Cosmos are one of the most popular summer garden flowers, known for their cheerful blooms and airy foliage. With their delicate appeal, it’s no wonder you’re be hoping to keep your cosmos plants alive past the first fall frost.
You’ll have to plan for other flowers to provide blooms through fall frosts, though, as summer cosmos aren’t up to the job.
Cosmos are not a cold hardy plant because they are what’s called a tender annual. A light frost will damage the leaves of the plant, and a hard frost will kill the plant entirely. If you live in an area with cold winters, you will need to treat cosmos as annuals and replant them each spring.
Even with a short life span, cosmos are worth growing for their cheerful blooms. Try planting some in containers that can be brought onto a covered patio or deck when frost is threatened.
While frost will still ultimately kill the cosmos, there are a few tricks you can use to offer your plants some protection that can help postpone the inevitable just a bit longer.
Can cosmos tolerate frost?
Because cosmos are warm-season annuals, they are sensitive to frost. The leaves and flowers will suffer damage from a light frost and the plant will be killed if it experiences a hard frost or freeze. If you garden in zones 8 or below, cosmos must be treated as annuals and replanted each spring.
Cosmos grow best when temperatures are at least 60 degrees Fahrenheit. To achieve optimal germination, cosmos seeds should be sown in soil that’s at least 70 degrees, which is an indicator of how much cosmos prefer warmth over cold.
If the seedlings experience a frost in the spring, they may survive if it’s a light frost. You can offer cosmos seedlings protection by using row covers while there’s still a risk of frost.
Row covers are thin pieces of fabric or plastic that are placed over plants to protect them from the cold. They work best on tender seedlings and can be removed during the day when temperatures are warm enough.
Once the cosmos plants are mature, they’re still vulnerable to frost. During the summer the plants will produce an abundance of blooms up until the weather starts to cool as fall approaches.
Once nighttime temperatures drop to the 40s or 50s Fahrenheit, you may notice that bloom production slows down considerably. Once temperatures reach freezing at 32 degrees Fahrenheit, your cosmos are done for the season and can be removed and composted.
They aren’t the only flowers that will succumb to frost. You can learn which other frost sensitive flowers you should make sure to grow during the heat of summer in this article, Will A Fall Frost Kill Your Cut Flowers?
How can I protect my cosmos from the cold weather?
Although cold protection won’t preserve your cosmos forever, it can help extend the season by a few weeks.
- Cover your cosmos seedlings with a row cover when a frost is expected, as it will keep the air immediately surrounding the plant just a little warmer than without the cover.
- Plant cosmos in pots and move them under a covered area on cold nights to provide some protection.
- Move potted cosmos indoors or to a garage on very cold nights, moving them back outside in the morning.
- If you have a greenhouse, consider moving your cosmos plants inside to extend their growing season.
By using a little ingenuity, you can try to keep your cosmos plants alive past just a bit longer.
If you need some ideas for cut flowers that are more cold-hardy than cosmos, check out this list: 11 Fall Flowers That Will Bloom Beyond A Frost (with pictures).
What does it mean when a plant is or isn’t cold hardy?
When a plant is described as being “cold hardy,” it means that it is able to withstand colder temperatures than most other plants. This is an important consideration for gardeners in areas with winters, as many plants will not survive if exposed to frost or snow.
In general, cold-hardy plants are native to cooler climates and have evolved over time to develop ways of protecting themselves from the cold.
For example, some plants have a waxy coating that helps to insulate them from the cold, while others can go into a state of dormancy in order to survive extreme conditions.
By contrast, plants that are not cold hardy have no such defense mechanisms and will often die if exposed to freezing temperatures. As a result, it is important to only plant cold-hardy plants in areas where you know they will be able to survive the winter.
Cosmos are an example of a plant that isn’t cold tolerant. They will die when temperatures drop below freezing. Their ferny leaves and thin stems are no match for the cold weather and freezing temperatures.
What weather do cosmos grow best in?
Cosmos are heat-loving plants that will grow to their prime during the hot summer months due to their heritage as native plants of Mexico and the southwestern United States. Their ideal growing conditions are in full sun with long, warm days and regular water.
Cosmos can tolerate summer rainstorms and humidity as long as their light requirements are met and the days are consistently warm.
When can cosmos go outside?
Wait to plant cosmos seeds and seedlings until all risk of frost has passed, which for most gardeners is between April and May. At that point, the soil will have warmed up enough to allow for seed germination and the seedlings shouldn’t experience any freezing temperatures.
If you have a short growing season, try starting cosmos seeds indoors to get a headstart on the season. 4-6 weeks before your last expected frost date, sow the cosmos seeds in soil blocks or seed trays.
This additional growing time will yield earlier blooms so that by the time frost kills the plants, you’ll have had an additional month of flowers from your cosmos.