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Cosmos Flower Growing Tips

By: Sherri Ribbey
Looking for a flower that’s easy to grow and looks great? Give cosmos a try — it blooms from late spring to frost and won’t disappoint!

Sensation Mix Cosmos: Flowers are most often single like the popular ‘Sensation Mix’ tall cosmos shown here.

Grow lots of cosmos flowers

Cosmos has been a staple in cottage gardens for generations, but this lovely heirloom annual is a first rate addition to just about any casual garden you have right now. Cosmos flowers sit on long stems that gently nod in the breeze — a great addition to meadows and cutting gardens, too.

Two types of cosmos

The two most common cosmos are tall cosmos and sulphur cosmos. You’re probably most familiar with tall cosmos. This is the one you see in the grocery store seed rack every year. Plants grow 1 to 4 feet tall and have slender, ferny leaves. The 2- to 4-inch blooms come in shades of pink, red, white or violet and bicolors. Flowers are most often single but there are also doubles and other interesting variations — meet a few of my favorites in the gallery below.

Sulphur cosmos has 2- to 3-inch orange, red or yellow single blooms and grows 1 to 6 feet tall. Its foliage is coarser than that of tall cosmos and it tends to grow taller. ‘Bright Lights’ is a dwarf variety in a mix of yellow and shades of orange. But if you prefer a particular color, look for ‘Cosmic Yellow’ or ‘Sunset Orange’.

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Cosmos flowers are great in bouquets

Besides looking great in the garden, cosmos is a wonderful cut flower. To get the most from your summer bouquets check out these tips:

  • Cut your cosmos as the flower just opens and the center is still tight.
  • Use sharp scissors to cut the slender stems at a leaf node — that’s the spot where the leaf joins the stem. The plant will branch from there and form even more flowering stems.
  • Strip off the leaves that will sit below the water before putting your stem in a vase. Otherwise it will rot and shorten the life of the bloom. Do leave some of the frilly foliage at the top of the stem near the flower — its fine texture will be a nice contrast to larger leaves in your bouquet.

Deadheading cosmos flowers

Cosmos usually starts blooming in early summer and continues until frost if you deadhead. While you don't have to deadhead, doing so keeps the planty looking tidy and encourages a quick rebloom. Here’s how to do it: Cosmos produces multiple flowering stems near the top of the plant. The center one opens first. As that one fades, clip it out and the side stems will take off more quickly. When they’re all done cut the whole group off above a leaf node to encourage more growth and more blooms.

deadheading cosmos flowers: To deadhead, use sharp scissors and grab a handful of stems. Cut them off above a pair of leaves if you can, but it’s not essential.

Shearing back

If you have a large planting or some back-of the-border beauties that are hard to reach, you may or may not want to be as detailed. The photo above shows a less fussy method. You can even go so far as to cut your cosmos back to within 12 to 18 inches of the ground, and the plants will rebloom again in a few weeks.

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Save cosmos seed

While it’s a good idea to deadhead your cosmos to keep them blooming, be sure to let a few go to seed toward the end of the season so you’ll have more plants the following year. Let them grow wherever they land or save seeds so you can choose where to grow them. The video above shows you what to look for and how to harvest cosmos seed. They may not come true, or look like the parent plant, though. Check out the simple steps below on how to save cosmos seeds.

Mature cosmos flower seed: When cosmos spikey seeds turn brown they are ready to harvest.

How to save cosmos flower seeds

  1. You’ll know it’s time to harvest when the spiky seeds are dark brown. Carefully grab a handful — they fall off quite easily at this point — to add to your stash.
  2. Once you get them indoors, sort out the seed from any debris and put them in a shallow bowl or on a tray out of direct sunlight for about a week to dry. This helps induce dormancy and harden the seedcoat.
  3. Store the seed in a plastic bag, jar or other handy container and keep it in a cool, dry place that stays 32 to 41 degrees F (the refrigerator is fine.)
  4. And don’t forget to add a label with the date so you know what you have next year.

Cosmos flower varieties to try

Looking for some cosmos to brighten your borders and liven up bouquets? Besides the traditional single flowers that bloom in pink, magenta, white or bright orange, there are plenty of exciting new looks for cosmos. Now let’s take a look at a few gorgeous cosmos varieties that you’re going to want to try.

Cosmos sources

‘Sensation Mix’ tall cosmos (Cosmos sulphureus )

Type Annual Blooms 3 to 4 in. single blooms in lavender, pink, magenta and white Light Full sun Soil Poor, well-drained Size 48 to 54 in. tall and 12 to 14 in. wide

'Seashells’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms An heirloom with fluted petals, this classic has stood the test of time. Flowers come in pink, red and white Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 16 to 20 in. wide

‘Bright Lights’ sulphur cosmos (Cosmos sulphureus)

Type Annual Blooms This variety is a flower powerhouse — it blooms nonstop with 2 ½-in. blooms in shades of bright orange, yellow and red Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 15 to 18 in. wide

‘Xanthos’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Soft yellow 2 ½-in. open-pollinated blooms with a white center ring will come true if it reseeds Light Full sun Soil poor, well-drained Size 20 to 24 in. tall and 14 to 16 in. wide

Sonata White tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Part of the Sonata series 2 to 3 in. flowers come in white, pink, magenta, purple and a mix. Sometimes you’ll find this series in multipacks at the garden center Light Full sun Soil Poor, well-drained Size 18 to 24 in. tall and 12 to 14 in. wide

‘Double Click Rose Bonbon’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Tall enough for the back of the border, the frilly pink semidouble to double 3-in. blooms add a romantic look to the garden Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 18 to 24 in. wide

‘Sensation Mix’ tall cosmos (Cosmos sulphureus )

Type Annual Blooms 3 to 4 in. single blooms in lavender, pink, magenta and white Light Full sun Soil Poor, well-drained Size 48 to 54 in. tall and 12 to 14 in. wide

‘Xanthos’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Soft yellow 2 ½-in. open-pollinated blooms with a white center ring will come true if it reseeds Light Full sun Soil poor, well-drained Size 20 to 24 in. tall and 14 to 16 in. wide

'Seashells’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms An heirloom with fluted petals, this classic has stood the test of time. Flowers come in pink, red and white Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 16 to 20 in. wide

Sonata White tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Part of the Sonata series 2 to 3 in. flowers come in white, pink, magenta, purple and a mix. Sometimes you’ll find this series in multipacks at the garden center Light Full sun Soil Poor, well-drained Size 18 to 24 in. tall and 12 to 14 in. wide

‘Bright Lights’ sulphur cosmos (Cosmos sulphureus)

Type Annual Blooms This variety is a flower powerhouse — it blooms nonstop with 2 ½-in. blooms in shades of bright orange, yellow and red Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 15 to 18 in. wide

‘Double Click Rose Bonbon’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Tall enough for the back of the border, the frilly pink semidouble to double 3-in. blooms add a romantic look to the garden Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 18 to 24 in. wide

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Product Recommendations

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