Growing Cosmos From Seeds Indoors

Cosmos are a popular annual. Although they only bloom for 1 year, they will bloom all summer long, are easy to grow from seed, and are beautiful with their brightly-colored blooms and lacy foliage. Butterflies love them and they also work well as cut flowers. Cosmos Sulphureus are the most compact species of cosmos and are the best to use for growing indoors. They reach a height of less than 2 feet, unlike the Cosmos Bipinnatus, which can grow up to 6 feet, making them very difficult to grow indoors. Cosmos Sulphureus come in warm reds, oranges and yellows.

Cosmos seeds are quite distinct--crescent-shaped, with a row of beading along one edge. If you are growing cosmos indoors, you can plant them up to 4 to 6 weeks before the last frost is due in your area. Use pots to grow cosmos or a long, shallow trench. Fill the pots or trench with any indoor potting soil; cosmos are not terribly picky about soil. Using a stick, draw it across the soil in the trench, creating a planting ditch about 1/2-inch deep. Sprinkle the seeds in the ditch, about 1 inch apart or so (again, cosmos are not picky growers). Then, pinch the ditch closed over the seeds, and press down lightly. Add a little water. To plant the seeds in the pots, simply sprinkle them on top of the soil, then cover lightly with another shallow layer of soil. Add water. Place the pots or trench under a growing light set between 6 to 12 inches above the soil. Cosmos like a lot of sunlight, so make sure that they remain under the light for a good 12 hours per day. Purchase growing lights at any garden center or nursery.

Once the seeds have sprouted and are well-established, (about 2- or 3-inches tall), remove them from under the growing light and place them in a window, as long as they will still receive several hours of sunlight per day. If the cosmos seem crowded, now is also a good time to thin them out. Depending on the species, thin them anywhere from 2 to 12 inches apart. For Cosmos Sulphureus, 2 inches is fine. Encourage cosmos to bloom again after the first blooming period by pruning them back when the blooms start to wilt. Cut each branch of finished flowers back by about one-third, then use any annual fertilizer to feed them. Water and place back in the window, or place the cosmos back under the growing light to give them an extra head start on blooming.