Growing Cosmos From Seed: Common Questions Answered

cosmos seedlings in soil blocks

Growing cosmos from seed is a simple process that rewards you with an abundance of plants for the investment of just a little time and attention. Plus, there are dozens of varieties you can grow from seed, whereas the garden centers and nurseries usually only sell a handful of standard varieties.

Even though starting cosmos from seed isn’t complicated, you might still have a few questions before getting started. Find your answers here, then jump over to this article, Easy Steps To Plant Cosmos Seeds, Indoors And Out, to get step-by-step instructions for planting cosmos seeds.

Key Takeaways For Starting Cosmos From Seed

  • EASY TO GROW – Cosmos are an amazing beginner flower because they’re just so easy to grow. No fuss, no muss. Expect to see your seeds pop up 7-14 days after planting.
  • DIRECT SOW – Clear an area in your garden to scatter seeds or sow them in rows after all risk of frost has passed. Cover the seeds with 1/4 inch of soil and keep moist until they germinate.
  • START INDOORS – 4-6 weeks before you want to transplant, start your cosmos indoors in a seed tray or recycled container. They’ll be ready to transplant when they have 3 sets of true leaves.

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Is it easy to grow cosmos from seed?

Cosmos are very easy to grow from seed, whether they’re started indoors or direct sown in the garden. The large seeds are easy to plant and don’t require special treatment. As long as the seeds are kept moist until they germinate, starting cosmos from seed is straightforward.

You can find cosmos seeds at any garden center, online seed catalog, and even some hardware stores if they sell seeds. If you need a source for seeds, check out my favorites on this list, 10 Best Places To Buy Quality Flower Seeds Online.

Each packet will have the growing instructions, which will be something along the lines of:

  • Choose a site in average soil and full sunlight
  • Sow seeds after danger of frost has passed
  • Seeds will germinate in 7-14 days
  • Once seedlings are several inches tall, thin plants to 9-12 inches apart
  • Days to maturity (bloom) range from 60-110 days, depending on variety

You can modify the planting process by starting seeds indoors, using succession sowing, or growing in containers, but the basics of growing cosmos from seed are the same. There’s not much fuss like cold stratification, slow growth, or complex fertilizing routines.

From seed to flower, cosmos are easy to grow. To find out how to do it, check out this article, Are Cosmos Easy To Grow? Yes, Especially From Seed!

How long does it take for cosmos to grow from seed?

Cosmos are fast to grow from seed, germinating in 7-14 days and blooming in about 12 weeks. To speed up growth, use a heat mat for your seed trays during germination and set out transplants once the soil has warmed and all risk of frost is gone. Cosmos love heat, so these steps will help them grow quickly.

If you’re looking to speed up the germination process, try using a heat mat.

Heat mats are like a heating pad for seed trays. The mat will bring the soil temperature up to a certain degree. For cosmos, seeds germinate best at 68-72 degrees Fahrenheit. With a heat source, be sure to keep an eye on the soil moisture since the tray will dry out more quickly than without a heat mat.

Conversely, if you start cosmos seeds in too cold soil, it will take longer. The seeds will wait until temperatures rise adequately to sprout, which may take a while depending on the scenario.

Cosmos seeds that have fallen from the parent plant in the garden might wait for months before sprouting if the fall temperatures are too low. The seeds lay dormant in the soil, just as they’ll do if you sow them too early in the year before the soil has warmed up.

If you sow seeds in a tray and put them in your cold garage without a heat mat, bear in mind that it might take several weeks, or even longer, for the cosmos seeds to germinate. Just be sure to keep the moisture constant during that time so that when they do sprout, there’s no risk of them drying out before they even get a chance to grow.

Can I just scatter cosmos seeds?

You can scatter cosmos seeds directly in the garden. To start, clear a patch of soil of any grass or debris. Scatter the seeds so they land with a few inches between them, then cover them lightly with a quarter-inch of soil. Press the covered seeds down with your hand to make contact between the soil and seeds, then water daily until they germinate.

This video by Beautiful Nest Home & Garden shows how to scatter cosmos seeds straight into the garden:

If you scatter seeds instead of starting them indoors, try covering them with a piece of burlap or an old bedsheet until they germinate. The fabric will help hold in moisture and prevent a crust from forming on the soil’s surface, which is difficult for seeds to penetrate.

Additionally, the barrier will stop birds from scratching at the bare soil. While they won’t damage ungerminated seeds (as long as they don’t eat them!), if the seed has cracked open and a sprout is emerging, the movement could break the tiny seedling and it will die.

If you do cover the scattered seeds, water over the top of the fabric and it will penetrate through to the soil below. Remove the burlap or sheet as soon as you see the sprouts poking out so they get full access to sunlight.

What month do you plant cosmos seeds?

April or May is the best month for most gardeners in the US to plant cosmos. The soil will be warm enough for germination, and the risk of frost has passed. Cosmos seeds can be planted through the summer until July or August, as long as there’s enough time left in the season for them to bloom before a frost.

If you have a shorter growing season with only a few months between the last spring frost and the first fall frost, be sure to choose a cosmos variety with the fewest days to maturity. Many cosmos varieties mature in 75-90 days, but a handful will bloom about 60 days after planting.

On the other hand, if you have a longer growing season between frosts, or if your area doesn’t freeze at all, you have more wiggle room to grow varieties with a longer time to maturity. Take advantage and grow a variety like Versailles Flush, which takes a bit over 100 days to bloom.

VarietyDays to maturity
Apricot Lemonade56-84
Cosimo Collarette56-84
Afternoon White75-90
Double Click Mix75-90
Sensation Mix75-90
Cupcakes White90-100
Versailles Flush90-110

For full details about when to plant cosmos seeds, along with a table showing you how to find your planting dates, check out this article: When To Plant Cosmos: The First And Last Planting Dates.

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