The beauty of Cosmos is that is such a prolific flowering plant. Flowers come in a range of colours, and the fine ferny foliage is attractive as well.
The two common varieties are “Cosmos sulphureus” and “Cosmos bipinnatus” and these are annuals, however you can collect your own seeds and sow them again the next spring.
You can sow the seeds into tray, or directly into the ground.
Either way you need to wait until the frost are over before planting into the garden.
A few other Cosmos species are garden worthy, including the sought after ‘Chocolate Cosmos’ – Cosmos astrosanguineus.
How to grow Cosmos from seed
- Cosmos seeds are fairly fine, however they do germinate easily.
- They will take around 1 to 2 weeks germinate. Keep them warm, and water regularly. Try not to over water, moist soil, not soggy.
- We like to water in with a liquid seaweed fertiliser.
- Seedlings need to be planted at around 2 inches apart and then thinned to around 10 inches.
- Full Sun.
- Humus rich moist soil
- Well drained position
- Propagate from seed
- Although they will self seed, you will get better results from collected the seeds.
Collecting Cosmos Seeds
To collect the seeds look for old flower heads at the end of the season. Wait until they are a brown in colour. Simply rub the seed pods between your fingers and the seeds will fall out.
The seeds can be stored in a brown paper bag until the next season. Keep them in a cool dry place over winter.
How to Stake Cosmos
As they get taller Cosmos plants tend to want to fall over, you can stake them with bamboo canes if you wish.
- It is best to stake plants early, as once they fall over in the wind the stems tend to bend and they become difficult to stake.
- Tie all of the stem of each individual plant. Use stakes that are a little shorter that the plant itself and you will hardly notice them amongst the foliage.
- You can also use a grid of open netting set at around 12 inches above the ground, let the Cosmos group through it and this will hold it upright.
Pruning and Deadheading
To keep Cosmos flowering for as long as possible it is essential to deadhead it as the flowers die back. By regular deadheading you can flowers right through until winter. Always deadhead back to below a leaf near the main stem.
You can actually see the new buds, and that is where you cut back to.
For cut flowers cut back lower, again to near a leaf union on a strong stem.