Cosmos are thankfully fast-growing flowers that are perfect for planting in the spring when you want to get some growth and color in your garden. They are easy to start from seed indoors or straight in the garden, but you have to make sure you plant them at the right time to set them up for success.
Cosmos should be planted after all risk of spring frost has passed and the weather is consistently warm. For most gardeners, this is sometime in April or May. Looking up the last average frost date for your area can help pinpoint when you should plant cosmos in your garden.
Let’s take a look at the best tool you can use to find your last frost date and what that means for when you should plant your cosmos.
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When should you plant cosmos?
The best time to plant cosmos is in the spring after all risk of frost has passed, usually around April or May. This is when most areas complete the transition to spring, and the weather starts to be consistently warm. You can plant through the summer until as late as August to get flowers before the first fall frost.
It’s easy to find out when you can expect that warm weather to stick around. You’ll use two tools, one called the last average frost date, and the other called USDA garden zones.
Frost dates: The last average frost date predicts when the risk of spring frost in your area should be gone. It indicates when temperatures will no longer fall to 32 degrees Fahrenheit or lower, which is when frost and freeze occur. It’s also called the last expected frost date, and you can look it up using your zip code.
The first average frost date predicts when temperatures will cool enough again in fall, bringing the return of freezing temperatures. The time between these two dates is known as the growing season.
Keep in mind that these dates are just estimates. In fact, the frost dates are usually given as a range of a week or two since it’s impossible to predict the weather. Many factors can influence it, such as a harsh winter or warm snap in the spring. There’s always a risk that a surprise frost can hit before or after the estimated date.
If you want to err on the side of caution, then plant a week after the estimated spring date, and you’ll reduce the risk of losing plants to frost. The same goes for the fall; plant a week or two sooner to be on the safe side.
USDA garden zone: Garden zones are another factor that determines when it’s safe to plant warm-season flowers like cosmos. The zones run from 3-10, with the lower numbers assigned to regions with harsher winters and the higher numbers for areas with very mild (or non-existent!) winters.
Garden zones are significant for plants like perennials, shrubs, and other overwintering plants. For cosmos, knowing your zone will help you understand the temperature trends for your area, but honestly, knowing your last frost date is more important.
You can look up your garden zone here on the USDA’s website. Then find your last expected frost date here at Dave’s Garden. You can see weather trends from the National Weather Service, as well as a chart of the probability of frost as you adjust the date for your area.
In the table below, I give a couple of examples of how to use the two tools to find your planting dates. The example uses a location in zone 8a/8b. I’ve listed a range of dates from the first to the last opportunity of the growing season. Remember that the growing season is the time between the last and first frost dates of the year.
|Last/first frost date
|Start seeds indoors
|Direct sow/transplant seedlings
|April 20/Oct 15
|March 9-June 25
|April 20-July 23
Once you have found your planting date, it’s time to decide if you want to start seeds indoors to get a jump on the season or direct sow seeds in the garden to keep the process simple.
To start cosmos seeds indoors, sow them in soil blocks or cell trays 4-6 weeks before your last spring frost. This will give the seedlings enough time to germinate and grow to about 3 inches tall with several pairs of true leaves. You can expect to get blooms a month or so sooner than cosmos that you direct sow in the garden.
The Spruce has an in-depth article, complete with photos of each step, that shows you how to start seeds indoors. Check that out here.
If you direct sow, plant seeds according to the date that shows the risk of frost is gone. If the seeds germinate and then a frost hits, all the seedlings will die since they are not frost-hardy at all. To know when you’ll get blooms, refer to the days to maturity on the seed packet.
When is it too late to plant cosmos?
It is too late to plant cosmos when there aren’t enough days left in the season for the plant to mature and bloom. For most areas, this is the month of July or August, since cosmos need an average of 75-90 days to mature.
Keep in mind, though, that some years the weather is unreliable, and factors such a late spring, a long summer, or a predicted early frost in the fall can result in shifting planting dates earlier or later than expected.
A long, warm summer is reason to push the envelope and squeeze in an extra planting, even if your calculated cut-off date has already come and gone.
On the other hand, if you’ve had a short summer and fall rains have returned sooner than in other years, it might not be worth wasting the seed and effort to plant out in the fall. If soil is too cold or wet, it’s unlikely the seeds will germinate since cosmos need warmth to grow and thrive.
To dive deeper into planting flowers late in the season, get all the details you need in this post, Too Late To Plant A Flower Garden? A Guide To Summer Planting.