Growing Annuals from Seed
Growing annuals like zinnias, marigolds, and tomatoes from seed is easy and economical. The exact time to start seeds varies with the plant and with your location. In general, it's about six weeks before the date of the average last frost in your area. Seed catalogs and packages usually list the number of weeks needed when starting seeds indoors.
Use a potting mix specially formulated to start seeds; you can find such mixes at almost any garden center or hardware store. The mixes are sterile, provide the drainage that seedlings need, and are sometimes enriched with fertilizer. Fill peat pots or other seed-starting containers with the mix and thoroughly moisten it. Then sow your seeds of choice following the directions on the packet. Place the containers on a tray in a very sunny window or under a grow light. Never allow them to dry out. As soon as the seedlings develop four leaves, use scissors to thin out your seedlings, leaving just the healthiest looking ones.
After all danger of frost has passed, place the trays of seedlings outdoors in a sheltered place for several days, gradually lengthening the time they are exposed to the sun and wind to prevent scorching the leaves, before transplanting them into the garden.