Gardeners most often grow two species of cosmos in their flower gardens. Cosmos bipinnatus grows 3- to 5-feet tall and bears white, pink or crimson-burgundy flowers. Species Cosmos sulphureus matures 3- to 6-feet tall with gold, orange or orange-red blossoms. Both native to Mexico, these cosmos plants grow as annuals -- germinating, flowering, dropping seed and dying within one growing season. Plant seeds or young transplants only when there is no threat of frost in your region.
Sow the flaky brown seeds of cosmos outdoors after the last expected frost date in spring. You may sow seeds one week before the last frost date, but if any germinating sprout emerge and frost threatens, the seedlings must be covered overnight; otherwise they will be killed. Cosmos grow quickly, so you should sow successive plantings every four weeks across the growing season. This ensures a wide array of plant stages and a more continual production of flowers. Do not sow cosmos seeds within 70 days of the expected first frost date in fall; otherwise they may not grow enough to produce flowers before the frost kills the plants.
Starting Plants Indoors
Easy to grow from seed, you may want to start cosmos seeds indoors in soil trays 2 to 4 weeks before the last spring frost date. Place the trays in a warm, sunny window, and keep the soil evenly moist. Within 7 to 21 days, the seedlings emerge from the soil. Germination and growth is fastest when temperatures reach in the 15.6 to 23.9 degrees Celsius range. Grow the seedlings in as much sunlight as possible, rotating the plants to prevent leaning toward the sun. Transplant them outdoors where they are to grow after the threat of frost passes.
Many cultivars of both species of cosmos exist, providing you a wide array of plants with varying blossom colours or mature plant heights. Plants grow faster in moist, fertile soil that is warm. Hotter air temperatures also increase growth rates. Butterflies and bees pollinate cosmos flowers, leading to abundant production of seeds. Often these seeds drop to the soil and sprout. They replace the mother cosmos plant that may wither away. Clip off old flower heads if you want to prevent uncontrolled seeding.
In most of the continental U.S., grow cosmos from late spring to early fall. However, in high mountain elevations or other cool summer areas, ample soil and air warmth occurs only in summer. In subtropical areas, like the Desert Southwest and peninsular Florida, frosts are infrequent but summers are very hot. Plant cosmos in these regions from fall to late spring. In Hawaii and Puerto Rico, plant year-round, but try to avoid the wet season as it leads to rotting of cosmos' plants and seeds.