Many gardeners choose to get an early start on the growing season by planting vegetable seeds indoors and then transplanting the seedlings to the outdoor garden after the danger of frost has passed. In climates with a shorter growing season, slow-growing vegetables such as broccoli, eggplant and tomatoes are often started indoors so they can be harvested before the cold weather arrives in the fall. Growing plants from seeds can also be less expensive than purchasing plants at a nursery or garden store.
Select the containers. The containers should be at least 2 or 3 inches deep. Shallow containers can dry out too quickly. Containers with drainage holes are best to ensure the roots of the plants will not be flooded after watering. Plastic trays or flats, pots, multi-celled containers and even simple materials such as milk cartons all make suitable containers. Containers should be washed before use in warm soapy water and then rinsed with a 10 percent bleach solution.
Choose the growing medium. Don't use soil from your garden. It may contain organisms that could lead to plant disease. Horticultural vermiculite is recommended, or a prepared mix from a garden supply store that often contains vermiculite, perlite, peat and fertilizer. This material is loose enough that water and air can reach the seeds, encouraging root growth.
Plant the seeds. After filling the container with the growing medium, tap it a few times against a hard surface so the medium will settle. Water the container and let the excess water drain. Make shallow furrows in the container two inches or more apart so the seeds have room to grow. Check the seed packet for the correct depth of the furrow for the particular plant. Press the seed lightly into the soil. Cover it with a small amount of growing medium. Using a spray bottle, moisten the seeds after planting.
Set the containers in a warm place. Start the seeds in a warm area such as the kitchen. Cover them with newspaper, foil or plastic wrap. After seedlings sprout, move them to an area with a source of light. They need 12 to 16 hours of light each day. If the natural light from a window is not sufficient, put them under a fluorescent light, about 6 inches from the light source.
Thin the seedlings. Seedlings are thinned so they have room to grow. After thinning, plants should be spaced at least one inch apart.
Transplant the seedlings. Seedlings with two to four leaves are ready to be transplanted to larger containers. Putting them in deeper containers encourages the roots to grow deeper, strengthening the plant. Liquid fertilizer at half strength should be applied, and the plants should be watered to keep the soil moist. Prepare the young plants for the outdoor garden by taking the containers outside for gradually longer periods, from three hours up to 24 hours.