What are Grafted Vegetables?
Grafted vegetables are superhero vegetables: stronger, bigger, faster, more able to fend off foes than regular vegetable plants – and they deliver a more abundant harvest! Grafted vegetables are created by attaching the top part of one plant (the scion) to the root system of a separate plant (the rootstock). When their tissues heal, they fuse into one Super plant with the best qualities of each: the rootstock contributes vigor and disease resistance while the scion is chosen for exceptional fruit flavor or quality. The resulting plant is more vigorous and productive, going above and beyond the call of duty to bring big, beautiful, wholesome harvests to gardeners everywhere.
After experimenting with several methods, we decided to use the Japanese top-grafting (or tube-grafting) method, a new technique that is fairly simple but must be performed in carefully controlled conditions to avoid stressing the vulnerable, freshly grafted plant. We use a number of different rootstocks from Holland, according to which is best suited to the scion variety’s growing habit and kind of fruit. When both rootstock and scion have reached the ideal size, we sever the seedlings, then use a special clip from Japan to attach the upper stem and leaves of the scion to the lower stem and roots of the rootstock. The stems must be the same diameter and shape so their vascular tissue can align, remap, and reorganize, allowing water and nutrients to flow up the stem.
Once the scion and rootstock have been clipped together, we place them in a healing chamber which regulates temperature, humidity, and light to create the best conditions for the vascular tissue to grow together and the scar heal over. Over the next week or two, we slowly reintroduce the seedlings to natural conditions. After another week or two in the greenhouse, they are ready to be transplanted into the garden.
The graft is strong as nails once it is completely healed, but the plants must be handled carefully throughout the grafting process to avoid disturbing the graft union and ensure optimal conditions for the new plant to heal.
History of Grafting Vegetables
For centuries, grafting has been used in agriculture to enhance the health, yield, and fruit quality of woody species like fruit trees and grape vines. Large scale production of grafted vegetables emerged in Asia, where land has been intensively cultivated for many years. In the 1920s, growers there found that grafting watermelon plants onto squash or gourd rootstock significantly reduced the incidence of fusarium wilt. Today, 81% of Korean vegetables and 54% of all Japanese vegetables (95% of Japan’s watermelons, oriental melons, greenhouse cucumbers, tomatoes and eggplants) are produced on grafted plants. Vegetable grafting is also popular throughout Europe, especially in Greece, Spain, France, Italy, and Morocco.
In the U.S., the technique’s potential for improving plant health and fruit yield without harmful pesticides, chemical fertilizers, or soil fumigation/steam sterilization is catching the attention of greenhouse produce growers and organic farmers, especially on the East Coast with its short growing season. Together, the environmental and economic benefits have made grafted vegetables a worldwide movement among commercial produce growers.
Benefits of Grafted Vegetables
Advantages of grafted vegetables include enhanced plant vigor, better disease resistance, tolerance of environmental stresses, and heavier crops that are produced over an extended harvest period.
Disease resistance – This is probably the most important reason commercial growers initially turned to grafted vegetables. After Asian growers’ success with grafted melons in the 1920s, tomato grafting emerged in the 1960s as a strategy to avoid soil-borne diseases like bacterial wilt, which can be hard to eradicate in a tomato crop because of its wide range of hosts and ability to persist for years in the soil.
When a grower raises tomatoes and other solanums (potatoes, eggplant, peppers) in the same fields or in the ground in greenhouses year after year, a range of fungal, bacterial, viral, and nematode diseases can become established in the soil, leading to a poorer yield with each subsequent harvest. Grafting has been found effective against verticillium wilt (Verticillium albo-atrum, V. dahliae), fusarium wilt (Fusarium oxysporum), Fusarium crown rot, corky root rot (Pyrenochaeta lycosersici), root-knot nematodes (Meloidogyne), bacterial wilt (Ralstonia solanacearum), Tomato mosaic virus, and Tomato spotted wilt virus.
Grafting may also help plants ward off three other big tomato problems: early blight (Alternaria solani), late blight (Phytophthora infestans), and blossom end-rot (a physiological disorder caused by low calcium levels). Tomato plants that are less vigorous to start with or weakened by nematodes are more susceptible to early blight, so super-vigorous grafted tomatoes with enhanced resistance to pests and disease should fare better. Blossom end rot is exacerbated by fluctuations in soil moisture or when plants are stressed by drought. Grafted tomatoes, with their superior root structures, allow continued uptake of moisture and nutrients even in less-than-optimal conditions. It’s not clear whether grafted tomatoes are resistant to late blight (infamous for causing the Irish Potato Famine of the 1840s and still responsible for widespread losses of potato and tomato crops each year). The spores of the late blight fungus can’t survive in soil but are carried through the air, bypassing grafted tomatoes’ protective root system and landing directly on fruits or foliage. However, we expect that grafting would confer some level of protection because in general, the healthier and more vigorous the plant, the greater its ability to fight off a disease or pest. (See this article from Ohio State University for more research)
Tolerance of environmental stresses – Grafted plants are also more tolerant of environmental stresses like salinity or temperature extremes. Regular tomatoes often react to heat (temperatures over 86 degrees Fahrenheit) by dropping their blossoms, but we’re closely watching our grafted tomatoes to see if they can come through a heat wave with flowers intact. And with the ability to withstand hotter and cooler temperatures comes an added bonus – an extended growing season. Cool-season gardeners should be able to set out plants a little earlier in the spring and continue harvesting ripe fruits longer into the autumn, while warm-climate gardeners who sometimes get tomatoes to overwinter can increase their chances of winter and spring harvests with hardier grafted varieties.
Increased vigor and yield – Even for those growers and gardeners fortunate enough to have fresh soil and ideal growing conditions, grafting has its advantages. The vigorous rootstock increases the uptake of water and nutrients, for healthier and more beautiful plants and greater harvests without using chemical pesticides or fertilizers.
Overall, grafted vegetables lead to bigger harvests of better quality fruits over a longer period with fewer harmful inputs. All tomatoes can benefit from grafting, but heirlooms, which are generally less disease resistant, can increase yields dramatically when grafted on special rootstock. Organic farmers find that they can achieve better resistance to pests and disease without soil fumigation or chemical pesticides, as well as higher yields without chemical fertilizers. Greenhouse produce growers are turning to grafted vegetables because the plants thrive and produce even when crops can’t be rotated frequently to fresh soil.
Growing Grafted Vegetables
Handle carefully when planting to avoid bending or putting pressure on the graft. If your plant comes with a grafting clip, you can either remove it carefully or leave it on and it will slip off as the plant grows. Mighty ’Mato should be planted at the same level as in its original container so that the graft remains above the soil. If planted too deeply, Mighty ‘Mato’s adventitious roots will develop from the scion, negating the disease and nematode resistance of the rootstock.
Avoid over-fertilizing your Mighty ‘Mato because this will promote extra foliar development and delay fruit production (just feed it as you would a regular tomato). With its large and vigorous root structure, Mighty ‘Mato is happiest planted in the ground, but if you decide to grow it in a container, use at least a 15-gallon size or a half wine barrel.
Pruning and Support
Pruning is crucial to contain and direct the plant’s energy to fruit production. For all grafted tomatoes, remove any suckers (side shoots) that form below the graft. After that, how much you prune will depend on whether your grafted tomato is a determinate or indeterminate variety.
Pruning and staking determinate tomatoes – It’s generally agreed that these tomatoes should be pruned very slightly if at all. Because they are programmed to produce a certain amount of fruit and then stop, pruning them isn’t necessary to encourage further growth or additional fruit set. In fact, removing blossoms or suckers always reduces the overall number of tomatoes. However, to improve air circulation and keep the foliage away from soil-borne diseases, you may want to remove all of the bottom branches and foliage, 10 or 12 inches up the main stem. This is especially important in wetter or cooler climates. Some gardeners also recommend pinching off the first few sets of blossoms, until plants are well-established in the garden; and some suggest removing about half the total blossoms, believing that the fruits left to ripen will be larger and more intensely flavored.
Determinate varieties usually grow only 3 or 4 feet tall, and many sources say supports aren’t necessary for these bushier, sturdier plants. However, a short stake or cage will prop up the branches as they start to sag under the load of ripening fruit, holding them up off the ground and away from slugs and moisture that can cause rotting or spread disease.
Pruning and trellising indeterminate tomatoes – Opinions vary widely on how to prune indeterminate tomatoes. But without pruning, a supervigorous grafted tomato will just keep growing, producing a riot of foliage; by strategically removing some of this excess, you will allow plants to ripen larger, tastier, earlier tomatoes and keep foliage up off the ground, reducing losses to pests and disease. As with determinate tomatoes, you can improve air circulation by removing all branches and suckers (the little shoots that emerge where each branch meets the main stem) from the bottom 10 or 12 inches of the stem.
Our favorite way to grow a grafted tomato is to prune it to two laterals (the main stem plus the side shoot or sucker just below the first fruit cluster) which are then trained up a trellis into a V shape.
We make our trellis from two eight-foot-tall poles, placed at either end of a garden row, joined at their tops by a crosspiece of wood (or a taut piece of string or wire) that parallels the ground. We anchor a wooden stake in the ground next to each tomato plant. Then, starting at one of the end poles, we run a string from the top of the trellis down at an angle to the first stake, then back up at an angle to the top of the trellis, repeating the process (and connecting the string to each stake) until we reach the top of the pole at the other end. This creates a repeating V-shape in the trellis, giving each plant’s two vines its own rope to climb.
Throughout the summer, continue to pinch off all the suckers that appear on both leaders. If plants seem to be getting too dense and bushy, you can remove a branch of foliage here and there to encourage air circulation. But don’t get carried away and prune too much – plants need leaves to manufacture the energy that creates sugar in the fruits; and tomatoes can be subject to sunscald if too much of the plant’s leafy canopy is removed. Depending on your soil and climate, you may need to prune more or less – we look forward to hearing about regional differences in growing Mighty ’Mato!