Originally from the tropics and subtropics, Cosmos (or Mexican Asters) are definitely not fans of the cold. In fact, they start getting grumpy long before the first frost hits! While perennial varieties are a bit tougher than the annuals most of us grow, neither can survive a true winter freeze.
What’s the “comfort zone” for Cosmos?
Once the thermometer dips below 59°F (15°C), you’ll notice your Cosmos starting to drop those gorgeous edible blooms. They are definitely not what we’d call “hardy.” If temperatures fall below 41°F (5°C), they are in the danger zone for frost damage. If you’re growing perennial varieties, you’ve got to act before the first frost to overwinter them successfully.
How do you protect Cosmos during the winter?
To be honest, annual Cosmos won’t survive the winter even with protection. Your best bet is to harvest the seeds, dry them out, and save them for planting next spring. To keep those seeds viable, store them somewhere that is:
- Cool,
- Dry, and
- Dark.
Perennial Cosmos are a bit different—they grow tuberous roots (kind of like Dahlias). You’ll need to dig these up and store them in a cool, frost-free spot over the winter. Once the danger of frost has passed in the spring, you can pop them back in the ground. If you want a head start, you can even pot them up indoors starting in March.
If you have your Cosmos in containers, just bring the whole pot inside to a frost-free area. Give the plant a good, hard prune to prevent any pests or diseases from hitching a ride, and just water it occasionally—don’t overdo it!
Are Cosmos seeds winter-hardy?
Here’s the good news: while the plants are delicate, the seeds are tough as nails! They handle the winter just fine and love to sprout as soon as it warms up. This is why you’ll often see “volunteer” Cosmos popping up in your garden the following year. They’re pretty chill about being moved, too, so you can easily transplant these unexpected guests to wherever you want them.
