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Garden cosmos are annuals derived from several Cosmos species native to Mexico and other areas of North and South America. They have long been treasured for their simple but profuse flowers, which can occur throughout summer and into autumn, and for the frothy mass of fine foliage that they create.
Recent years have seen diverse selections come to market, moving well past the typical tall pink- or white-blooming plants of the past. Cosmos can now be had with varied heights, colors and flower form.
Cosmos are easy to grow from seed started indoors or right in the garden. Indoor sowing should be timed so that the young plants can begin to move outside within just a couple weeks, because otherwise they can become spindly. In Mastering the Art of Flower Gardening, author Matt Mattus recommends sowing a handful of cosmos seeds in the garden every few weeks until early August, a regimen that will provide plenty of flowers for both the garden and fresh-cut bouquets throughout the summer.
Cosmos require full sun and warm temperatures, but their billowy shape necessitates some protection from wind. This may come in the form of a fence or screen, or simply some supportive or sturdier companion plants. Good drainage is important for cosmos, but they will grow in otherwise poor soil conditions. Too rich a soil can spur lanky stems, an abundance of leaves and a lack of flowers. Regular water is appreciated, although cosmos can withstand a dry spell.
Deadhead cosmos to promote continuous bloom. The whole plant can be sheared back by a third to cue fresh growth and flowering.
Cosmos are versatile companions, thanks to their (usually) simple flower shape, upright habit and feathery foliage. They match well with dahlias, zinnias and marigolds—all of which also trace their roots to Mexico and whose flowers can have a similar shape and size to cosmos (depending on type). Other daisy-type summer bloomers like perennial coneflowers (Echinacea) and black-eyed Susans (Rudbeckia) can punctuate a drift of cosmos, too, and their chunkier blossoms and coarse leaves offer some contrast.
Cosmos also mix well with plants that have a similar wispy frame but flowers that look different, like sweet peas, with their tendriled stems and stout pea flowers, or Verbena bonariensis, with its tufts of tiny purple flowers atop airy stems. (Note: This plant is reported as invasive in parts of the United States' Southeast and West Coast. See its listing at invasiveplantatlas.org.)
For a third look, try mixing cosmos with very coarse plants, such as the tall, large-leaved Nicotiana sylvestris, whose chandelier of flowers provides even further contrast; or canna, whose bold, shiny and sometimes colorful leaves make a good foil for feathery cosmos foliage. Warm-season ornamental grasses are another interesting match for cosmos, especially those with rich color, like Cheyenne Sky switchgrass.