Heirloom tomatoes are open-pollinated varieties (they will grow true from seed) that have been passed down through generations of gardeners. Heirlooms are popular because of their excellent taste and sometimes unusual appearance.
Heirloom tomatoes have two major shortcomings. Most are susceptible to tomato diseases, of which there are many, and they are not usually prolific producers. Both of these factors can be overcome by grafting heirloom seedlings to special rootstocks.
Rootstocks and grafting
When a tomato is grafted, the top part (called the scion) acquires the disease resistance and productivity of the rootstock. The most popular rootstock varieties are Maxifort, Beaufort, and Multifort. Maxifort and Multifort are extremely vigorous and are best suited to twin-leader cultivation. Beaufort is suitable for single-leader plants. All three of these rootstocks have resistance to (or at least tolerance of) a wide variety of tomato diseases.
Sow the rootstock seeds about a week before the scion seeds. Mark and date the trays of seeds.
When the seedlings reach the third true-leaf stage they are ready for grafting.
Start with six rootstock seedlings. Using a common double-edge razor blade (broken lengthwise into two pieces), make a 45-degree angle cut, about ¼ inch below the cotyledon leaves, and discard the tops.
Now take six heirloom variety seedlings, and cut those off in the same way, cutting below the cotyledon leaves. Take care to cut the stems at the same 45-degree angle at which the rootstocks were cut. Retain the tops (the scions), and discard the roots.
Place a grafting clip on each rootstock stem so that about half the clip grips the stem. Now gently slide the heirloom scion into the other half of the clip so that the two 45-degree cuts meet exactly. Very lightly mist the grafted seedlings with room-temperature water and set aside.
Repeat this process, working with about six transplants at a time, until all your seedlings have been grafted.
Place the grafted seedlings in a darkened and humidified space, such as under a greenhouse bench. Drape black plastic around the sides of the bench to keep the plants in the dark, and once or twice a day lightly mist them with room-temperature water. Keep them in this darkened "healing chamber" for four to five days.
At the end of this period, if the grafts have "taken," all the tops should be standing up and looking healthy. Discard any that have a broken graft union, or where the top is drooping.
Remove the grafted plants from the healing chamber and place them in a sheltered spot, out of direct sun, for about three days. Continue misting them once or twice a day. If any droop, put them back in the healing chamber for a couple of days to help them recover.