How to Grow: Cosmos

Pronunciation: koz-mose

Dreamy. Whimsical. Romantic. Swoon-worthy. Delightful.

Cosmos are all this and more. Because they are such a common and easily grown flower, I think they are often overlooked and underestimated for their value as a cut flower. They are not necessarily big or bold or flashy… but they are still one of my favorites!

They are reliable, productive and simple to grow. The more you cut them, the more they produce. As a bonus, they really shine in late summer, when some of the other flowers are starting to get tired. Every cutting garden should have some Cosmos!

PROS

They come in a variety of colors and sizes.
They are very easy to grow.
The plants are productive for a long period of time (if harvested regularly).
They are a beautiful addition to bouquets.
They have a good vase life of about 5 days, if harvested at the proper stage.

CONS

They are susceptible to Powdery Mildew. This usually does not affect the blooms, but eventually the plant will die.
Deadheading (removing old blooms) can be difficult to keep up with! If plants are not regularly harvested or deadheaded, they will stop blooming.

CHOOSING SEEDS

Nearly every type of Cosmos can be used to cut flower use, but there are some varieties that have been bred specifically for cut flower use, rather than ornamental use.

Some Cosmos have “single” petals, while other are “double” petals. Some Cosmos even have petals that look like cupcake liners!

Most Cosmos grow quite tall, often reaching 4’ tall, but recently new shorter varieties have been introduced as well. Make sure you purchase varieties that reach at least 24” tall.

Cosmos bipinnatus is the class of Cosmos that includes the Cosmos you are likely familiar with, in shades of pink, white, magenta and burgundy. However, Cosmos sulphureus, know as “Sulfer Cosmos”, are another great addition to the cutting garden. They come in shades of bright yellow and orange.

As a bonus, Cosmos foliage is beautiful and useful in bouquets/arrangements. Regular Cosmos have lacy, delicate foliage, while Sulfer Cosmos have bold, sturdy foliage (pictured below). When growing Sulfer Cosmos, I find myself using the foliage more than the blooms!

Here are a few of my favorite Cosmos to grow for cut flower use:

“Double Click” Series (all the colors are beautiful, but I especially like the “Double Click Bicolor Pink”)
“Tip Top Picotee” (beautiful bicolor blooms)
“Afternoon White”
“Versailles” (classic, sturdy variety bred specifically for cut flower use)
“Bright Lights” (Sulfer Comsos)
“Xsenia” (gorgeous color, stems are short)
“Xanthos” (unusual pale yellow, stems are short)

HOW TO SOW

Cosmos are a summer blooming flowers. They should be planted after your last spring frost.

Transplants or Direct Sow?

Cosmos are versatile and can be transplanted or direct sown.

Transplants are fine, but make sure to start the transplants only about 3-4 weeks before planting outside. Larger, older transplants tend to be “root bound” and may be permanently stunted in their growth and vigor.

Personally, I find Cosmos grow faster, are more productive and healthier when they are direct sown. You might as well save yourself some time and space, and simply direct sow the seeds!

Transplants/seeds should be spaced at about 12” apart.

GROWING ON

After the transplants are established and growing, it’s helpful to “pinch back” the plants when they are about 8”-10” tall. To pinch, simply use clippers to remove the top of the plant, only leaving 2-3 sets of leaves behind. I know this seems counterintuitive and you probably just freaked out, but trust me!

Plants that are pinched are more productive than plants that are not. Experiment for yourself - pinch half the plants and leave the other half alone. The unpinched plants will bloom earlier, but the pinched plants will produce more branches.

If flowers are harvested and spent blooms are “deadheaded” (removed), the plants will continue to send up blooms until frost or cold weather arrives.

STAGE OF HARVEST

Many people say that Cosmos don’t last in the vase… but this is because they are harvesting the blooms too late.

Once a Cosmos bloom opens, it is immediately pollinated by bees. Once pollinated, the flower quickly drops its petals so it can begin its next stage of life - forming seeds.

In order to have long lasting blooms, you must cut Cosmos BEFORE the blooms have been pollinated, looking for buds that are cracking open or just opened. The flower will open up in the vase over the next few days. When harvested at this stage, you can expect them to last about 5 days.

Below, you can see the difference between an “Unpollinated” bloom and a “Pollinated” bloom. If the bloom has been pollinated, you might as well leave it in the garden, as the petals will quickly fall off in the vase.

Cut deeply into the plant between 2 sets of leaves, cutting stems at least 18” long. Each stem should have multiple blooms/buds, a “spray” of flowers. Deep cuts are especially important with Cosmos, as they tend to become a tangled mess if you don’t make deep cuts!

POST HARVEST CARE

Cosmos require no special treatment.

Questions?

Questions or comments? Leave them below and I’ll get back to you!

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