Start cosmos seeds. A brief context to set expectations.
Start cosmos seeds: Quick notes
Every spring, I can’t wait to start cosmos seeds. But if I jump the gun and start too early, my seedlings can succumb to a late frost. If I start them too late, their ability to flower is limited by summer’s heat. So over time, I’ve learned to start them at just the right moment.
Cosmos are important plants in the veggie garden. Growers all over can start these plants from seed and bring in the butterflies, bees, and wasps that pollinate fruiting plants. They’re a boon to flower gardeners, who look to cosmos for their productive nature and delicate form.
If you want to grow cosmos for pollinators, stick to single-petaled varieties. Outside these limits, there’s so much to enjoy. Multiple colors and differently-shaped petals add interest to gardens, growing tall and strong through the season. They seed out as the first frost rolls in, and re-emerge in spring in the same spot in most regions.
Apricotta Cosmos
Candystripe Cosmos
Xanthos Cosmos
The Bottom Line
Your last frost date will determine when you can sow.
Use your last frost date and your season length to guide your timing. In general, gardeners in all zones should start cosmos seeds directly in the garden one to two weeks before their last frost, when soil temperatures are at least 60°F (16°C). It takes roughly two to three months for cosmos to reach their blooming stage.
Most cosmos varieties emerge within a few days and up to a little over a week after planting. After they bloom, deadhead them for another flush of flowers. They’ll continue to bloom into fall in areas where the cold comes slowly. In colder areas, frost tends to kill them off.
A Regional Guide
Only start seeds indoors if you need the extra time in the growing season.
Now that we’ve covered the basic rules for when to start cosmos seeds, let’s discuss some of the regional nuances. Note that the last frost dates we provide here are approximate. You should always search for your average last frost date based on your zip code. Even that date is approximate, given today’s weather extremes.
One more thing! In zone 1, the season isn’t long enough to directly sow your seeds. You can start them indoors under a grow light. Use a heat mat and sow them in paper pots to ensure the plants don’t succumb to transplant shock, and get them in the ground outdoors ASAP to enjoy the blooms before it gets cold again.
Zones 2-4
Don’t wait too long in these zones or you’ll miss the flowers.
In zone 2, the projected last frost is in mid to late May, from the 15th to the 22nd. Therefore, plant your cosmos seeds early in the month, preferably in the first week. Zone 3 isn’t far behind with last frosts occurring between the 1st and 15th. Zone 4 has frosts occurring from April 24th to May 1st.
In zones 3 and 4, you don’t have to get your cosmos in the ground immediately. Both have roughly four months of growing season, so as long as you get them in the ground within a week or two after your last frost, you’ll have plenty of blooms before the first frost rolls through. But don’t wait too long, or you’ll miss out!
Zones 5-8
To know when to start cosmos seeds here, check your exact last frost date.
In zones 5, 6, and 7, the last winter frost occurs between March 22nd and April 7th. Just as zones 3 and 4 have well over the two months needed to grow cosmos from seed to flower. Zones 5 and 6 have five months, and zone 7 has six. Start cosmos in these zones at the beginning of April, or in the last week of March.
Here in zone 8, I’m about to start cosmos seeds. It’s a dicey endeavor, though, as our last frost – though projected to occur in mid to late March – can sometimes occur in the first or second week of April. Here is a case in point about last frost dates. They’re very localized.
So other gardeners in zone 8 could start their cosmos seeds now, but they’ll need to provide some cover in case of a snap freeze.
Zones 9-12
Frost is rare in these zones, so when to start cosmos seeds is largely up to you.
If you live in zones 9, 10, 11, or 12, you can start your cosmos now, and you may have already started them. In these zones, frost is rare, and it makes sense to maximize your cosmos enjoyment by sowing as soon as you can.
Even if you have some left over from summer, protect them during frost to see if you can continue to grow them when conditions improve. Day length does lessen in winter, but the temperatures and potential rain allow for tons of lovely flowers.
Key Takeaways
Remember, your season length and your last frost date are the keys to the right timing. Use your zip code to determine a more accurate but still approximate date, and sow your cosmos one to two weeks before your last frost.
When you’re deciding how long to wait to start cosmos seeds, consider this chart.
USDA Hardiness ZoneWhen to Start Cosmos Seeds21st week of May31st week of May4Last week of April51st of April, or in the last week of March61st of April, or in the last week of March71st of April, or in the last week of March8Mid March9-12Any time; February is a good starting point
Note that you can start your cosmos seeds before your official last frost date, but if you live in a zone where snap freezes can occur after that date, keep a cover handy to protect seedlings through the freeze.
