Heirloom tomatoes are those that have been grown from seeds that have been kept as pure as possible over the years, sometimes for centuries, though some genetic change is inevitable. There are now thousands of tomato varieties in America, most of which have been hybridized, which is why heirlooms are so special. Growing heirloom tomato plants is a way to preserve history, while enjoying more pure versions of tomato varieties which often have more interesting colors and tastes.
Acquire seeds from a reputable heirloom seed source such as a seed exchange like Seed Savers Exchange, or from specialty seed houses. Brandywine and Yellow Pear cultivars are a few heirloom tomatoes recommended as suitable for outdoor growing by "Rodale's Illustrated Encyclopedia of Organic Gardening."
Sow seeds or seedlings in trays during mild temperatures, six to eight weeks before the last frost. These can be planted in trays, cell packs or small pots 3 1/2 inches in diameter. Water daily, but do not drench. Keep in an unheated greenhouse.
Move the seedlings into the ground when all danger of frost is past and soil temperatures reach about 50 degrees Fahrenheit and air temperature is at least 45 degrees Fahrenheit. Allow 3 feet between rows, and do not plant the seedlings closer than 15 inches apart.
Keep watered, and place a 1/2-inch wood dowel within a few inches of each plant so that it can be tied to it as a support as it begins to gain height during growth. It can take seven to 12 weeks from seed to harvest depending on the variety and the conditions.
Save seeds from the fruit that is most true to type to continue growing the varieties you like and even participate in the Seed Savers Exchange. Gather seeds from several different plants over the course of the season by scooping out seedy jelly in the center of tomato fruits into jars of water. Let them ferment for three to five days. The best seed will sink to the bottom. Pour off the scum and bad seeds and spread the good ones on dry paper towels to dry for two to three weeks. Store in airtight, dated containers.