Dreaming of a vibrant cottage garden without the hassle? This guide shows you how to grow the stunning cosmos flower from seed to bloom. Learn the best way to start cosmos from seed with these simple tips.

Are you ready to sprinkle a little magic into your garden with some stunning cosmos flowers? Well, you’re in luck because today, we’re diving into the wonderfully simple world of growing cosmos flower from seed!

Now, if you’re thinking, “But I don’t have a green thumb,” I got you! Cosmos flowers are here to change that narrative with their easy-going nature and high germination success rate. There are two ways to start cosmos seeds and both are incredibly simple to do.

Whether you’re a seasoned gardener or just starting to dip your toes into flower gardening, cosmos are fun blooms to grow for use both in the garden and out.

Wait until you see how easy they are to grow.

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About Cosmos Flower

Cosmos are flowering annuals with cheerful daisy-like blooms that add lots of dainty blooms and beautiful texture to your gardens. These low-maintenance blooms are not only easy on the eyes, but also easy on gardeners.

Their long, sturdy stems make them fantastic cut flowers, lasting for days in a vase and bringing a touch of summertime cheer indoors. Additionally, their airy foliage and freely branching forms create a relaxed, whimsical feel, perfectly suited for a charming cottage garden overflowing with color and life.

I don’t love cosmos as cut flowers as much as I enjoy them in my cottage gardens but love the airy texture they add to my flower beds.

Before you start cosmos from seed, keep in mind that they need full sun (6-8 hours) in well-drained soil to grow, bloom, and be healthy. So make sure you have a spot like that in your garden before you start the seeds.

Taller cosmo varieties will need support to keep the plants from flopping over in your garden. To keep them blooming, you’ll need to deadhead and cut flowers often because the more you cut the more they’ll flower.

Deciding Whether to Start Cosmos Flower From Seed Indoors vs Outdoors

The biggest question you’ll need to solve for yourself is HOW you want to start your cosmos seeds because there are two ways to get your cosmos journey rolling: starting seeds indoors or sowing them directly outdoors.

Deciding whether to sow cosmos seeds indoors or directly outside depends on a few things. So consider these factors carefully.

Climate

  • Cooler climates: Starting cosmos flower seeds indoors allows you to get a head start on the season and enjoy earlier blooms before the last frost. This is especially beneficial in areas with short growing seasons.
  • Warmer climates: Direct sowing cosmo flower seeds is perfectly feasible as long as the threat of frost has passed. It’s simpler and requires less equipment, making it ideal for warmer regions.

Personal Preferences

  • Convenience: Direct sowing cosmos seeds outdoors is generally quicker and less work compared to starting seeds indoors, which requires containers, grow lights, and more attention to watering and temperature.
  • Control: Starting cosmos flower seeds indoors gives you more control over germination conditions and seedling care, potentially leading to stronger plants. It also allows you to choose specific varieties that might not be readily available as seedlings.
Double Click Snow Puff cosmos
Double Click Cosmos with light pink flowers

Other Factors to Consider

  • Planting time: If you want blooms earlier in the season, starting indoors is necessary. For later summer blooms, direct sowing might suffice.
  • Cost: If you are on a budget, starting seeds outdoors may be more cost-effective than purchasing expensive grow lights and other equipment needed to be successful.
  • Variety: Some cosmos varieties might germinate better indoors. Check the specific needs of your chosen variety.
  • Experience: New gardeners might find starting indoors easier with more control, while experienced gardeners might be comfortable with direct sowing.

Ultimately, the best way to decide is to consider your climate, personal preferences, and specific goals for your cottage garden. Experimenting with both methods can help you discover what works best for you and your local conditions.

FactorStarting Seeds IndoorsDirect Sowing Outdoors
ClimateWarmer climates, short growing seasonsWarmer climates, longer growing seasons
CostMore expensive, need suppliesLess expensive, don’t need as many supplies
ConvenienceLess convenient, requires more attentionMore convenient, less work
ControlMore control over germination and seedling careLess control over germination conditions
Planting TimeEarlier bloomsLater blooms
VarietyMay be necessary for some varietiesWorks well for most varieties
ExperienceWorks well for experienced gardenersMay be easier for beginners
Things to Consider When Deciding Whether to Start Cosmos Seeds Indoors or Outdoors

How I Start My Cosmos Flower Seeds

Every year, I prefer to start my cosmos flowers outdoors because I start so many other types of flowers indoors, that I need the indoor seed starting space for those.

Because cosmos flowers are quick to grow and germinate, to me, they are easy to directly sow outdoors in the garden. They bloom a bit later this way, but germinate so quickly and grow so fast, the difference isn’t much.

A few years ago, I started cosmo flower seeds indoors and the plants did great, so I’ve found that both methods work equally as well.

Picture of Pink flowers on double click cosmos
Cosmos

Starting Cosmos Flower Indoors

If want to start cosmos indoors, it is pretty easy to do because they grow from seed so well! Seeding cosmos indoors is a cost-effective and rewarding way to grow these beautiful flowers, as well as get a head start on the season, particularly if you live in a cooler climate. You’ll have greater access to the many varieties of cosmos available, so they are worth growing from seed if you are up to it. I’ve seen cosmos flowers available at the local nursery, but the options are very limited.

If you are not sure when to plant cosmos seeds, you’ll need to know your last frost date and count about 2-4 weeks before. But check your seed packet for specific sowing date instructions. Some varieties may even recommend sowing directly outdoors.

Must-Have Seed Starting Supplies I Use

Directions for Seeding Cosmos Indoors

Sowing cosmo flower seeds indoors is pretty easy to do and a fun way to wrap up winter. This year, my last frost date is April 24 which means if I were sowing my cosmo seed indoors, I’d need to do it by the week of April 7 at the latest. If you aren’t sure when your last frost date is this year, you can check it here.

How to Start Cosmos Seeds Indoors

  • Cosmos can be started indoors about 4 weeks before the last frost date in your locality. Use a seed starting tray or small pots with drainage holes filled with a quality organic seed starter mix that is pre-moistened. I started mine under grow lights and have found great success starting them in both my basement a few years ago.
  • Make a small hole then sow the seeds then lightly cover them with vermiculite. Sow only one variety per cell tray as the different plants will germinate at different times.
  • Cover seed trays with a clear dome and move them under the grow lights on a heat mat to speed up germination. They typically germinate within a few days to a week.
  • Remove the cover when the seeds start to germinate. It’s OK if they didn’t all germinate, more will grow after the cover is removed.
  • Keep soil moist by bottom watering only.

How to Move Cosmos Seedlings Outdoors and Plant Them the Right Way

  • Before transplanting cosmos seedlings outdoors, harden them off by gradually exposing them to outdoor conditions over a period of 14 days.
  • When seedlings are hardened off and ready to plant in the garden, space cosmos transplants in your flower garden about 9 inches apart after the last frost date. Spacing is really important so there’s enough airflow between plants to prevent problems like powdery mildew.
  • Water young plants regularly, keeping the soil moist but not waterlogged. Feed with an organic fish fertilizer for a few weeks and then use a slow-release fertilizer so feeding them is set-and-forget. This will help promote healthy growth and lots of blooms.
cut flower patch with cosmos, zinnias and tall phlox
Cosmos, Zinnias and Tall Phlox in my cottage garden

Direct Sowing Cosmo Flower Seeds

Direct sowing cosmos seeds outdoors is a simple and rewarding way to enjoy these vibrant blooms in your garden. But you’ll need to wait until after your last frost date to do it. This is my preferred method for getting them started but every gardener and garden is different, so do what’s best for you.

Preparation

  • Timing: Wait until all danger of frost has passed, typically 1-2 weeks after the last frost date to be safe. Aim for a soil temperature of above 60 degrees for optimal germination. If you aren’t sure how warm your soil is, you can use a soil thermometer like this one.
  • Choose a sunny spot: Cosmos thrive in full sun, receiving at least 6-8 hours of direct sunlight daily.
  • Ensure well-drained soil: Poor drainage can lead to root rot. Amend your soil with compost, leaf mold, or other organic matter to improve drainage if needed.
  • Clear the planting area: Remove any weeds, debris, or previous plant material.
  • Loosen the soil: Rake or till the soil to a depth of 6-8 inches to create a good planting bed.
  • Mix in compost or organic matter: Amend the soil with 2-3 inches of compost or aged manure to improve fertility and drainage.
  • Rake the soil smooth: Create a level surface for planting.

Sowing Cosmos Seeds

  • Follow spacing instructions: Refer to the seed packet for the recommended spacing between seeds and rows. Common spacing generally ranges from 9-18 inches.
  • Sow seeds thinly: Avoid overcrowding, which can lead to competition for resources and promote disease problems.
  • Plant at the right depth: Gently press seeds into the soil to the depth specified on the seed packet.
  • Cover lightly: Sprinkle a thin layer of soil or vermiculite over the seeds.
  • Water thoroughly: Water gently to settle the soil and ensure good moisture contact with the seeds.
pink and white cosmos

Caring for Cosmos Seeds After Planting

  • Keep the soil moist: Water regularly, especially during hot and dry periods. Aim to keep the soil evenly moist but not soggy.
  • Thin seedlings (if needed): Once seedlings have their first true leaves, thin them to the recommended spacing if they are too close together.
  • Pinching back: Pinch back plants after the first 4-5 sets of leaves to encourage branching with more flowers.
  • Deadhead spent flowers: Regularly remove spent blooms to encourage more flowering throughout the season.
  • Enjoy your cosmos! With proper care, your cosmos should flower in about 6-8 weeks after sowing.

Pro Tips

  • Label your plantings: Keep track of the different varieties you sow by using plant markers for every single seedling so you don’t forget.
  • Protect seedlings from pests: Consider using insect netting or row covers to deter pests like birds and insects while seedlings are young.
  • Mulch around your plants: A layer of mulch can help retain moisture, suppress weeds, and regulate soil temperature. But hold off adding it until the seedlings sprout and have some growth to them.
  • Consider succession planting: Sow seeds every 2-3 weeks throughout the season to extend your blooming period and have fresh flowers continuously.
double click cosmos

Reaping the Rewards of Your Cosmos Flowers

With proper care, your cosmos should start blooming in 6-8 weeks from sowing. Remember to:

Favorite Cosmos Plant Varieties

While I love to grow cosmos, I don’t grow that many varieties of them in my gardens. So here are the few that I grow and enjoy.

  • Double Click Snow Puff
  • Versailles Mix
  • Cupcakes White
  • Apricotta
  • Double Click Bicolor Pink

I hope this blog post helps you get started on your cosmos flower adventure! If you have any questions or specific tips you’d like to add, feel free to share them in the comments below. Happy gardening!

red and white cosmos

More About Starting Cosmos Flower From Seed

Have you grown cosmos from seed before? If so, what’s your favorite variety? Do you have any tips to share? I would love to know more in the comments below.

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Garden Supplies I Use

Since I’ve been gardening for well over twenty-five years, I’m often asked about the garden supplies and tools that I use most. Here are some of my favorites that I use in no particular order.

dahlia kogane fubuki in the potager garden

Click here to shop my favorite garden supplies!

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Enjoy a beautiful day! xo

Stacy Ling

Want to learn more about me? I’m a master gardener who’s been gardening and growing things for over 25 years and author of the best-selling book, The Bricks ‘n Blooms Guide to a Beautiful and Easy-Care Flower Garden. Get the inside scoop about my background as a master gardener, education, and experience, as well as why I started blogging here.

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