Captivating Cosmos

Cosmos_DBG_LAH_7489Do you enjoy big flowers with bright, showy colors and carefree maintenance? It’s hard to beat annuals for season-long impact. Whenever I think of annuals, I immediately think of cosmos, one of the very best annuals for Colorado gardens.

There are currently thought to be 36 species in the genus Cosmos, but the two most often grown in our gardens are C. bipinnatus (left) and C. sulphureus. (There are two other Cosmos species in cultivation. One is a frost-tender, tuberous perennial known as Chocolate Cosmos, C. atrosanguineus. The other is Cosmos parviflorus, a wildflower of the western United States.)

Cosmos bipinnatus_FedWayWA_20090920_LAH_0731C. bipinnatus, commonly called Garden Cosmos or Mexican Aster, is the most popular. While native to Mexico, it has escaped cultivation throughout the western hemisphere, as well as Australia, Asia, and southern Europe. It’s so easy to grow that it is considered a weed in some places.

The plants are considered half-hardy annuals, and grow to between two and four feet tall. The finely dissected leaves are bright green, and held on stems so covered with blooms that they tend to collapse under the load. Daisy-like blooms come in shades of red, pink, and white, and may be single or double, with a fluffy yellow center. Extensive breeding has resulted in a variety of color combinations and forms, such as bicolors, “seashell” petals curled into out-facing tubes, double or “pompom” flowers, and flowers with picotee edging.

C sulphureus lives up to its common names, Sulfur Cosmos and Yellow Cosmos, with flowers in hues ranging from lemon yellow to a deep, red-orange. In both color and shape, they’re reminiscent of Coreopsis, a close relative. The two-inch blossoms are held above the ferny foliage by wiry stems. Height ranges from dwarf varieties only a foot tall to seven-foot giants.

Cosmos sulphureus_DBG_LAH_3245

Sulfur Cosmos is native to tropical parts of the Americas, from Mexico to northern South America, although it has since become established in most parts of the world. In fact, it’s a little too well established in some areas, and is considered an invasive species by the US Southeast Exotic Pest Plant Council.

While transplants are sometimes hard to find, cosmos are easily grown from seed. Choose a site in full sun. Optimal soils are well-drained, and neutral to slightly alkaline. Avoid overly rich soil. Don’t add fertilizer unless recommended by a soil test, as too much will result in lanky growth and few flowers.

Seedlings don’t transplant well, so wait until the soil warms and then sow outdoors where the plants are to grow. Be sure to cover the seeds with a thin layer of soil, as they need dark to germinate. Germination takes one to three weeks, and the plants bloom a couple of months after that. (Some older cultivars are day-length sensitive, and won’t bloom until days are short enough.) They’ll continue to flower until killed by frost. If left to go to seed, you’ll have plenty of volunteers next spring. If you don’t want them to reseed, remove the flowers as they fade.

Cosmos is drought resistant, and isn’t bothered by insect pests or disease. A bit of crowding helps the stems stay upright, or you can add supports as needed to keep them from flopping. Pull plants when they succumb to frost.


Cosmos’ informal appearance makes them the ideal choice for an old-fashioned cottage garden, and their airy appearance is perfect for filling gaps in a border—tall ones in back, shorter ones in the middle or as a front edging. They also do well in containers, or use them for a quick-growing, temporary hedge. With such a range of heights and colors, there’s a cultivar for every part of the garden. The flowers are attractive to butterflies, bees, and other pollinators.

I would definitely include cosmos in my garden, except for one problem. Rabbits find them delicious. With a plant this perfect, I knew there had to be a catch.

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