The cosmos are a genus of plants comprising around 26 species that are native to the subtropical to tropical areas of the New World. Only four species occur naturally in North America. Most species are annual, only a few are perennial.
Are cosmos hardy and how do you overwinter them?
Are cosmos hardy? No, cosmos are not hardy and are sensitive to cooler temperatures. Perennial species can overwinter by digging up the root tubers and storing them frost-free. Annual cosmos should be overwintered as seeds and grown in late winter.
The decorative baskets are hardy, that's what the cosmos are called, but not all of them. They are even quite sensitive to slightly cooler temperatures. Your “pain threshold” is around +15°C. If it gets colder, the plant loses its decorative and edible flowers.
Overwintering perennial cosmoses
Few species of Cosmea are perennial, for example the black-red flowering Cosmea atrosangiuneus. Because it smells like chocolate, it is also called chocolate flower or chocolate cosmea. These species form root tubers, similar to how dahlias do. That's why they can overwinter in the same way.
Dig up the root tubers in autumn and store them frost-free at a temperature of around 5 °C. Only in a very mild area can you try overwintering in the garden. Then you should protect the jewelry basket from the cold with a thick layer of mulch.
Overwintering potted plants is also quite easy with Cosmea. Cut the plant back before the start of winter, this leaves less room for diseases and pests to attack. Then place the container in a frost-free room. The Cosmea does not require much care in winter. It should only be watered occasionally.
What do you do with annual cosmos in winter?
So that you have beautiful cosmos in the next garden season, you can start growing the young plants in late winter. You can get the seeds (€2.00 on Amazon) from nurseries or online, but also from your own plants in the garden. Sowing is easy because the Cosmea germinates without any problems.
The most important things in brief:
- mostly annual
- rare perennial
- Overwintering of perennial species in a frost-free environment
- “Overwintering” annual species in the form of seeds