The basic requirements for starting seeds indoors are the same as for starting them outdoors – adequate light, well draining soil mix, water, and suitable temperature. In addition to that, you will need appropriate containers.
Containers – Many different kinds of containers can be used to start seeds. You can use clay pots or egg cartons, plastic pots, wooden flats, trays or peat pots. Or you can build wooden boxes or use containers found around your house – milk cartons, plastic cups or other empty food containers. Containers should have holes in the bottom to allow for drainage.
The best containers are those that can be fitted with a transparent plastic or glass cover to admit light, yet retain humidity. Commercially available seed propagators are available from either mail order or your local stores. These are basically plastic boxes with clear plastic lids that have holes punched in them for air circulation.
Soil – For most home gardeners, a commercial potting mix is perhaps the most convenient and reliable medium for starting most kinds of seeds. Some gardeners prefer their own soil mix recipes, such as some combination of sand, soil, peat moss, a general purpose fertilizer and limestone. Sometimes vermiculite or perlite are added to promote air circulation, drainage and water holding capacity. Garden soil should not be used unless it is first sterilized to kill weed seeds and soil diseases.
Fill containers with soil mix, lightly pressing it into place around corners and edges. Water to moisten and settle the mix in the container. If you are planting small seeds such as petunias, impatiens or begonias, carefully level the mix and then spread about a one half inch layer of vermiculite over the top. scatter the seeds over the surface and do not cover them with soil mix.
For larger seeds, such as marigolds, calendulas and morning glories, use a straight edge or trowel to make rows into the firmed mix. Plant the seeds to the depth and spacing recommended on the seed package. Planted too thickly, seedlings crowd each other and do not develop as well.
Mist or light water so that the seeds are not dislodged or washed away. Let the soil drain, then place the container in a location that provides optimum lighting and temperature conditions. Cover the container with glass or a plastic top so you can capture the humidity. Check the seeds each day to be sure the soil mix is moist and that seedlings have not emerged. Once the seedlings start sprouting, remove the cover.