Starting summer flower seeds indoors gives your garden a head start on gardens whose flowers go directly in the outdoor soil. The indoor start is especially nice for locations with short growing seasons. When you start the seeds in the house early, you'll have strong seedlings by the time the last frost has passed and it's time to transplant. The seedlings will be in full bloom by summertime.
Read the directions on the back of each flower seed packet. Every type of seed is different in depth of planting and time to reach germination. The germination time is important; you want the seeds to become seedlings by the time the last frost has passed. Asters, for example, need to be planted six to eight weeks before transplanting, whereas cosmos and marigolds take only four to six weeks.
Fill each seeding tray with commercial potting soil, available at garden centers and nurseries. Leave ¼ to ½ inch free at the top of each seeding segment.
Spray the soil with water, just enough to moisten it. Mix the soil until it is moist from top to bottom.
Plant a flower seed in each seed compartment, at the depth suggested on the seed packet. Smooth soil over the seed hole.
Tear off a piece of kitchen plastic wrap and lay it on top of the tray. This will keep the soil moist and warm, encouraging the flower seeds to sprout.
Mark each section of seeds by type of flower. This will help when you plant the seedlings outdoors, because they will not have blooms at that time.
Place the seed tray near a sunny window, preferably on the north or south side of the house. As the sun moves east to west, these windows will receive the most sunshine. Keep the room temperature from 65 to 75 degrees Fahrenheit.
Check the soil in the seed tray compartments each day. Spray them lightly with water as the soil begins to dry. Remove the plastic wrap as soon as the seeds germinate. Continue to spray the soil with water when it starts to dry.
Transplant the seedlings after they have grown the second set of leaves and the last frost has passed. Acclimate them to the outdoor weather by placing the seeding tray in a protected area such as a porch, patio or deck for a few hours. Increase the time outdoors each day until the seedlings have been out for a full 24 hours. Then they are ready for transplanting in the yard or in outdoor pots.