There is something comforting in a $1.49 plastic six-pack of cosmos seedlings or a paper packet loaded with cosmos seeds -- an even thriftier choice. At a time when "new" annuals like bacopa and angelonia are selling for several dollars a plant, it's a relief to adopt the familiar cosmos and know virtually for certain that they will flourish.
Cosmos have been reassuring gardeners ever since the 1930s, when breeders first coaxed cosmos to bloom earlier than the native Mexican species, and the flower-growing public has been hooked. Like almost every garden plant, cosmos have had fashionable and unfashionable periods, but basically have remained a staple if not always a headliner. Their versatility is certainly one reason why: They are easy to tend and productive over an extended period; make excellent cuttings for bouquets; work hard as long-blooming filler in summertime, when the perennial garden needs some annual reinforcement; make great background plants for the rear of a border; and perform nicely in containers, bedded out on their own as a mass planting, or sown informally over a large area to simulate a flowering meadow. And then there are their simple good looks: No wonder their name comes from the Greek word kosmos, meaning "beautiful thing."
Horticulturally, Cosmos is a simple genus. The annual ones we grow are generally from one of two species: Cosmos bipinnatus, or lace cosmos, familiar yellow-centered daisy flowers ranging from white to pale pink to reddish or purplish pink (the one called 'Yellow Garden' is the notable exception colorwise), and C. sulphureus, the sulphur cosmos, whose blooms span the sunny range from vivid yellow to bright orange to near red. The other obvious differences are in the species' flower size and foliage. Lace cosmos' blooms are about three inches wide; sulphurs' are a bit smaller but often have more rows of petals. The bipinnatus types have ferny foliage of a very delicate texture; sulphureus leaves resemble those of marigolds (another member of the Compositae, or Daisy, family to which Cosmos belongs).
The annual cosmos are incredibly easy to grow and don't even require a head start indoors to perform well. Simply sow the seeds under a one-eighth-inch covering of soil right where you want them to grow, after temperatures outdoors have stabilized to at least 60 degrees Fahrenheit (or indoors, if you insist, about six weeks before final frost). These heat-loving plants don't want much fertilizer, particularly nitrogen -- overfertilizing will produce bushy stems and lots of foliage but few blossoms. Cosmos will not flinch at dry summers, and in fact flourish where the soil is lean or rainfall sparse.
If spent flowers are deadheaded regularly, cosmos can be kept in bloom for many weeks. When, despite such efforts, the profuse flowers on the taller annual types begin to wane, usually in midsummer, cut the whole plant back by about half its height to prompt another late-summer-through-fall display. Sowing a second, later crop of seedlings in midsummer to replace the original plants is another way to maximize bloom time in your garden.
Perhaps the only place you can go wrong with cosmos is failing to read the fine print -- on the label, that is. Although they look the same in infancy at the garden center, all cosmos varieties are not equal; even the closest cousins within a single species -- such as 'Sensation' and 'Sonata,' both derived from C. bipinnatus -- do not substitute easily. Height is the critical factor: Check to see how tall a variety will grow, or you may end up with an ungainly five footer as edging when you wanted a petite eighteen incher.
You want 'Sensation' for the back of a border, for cutting, or for any application where its impressive height (about three to five feet) would be welcome. Dainty 'Sonata,' on the other hand, averages one and a half feet, making it the choice for potting, edging, filling in a perennial garden of medium-size plants, or bedding out on its own or in combination with annuals such as purple Verbena bonariensis or lavender-blue Salvia farinacea 'Victoria' for a summertime of consistent color.
Similarly, among hot-colored sulphureus types, there are very double-flowered types like 'Diablo' and 'Sunset,' and singles such as 'Sunny Red,' though height again is the major differentiating factor (the former reach two to three feet; the latter about one foot). They combine beautifully with blue heliotrope, silver dusty miller, or any of the smaller dahlias, to make a long-blooming annual border.
Even at season's end, don't be too quick to pull up withering cosmos plants. Birds (particularly gold finches) love to snack on their seedheads in autumn, and the seeds that they miss may drop to the ground and reward you the next year by sprouting into a whole new crop -- just another of the many ways that cosmos aim to please.