Ask A Master Gardener – Classic Cosmos

Authored by: Carol Shirk

Why do you grow flowers? To attract pollinators and beneficial insects? For the sheer beauty? For cut flowers? To brighten your landscape with late-season color? To have some flowers for drying? If you answered yes to any of these questions, then you need to add some easy-to-grow annual cosmos to your list of favorite plants.

Cosmos are in the Asteraceae family and have the classic “aster” or star form in in their blossoms. There are over 20 species of cosmos as well as a multitude of varieties and cultivars. Native to Mexico and South America, they have a daisy-like flower in a wide range of colors. The name cosmos is derived from the Greek “kosmos” meaning order or harmony of the universe. Mexican priests cultivated these beauties because of the evenly placed, orderly petals, beautiful colors, and easy growth.

The two most common species for gardeners are Cosmos sulphereus and Cosmos bipinnatus. Yellow Cosmos (C. sulphereus) grows from 4 –7 feet tall. Cultivars include those in the ‘Ladybird’ and ‘Klondyke’ mix and range in color from yellow to orange to scarlet.

Cosmos bipinnatus grow from 1– 6 feet tall. Cultivars in this species include, but are not limited to: ‘Candy Stripe Mix’, Gazebo Mix’, and ‘Daydream’. These profuse blossoms are purple, pink, and white.

Cosmos seeds can be sown directly in the garden after the danger of frost has passed. But, first prepare the bed by turning the soil under to a depth of 6–8 inches, then raking it smooth. Amend the soil with some compost and work it into the top several inches before planting. Beware of making the soil too fertile; cosmos like soil a bit on the lean side. Too much fertilizer or soil that is too rich will yield abundant, fast plant growth with weak stems and few flowers.

Cosmos germinated best when the soil has warmed to 70°, so be patient in the spring before sowing. Scatter the seeds on the soil and lightly rake them in. A spot that is in full sun will yield the best blossoms. Thinning is not necessary and the additional plants will provide support for these very tall plants. However, some space between them is necessary to get the ideal plant size. Strive for about a foot between plants. Keep the seeds moist until they have sprouted, then reduce watering. Cosmos really do thrive on neglect; do not overwater or overfertilize or bloom yield will be reduced. However, do be vigilant about early weed control.

Because of their height, cosmos make stunning backdrops for other flowers or borders. They will attract a wide array of pollinators as well as beneficial insects like lacewing, tachnid flies, hoverflies, and parasitic mini- wasps.

There are very few problems associated with cosmos. They are deer resistant, pest resistant, and require minimal care. The most likely problem is their ability to reseed abundantly. Because they make striking cut flowers, take advantage of this feature and make frequent cuttings to keep seed production at a minimum. Deadheading spent blooms or cutting off the fresh blossoms will renew the plant continually so that it will rebloom frequently. Blooms can be used as fresh bouquets or dried.

Do something cosmic this summer and plant classic cosmos.

Carol Shirk

Certified Master Gardener



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