Cosmos (Cosmos bipinnatus) sometimes are called Mexican Aster or Cut Leaf Cosmos. They are sun-loving, bright, cheery flowers that are easy-to-grow additions for any garden. Cosmos are profuse bloomers and make for excellent cut-flower arrangements. Growing cosmos indoors from seed requires a little preparation, plenty of room and a sun-filled location.
Fill a germination tray with seed-starting potting soil. Compress the soil firmly using the base of a 3- or 4-inch pot.
Pour water into the germination tray until the soil is well-dampened. Place the tray where it will receive eight to 10 hours of light a day, preferably not direct sunshine, and where the temperature is 70 to 75 degrees F.
Create shallow, 1/2-inch deep rows in the germination tray using the long side of a ruler or any object that can create a straight line. Space each row at least 2 inches apart.
Plant the cosmos seeds 1 to 1-1/2 inches part. Fill in the shallow rows with 1/2 of an inch of the seed-starting mix. Fill a spray bottle with water and drench the surface of the soil in the germination tray. Check on the germination tray every day, misting the surface of the soil to keep the growing media moist as needed.
Thin out the scraggly cosmos seedlings by snipping them off at the soil-line when they are about 1 inch tall. Leave the hardy cosmos seedlings to mature, making sure that each seedling is spaced 1-1/2 to 2 inches apart in the germination tray.
Transplant the cosmos seedlings into 1-gallon pots when they are between 1 and 2 inches tall.
Transplanting Cosmos Seedlings
Scoop in enough potting mix to fill each of the 1-gallon pots to within 1/2 inch from the top of the rim. Pack the potting mix firmly by hand, a small piece of wood or the base of a 3- or 4-inch pot. Spray the surface of the soil until it's drenched.
Remove a cosmos seedling from the germination tray with a large spoon.
Dig a hole in the center of each 1-gallon pot that is twice the width of the root system of the cosmos seedling but about the same depth. Place the cosmos seedling into a previously dug hole. Fill the hole with potting mix.
Set each of the transplanted cosmos seedlings near a south- or southwest-facing window so that they will get at least 8 to 10 hours of sunlight a day. Leave the cosmos in the sunny location to grow and blossom, and provide water when the soil in the pots starts to dry out.