Planting Cosmos – How and When

Planting Cosmos – How and When

Growing cosmos is one of my top recommendations for the beginning gardener.  Cosmos are relatively easy to grow, and your efforts are often well rewarded with large numbers of flowers.  Cosmos are lovely flowers that can either be left on the plant to beautify your garden, or be cut for arrangements.  But let’s not get ahead of ourselves — we have to plant them first!

Cosmos are easy to start from seed.  You can either sow the seeds right into the ground or you can start them indoors about four weeks before the last hard frost.  (If you have a short growing season like the northeast U.S. coast or Canada, then it’s a good idea to start indoors or purchase young plants for your garden.)

Butterfly on cosmos

Just try to keep butterflies away from cosmos.

When purchasing cosmos make sure there is no sign of flowering!  This could be an indication that the plant has been stressed, and over the growing season may not produce as many flowers as you’d hope.  Plants that don’t start blooming until they are in their final flower beds typically produce better.  So don’t worry about not having any flowers right now — a healthy plant will produce blooms by the armful before you know it.

Sometimes cosmos can be hard to find in smaller garden centers.  If that is the case you may have to start from seed.  And if you’re new to starting plants from seed, cosmos are a great place to begin.   Cosmos seeds are also a great option for teaching children how to plant a seed.  They are large enough for little hands to pick up and hold.  Plus they are fast growing and produce lots of flowers, often in an array of colors.  (Which makes them way more fun than say, Brussels sprouts.)

Growing Cosmos From Seeds

The simplest way to start cosmos from seed is to directly sow them into the ground.  Wait until the last hard frost has passed, and scatter your seeds in a sunny location with soil that has not been amended with compost.  Cover them with a quarter of an inch of soil.  Water the seeds and wait a few weeks, then thin them to where your remaining plants are about a foot apart.

If you’re going to start your cosmos seeds indoors, do so about four weeks before your last frost date.  Place your seeds in a small container with drain holes, or use compostable peat or fiber pots.  Fill three quarters of the container with a seed starting mix or flower potting soil.  Drop a few seeds in each container or seed-starting cell, and cover them with soil.  Add enough water so that the soil or starting mix is damp, but not over-saturated.  Seeds will germinate and sprout best if the temperature is kept warm, but not hot…about 75-80 degrees Fahrenheit.  Once the sprouts have emerged — usually in about two weeks — place them under grow lights or in a sunny window with southern exposure, and keep them watered.  When the plants are about three inches tall, snip most of your seedlings down, leaving only one plant per container or cell.  Once your cosmos are about a foot tall, it’s time to transplant them into the garden.

Planting and Caring for Cosmos

Select a sunny location for your cosmos to call home for the next five or so months.  Plant your young (non-flowering) cosmos into a hole a little larger than the original container.  The entire root system should be underground, but the first set of true leaves should be above ground.  Water the new transplants and remove any competing weeds from your garden bed.  Once the plants become established in the garden, they should be able to take care of themselves.  All you have to do now is cut flowers for flower arrangements.  The more flowers you cut, the more flowers will grow.  If you choose to leave them any of them to spruce up your garden, just be aware: if you don’t want the cosmos to reseed themselves next year, you’ll have to stay on top of removing withering and fading blooms.

One last word: if you are growing one of the large varieties — that is, one that will grow taller than four feet — it will need a trellis for support.

growing cosmos

Cosmos have a wildflower charm.


Cosmos Growing Guide – Quick Reference

-Life span

Annual – tolerates a light frost, but not a hard freeze


Grows from seed

Can be transplanted or sown directly into the ground

-Bloom season

Summer to fall

-Bloom colors

Pink, white, lavender, red.  For a dramatic look try Chocolate Cosmos, which are deep red, verging on black

-Bloom size

Medium to large depending on variety


Outdoor plant, does not do well indoors

Full sun, at least six hours

Good for growing zones 3-10 (if growing below zone 4, direct sowing is not recommended)


Below average soil.  Not too much organic matter.  Great for new beds.  (Turn dead plants back into the soil to improve organic                  matter for other plants next year.)

pH range 5.0-8.0…Alkaline, Neutral, and Acidic


Great for cottage gardens, cutting gardens, back of flower beds, and English-style gardens

Flower arrangements and cut flowers

Gardening with children

Attracts beneficial insects

Smaller varieties grow well in containers


Remove old flower buds throughout the blooming season if you do not want flowers to reseed and come back next year.  They can become invasive.