-25ºC, or cover with a clear plastic bag. When the seedlings have two pairs of leaves, transplant into individual 7cm pots and grow on indoors.