Trays & pots of germinating seedlingsThe main reason to sow seeds in trays and pots is to start your plants early, when it's still too cold to plant a garden outside. Start them indoors -- maybe beneath artificial light, next to a bright window, or else outside in a greenhouse or a coldframe. Then when it's warm enough you can transplant the seedlings  into the garden.

An advantage to planting  your seeds in store-bought potting soil is that such soil should be free of disease organisms, weed seeds and insect eggs that may hatch into plant-eating bugs.

If you use good potting soil, then you just need to make sure your growing seedlings get the following:

Most seed packets tell you how deeply to plant the seeds, and how far apart to space the resulting seedlings when you transplant them. For example, leek seeds are usually planted ΒΌ-inch deep. Most seed packages also tell how many days it takes for the seeds to germinate.

Usually potting soil has enough nutrients in it so you don't really need to fertilize your seedlings. However, if you prefer to fertilize them, be sure to follow directions on the fertilize bottle or bag, for it's easy to kill plants with too much fertilizer!

It's nice to have pots and trays designed specifically for growing seedlings, such as the white, styrofoam tray with all the cells at the lower left in the picture, but really you can use all kinds of containers. Just be sure that each container has holes in its bottom to keep water from pooling in it. The holes should be about as wide as a straw. It's good to place trays and pots inside other shallow trays, such as the blue ones in the picture. Then you can pour water into the larger trays and the water will seep up to the seedlings or germinating seeds from below. This manner of watering disturbs fragile seedlings less than if you sprinkle water from above.

chili seedlings in a tray
Chili Seedlings
cabbage seedlings in a tray
Cabbage Seedlings

Sometimes I transplant certain seedlings twice -- especially those developing from very small seeds, and those whose seeds I'm not sure will germinate well. First I sow them thickly in a large, flat tray. Once the seedlings are up, I separate them and plant them in individual pots or in trays farther apart, so each plant will have plenty of growing room. Above you see seedlings in their second transplanting.

Notice that I always lable my trays. It's easy to forget which seedlings are which, and often seedlings of different kinds of plants look very much alike.