Don't plant outdoors until very cool weather is over, if you can help it. Of course, you want to get them started in the garden as early as you can in order to get ripe fruit that much sooner. On the other hand, if a cold spell occurs after they have been planted outside, they may suffer a serious setback. Sometimes it is difficult to tell for sure what the weather will be like two or three weeks ahead of time.
They need temperatures averaging aroung 60 to make growth. If it doesn't drop much below 55 for several hours at night, they should be all right. If frost is threatened, put caps made of newspaper over them for protection.
Choose plants 8 to 10 inches tall, dark green in color, with thick stems and no yellow leaves. If they are growing in small plastic or peat pots, pick off any flowers or fruit that may have developed. They will sap the plant's energy supply at a time when it is trying to become established in a changed environment.
Harden them before planting them outdoors. Do it by putting them outside for a couple hours the first day, and increasing the length of time each day for about a week.
Plant them outdoors on a cloudy day, or do the job in late afternoon or early evening, so the plants have a little time to adjust before getting hit by a day-long dose of bright sunshine.
If the plants come in peat pots, break off the top edges of the pot and plant into the soil. If there are no roots protruding, break off the bottom of the pot. It also helps to throughly wet the peat pot by soaking it in water. The idea is to make it easy for the roots to penetrate the pot and enter the soil.
Dig the hole for th plant an inch or two deeper than the height of the pot, so the plants will be a little further into the soil in the garden than they were growing in pots in the greenhouse or garden center.
Plant them in a location where they will get full sunlight, away from the roots of trees and shrubs which would complete with the tomato plants for nutrients and moisture.
Give them a starter solution when you plant them. After digging the hole and putting the plant in it, apply a pint of the starter solution around the roots. To make the starter solution, dissolve about three tablespoons of 5-10-5 fertilizer in a gallon of water.
Select varieties that resistance to Fusarium wit, a disease that can destroy your plants just as they are about to provide ripe fruit. These include Better Boy, Supersonic, Burpee's Big Girl, Harris' Moreton Hybird, Park's Whoppers, Beefmaster Terrific, He Man and Bragger.
Grow your tomatoes in cages. You can make the cages with concrete reinforcement wire and put them over the plants when they are large enough to require support. They should be 18 to 24 inches in diameter and 3 1/4 to 5 feet tall. When the soil gets good and warm, much the plants to control weeds and help maintain soil moisture.