Is It Too Late To Plant Cosmos? (Probably Not!)

cosmos seedlings in soil blocks

Spring is well and truly underway, and gardens are starting to bloom. But if you’re like me, you’re always looking for ways to add more color and life to your outdoor space. If that’s the case, I have good news for you: you might still have plenty of time to plant cosmos flowers!

Cosmos are frost-tender annual flowers, which means they only live for one growing season. This also means that they can be planted during a large portion of the growing season and still bloom beautifully because, as annuals, they are fast growers. 

Cosmos can be planted as late as July in areas with a long growing season. The plants need 70-100 days from seed to bloom, so a July sowing will mature in October. If you live in an area that doesn’t freeze, you can plant cosmos as late as August, though the blooms will be smaller as the days get shorter in fall.

Cosmos seeds are inexpensive, often sold in packets of 50 seeds for just a few dollars. It’s a small investment to experiment with the timing of your planting, and even if you only get a few blooms, they’ll be well worth it!

So if you’re looking to add a little more beauty to your flower garden beds, don’t wait any longer. Go out and plant some cosmos seeds and see what happens.

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What month do you plant cosmos?

Cosmos are typically sown in the late spring, starting in April. The big indicator that it’s time to begin planting is when the danger of frost has passed, which will vary by region. May and June are also great months to sow cosmos seeds directly in the garden.

The ideal soil temperature for germinating cosmos seeds is about 70 degrees, so the daytime temperature should also be in that range. In early spring, it can take the soil a while to warm up, but it stays consistently warm by summertime, leading to fast germination times.

Warm soil combined with full summer sun is a recipe for fast cosmos growth, making July and August viable sowing months, even if they’re considered late compared to spring plantings.

For more information on finding the best time to plant cosmos, bookmark this article: When To Plant Cosmos: The First And Last Planting Dates.

Sow cosmos up until 100 days from the first frost

With 100 days left in your growing season, you can still sow cosmos seeds and get blooms before that first cold snap.

tray of cosmos seedlings
Cosmos seedlings sprout quickly, making them easy to sow all summer.

Cosmos take 70-100 days to bloom, so in some cases, you might be butting right up against that first frost date. That date is just an estimate, though, so you could get lucky, and the first fall frost doesn’t hit until a week or two (or later) than when it was predicted.

In that case, you’ll get a couple of weeks’ worth of blooms as a nice bonus. For a cheap packet of seeds, it’s worth pushing the envelope with a late sowing, even if the typical planting window has already passed.

As I mentioned, one good thing about a late sowing is that the soil will already be warm from a month or more of the summer sun, so the seeds should sprout very quickly.

While you can technically sow cosmos seeds from late summer until the first frost date in fall, you might not get much from your efforts. The plants may sprout and grow quickly, but they won’t have time to produce blooms before the cold weather arrives.

Keep the soil moist during the germination process, which is easy to do with drip irrigation or a quick pass with the garden hose.

For the full run-down on starting cosmos from seed, check out this article: Easy Steps To Plant Cosmos Seeds, Indoors and Out.

Waited too late? Try winter sowing instead

If the last opportunity to sow cosmos seeds has passed, you can winter sow them with a little planning.

Winter sowing means planting the seeds outdoors in winter, allowing them to stratify (a process of cold-moist conditions that breaks seed dormancy) and then germinate in spring once the soil reaches its ideal temperature.

You’ll need a container (milk jugs work great), some potting soil, and your cosmos seeds.

Cut the jug in half, fill it halfway with potting soil, and then sow the seeds 1/4 inch deep. Water well, put the lid back on, and tape it shut with duct tape. Leave the cap off for air circulation and moisture to get in.

Once you’ve sowed your seeds, simply place the container outside where it will be exposed to winter conditions. Be sure to label the container, so you don’t forget what’s inside, and check on it periodically throughout the winter.

You should have a beautiful crop of cosmos seedlings ready to plant in your garden by spring!

You can also take the quick and dirty route and simply spring cosmos seeds outside in the soil where you want them to grow. You may lose some seeds to birds, so sow them thickly.

hand holding two cosmos flowers gone to seed
If you have any summer cosmos, let them go to seed and nature will make sure they sprout when the time is right.

Like in the milk jogs, you should have a bevy of cosmos seedlings that will grow quickly come spring.

For either method, make sure to wait to put the seeds out until freezing temperatures have arrived or it’s consistently cold enough that the cosmos seeds won’t sprout too early.

You don’t want them to germinate in October, only to have cold winter temperatures arrive the next week and kill all the tiny sprouts.

Can cosmos survive winter?

If you planted cosmos in spring or summer, the plants will die once winter and cold temperatures arrive. Cosmos are heat-loving plants known as tender annuals, so they can’t survive temperatures under 32 degrees Fahrenheit.

However, if you winter sow cosmos seeds, the seed will overwinter just fine. They’ll go dormant during winter and then come back to life in spring when the temperatures warm up again.

So even though you can’t keep your existing cosmos plants alive through winter, you can replant them each year by sowing new seeds in spring and summer.

If you’re determined to push the season for your cosmos, get some tips on making that happen in this article, Are Cosmos A Cold-Hardy Flower? (How To Extend Their Season).

For a full rundown of what a fall frost means for your garden, read this article next: Will A Fall Frost Kill Your Cut Flowers?

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