Tomato plants grow in the spring, produce in the summer and fall, and die back with the winter frosts. Though this plant is a perennial in warm frost-free climates, Northern Hemisphere gardeners grow tomatoes as annuals. Start seeds in a greenhouse six to eight weeks before the last frost date to get a head start on the growing season. Harvest standard variety tomatoes in 65 to 70 days after planting the seeds. Some early varieties come ripe for harvest in as few as 55 days while larger heirloom varieties take as long as 80 days to mature.
Fill a seed-starting tray or cell pack with compost. Cell packs of six 2-inch pots work well for starting tomato seeds. Pat down the compost and water until it is damp all the way through.
Put two to three seeds in each compartment or scatter them lightly over the top of a seed-starting tray. Cover the seeds with a 1/8-inch layer of vermiculite and place them in a sunny window to germinate. Keep the compost damp but not saturated with frequent light waterings.
Thin the seedlings, which will germinate in about two weeks, so that there is one healthy plant in each cell or, in a seed-starting tray, the plants are spaced 2 to 3 inches apart. Keep the seedlings in the sunny windowsill and water lightly every day or so to keep the soil damp.
Begin the hardening off process when the seedlings are 6 inches tall and the outside temperatures are above 60 degrees F. Move the seedlings outside for a few hours a day and bring them back in at night or if the temperature threatens to drop below 60.
Prepare to transplant the seedlings when they are about 6 to 8 inches tall and the soil temperature has risen to a consistent 60 degrees F or above. Plan to dig holes in the planting bed 24 inches apart for staked tomatoes, 36 inches apart for caged tomatoes, and 3 to 5 feet apart for low-growing bush tomatoes. If you are not sure about the growing habits of your plants, space them 36 inches apart.
Dig the planting holes 4 inches wide and 4 to 6 inches deep. When you transplant the seedlings you will bury the root ball and 2/3 of the stem so make the holes deep enough to accommodate your seedlings.
Gently shake or wiggle the tomato seedlings from the starter cells or planting tray. Place one seedling in each hole so that the stem is buried 2/3 in the soil. Tomatoes send out roots from their stems; by planting the seedlings deep, you will get stronger, healthier plants. Fill in the soil around the root ball and pat it down firmly to eliminate air holes.
Water the soil around the transplanted seedlings until it is damp to a depth of 6 inches and spread a 2-inch layer of mulch around the base of each plant; clean straw works well. Place stakes or cages around tomato varieties that require a structure to grow on.