Vegetable Grafting for Healthier Plants and Better Yields
My first exposure to grafting was to “help” my grandfather graft some fruit trees on the estate where he was gardener. And I laughed as a young man when an uncle created a “frankentree” by grafting several different varieties of maple branches to a single backyard maple tree. Whether done for practical reasons or for fun, grafting is an ancient art that has recently become a widespread and growing practice in the American vegetable gardening world. Why? The photo above shows a grafted heirloom tomato plant on the left and the same variety grown from seed on the right. (Courtesy of Royal Horticultural Society). That’s why. Keep reading to learn how.
History and Benefits
The practice involves selecting a rootstock, a plant variety that offers disease resistance and vigorous growth qualities, to be the in-ground portion of the plant. Then grafting it to a scion with desirable fruit characteristics to be the above-ground part. Properly executed, the best qualities of both varieties result. Resistance to soil borne diseases and pests without the use of chemical fumigants is one claimed benefit. A more vigorous plant with significant increases in fruit production is another. Commercial greenhouse growers and space-limited home gardeners who can’t rotate crops as a disease deterrent are potential beneficiaries. So are hydroponic growers wanting higher outputs and home gardeners who love the taste of heirloom tomatoes but are frustrated by soil borne disease and pest damage to their favorite older cultivars.
Several vegetable varieties, including nightshades and cucurbits, are commonly grafted commercially in the U.S. now. Tomatoes are probably the most common. Grafting a disease- resistant rootstock to a non-resistant but desirable scion, like a Brandywine, can make a critical difference in yield for a commercial grower and help a home gardener avoid the disappointment of disease or nematode damage to a favorite summer vegetable.
The earliest known references to vegetable grafting go back to fifth century China. Development advanced in East Asian countries due to intensive cultivation on limited arable land. By the early twentieth century, grafting of watermelon and cucumber was common in Japan. In the U.S. early vegetable grafting took place in the southern states during the 1930s and 1940s where tomato scions were grafted to jimson weed root stocks to combat regional disease and stress issues. In recent years, large scale greenhouse production with grafted plants has provided improved yield and product quality, and the practice has spread. It is estimated that 40 million grafted tomato plants will be grown in the U.S. this year, predominantly by commercial growers.
While home gardeners can participate by purchasing grafted plants from seed suppliers or retail garden centers, the adventurous among us can purchase rootstock seed and supplies and try our hand at home grafting. Let’s use tomato grafting as our example.
Rootstocks are the Key
Picking seeds for the scion for grafted plants is easy. Choose the variety you want to consume. Rootstocks are less obvious, and they have different characteristics and influences on plant vigor and output. Typical rootstocks are tomato varieties that are just this side of wild. Their fruits are not considered edible. They are classified as Generative, focusing energy on root growth, or Vegetative, adding above ground growth, stress tolerance and a longer season to the plant. A few of the more common rootstocks now available are:
- Estamino: a generative stock that balances vegetative growth and fruit production and is recommended for smaller fruited varieties
- Maxifort: a vegetative stock that provides strong plant vigor and is recommended for larger fruiting varieties
- Supernatural: a vegetative stock with good disease and nematode resistance and high stress tolerance.
There are over 70 tomato rootstocks available with different qualities and resistances. Know your local conditions and the scion you want in order to make the best choice.
The basic technique for growing your own grafted tomatoes is straightforward. Start by purchasing your selected rootstock seed as well as seed for your favorite home grown tomato. Plant them simultaneously, or maybe give the rootstock, which tend to have thin stems, a couple of days headstart. The goal is to graft plants having the same diameter when the stems are 3-4 inches tall with stems about 1/16”-1/8” in diameter and with 2-4 true leaves. Typical time from planting to grafting is 14-21 days. Obviously there is some judgement involved but equal diameter trumps the other variables.
When the plants are ready, choose one of three proven grafting techniques:
- Splice Grafting, also known as top, tube or slant-cut grafting. This is the most widely used technique because it is quick and simple. Cut the rootstock and scion at matching angles. Recommended angle ranges from 35-60 degrees; it is the match that is critical. Clamp them together with a grafting clip, of which there are multiple styles available. The silicon clip in the photo below is slid halfway down over the rootstock and the matching scion section is slid into it making the most complete possible contact between the two cut edges.
- Cleft Grafting: This technique involves cutting the rootstock horizontally, removing the top of the plant, and then making a ¼-inch vertical cut down the center of the stem. Then cut the scion tip into a matching wedge to be inserted into the rootstock incision. Hold it together with a spring clip or grafting film. This technique holds the graft together more tightly than a splice graft, so a film wrap can be used to secure the union. It is more time consuming and difficult to execute, however, and the wedge can split the rootstock stem if it is too wide. The technique can also be inverted with the root stock trimmed to a wedge point and the scion v-notched to fit over the wedge. This version is called a Saddle Graft.
Cleft grafted tomato sections to be joined and clipped
Photo: Royal Horticultural Society
- Side Grafting: Also called tongue or side-by-side grafting, this technique involves cutting matching 45° wedge-shaped incisions ¾ of the way through the rootstock and scion stems. The “tongues” formed are then joined so that the cut surfaces are in contact. The graft is secured with a spring clip or grafting film and left together for 5 days. On days 6 through 8, the rootstock top and scion root section are progressively severed until the two are completely separated. This technique has a high success rate because the scion can take moisture through its root system during the healing process. Larger plants can be used and stems can have slightly different diameters. Disadvantages are that the rootstock and scion must be transplanted into a common container prior to the graft and the incisions are more difficult to make. Don’t mix up rootstock and scion plants or the results are sure to disappoint.
Illustration of side grafting incisions and joined graft, prior to clipping
Photo: Rodale’s Organic Life and Plant Grafting.com
While there are several different grafting practices commonly used, basic technique follows similar steps. We already discussed matching stem sizes and cut angles. Here is additional guidance:
- Cut both plants just above cotyledons. Leave the top two leaves on the scion, pinch off other leaves to focus the plant on healing rather than respiration. Graft within a few minutes of cutting to prevent cut surfaces from drying out.
- Success will be less than 100% so seed and graft more plants than you want to ultimately plant in the garden.
- Water rootstock and scion 24 hours before grafting and water gently immediately after grafting.
- Sterilize everything involved in the process: sanitize hands, use clean, sharp razor blades for cutting, sterilize clips if reusing them.
- Prepare a “healing chamber” to house the just-grafted plants. It should be warm (80-85° F.), dark/shaded and humid. For the home gardener, a simple approach is to cover a container garden with a plastic bag. Spray the interior surfaces prior to placing the plants inside. Don’t disturb for two days. Open to add moisture on day 3. Open again on day 5, expose to air for 30 minutes, re-humidify and close. Increase the open exposure to 1 hour and 6-8 hours on days 6 and 7, adding moisture prior to closing each time. On day 8 remove the plants from the chamber. Specific times may vary with local conditions, but the principle is to slowly acclimatize the grafted plants to avoid permanent wilting.
- If the plant wilts in the first 48 hours, the graft has failed.
Usually the scion and rootstock establish a vascular connection in about 7 days. However it takes approximately 14 days for the union to fully heal. After removal from the healing chamber, allow the plants to rest in a greenhouse environment for a couple of days and then harden them off for 5 to 7 days prior to field planting. Watch the plants’ progress and minimize stress prior to final planting.
Wind is a grafted plant’s enemy. Leaving the clips on plants can protect against wind. Or clips can be replaced with a film wrap. Film and silicone clips will drop off as the plant grows and spring clips can be removed after a couple of weeks as the graft strengthens. Support the young plants with a bamboo skewer or toothpick if warranted.
A caution: the graft joint should be above the soil line. If it is buried, the scion may root into the soil and undermine the soil borne disease benefits of the grafting process.
Care and Maintenance
Grafted plants are typically vigorous growers. They require regular pruning to prevent undesirable rootstock growth while focusing the plant’s energy on scion growth and fruit production. At the least, trim leaves and shoots below the lowest fruit cluster. There is some guidance that claims that a vigorous grafted tomato plant only requires about 10-12 fully unfolded leaves for maximum fruit production.
Make, Buy or Eat?
The grafted vegetable movement is here to stay. It provides the benefits of hybrids without the long process of genetic hybridization with no compromise in fruit variety and it boosts output. We are all eating grafted vegetables. The question is whether we want to go further and purchase grafted plants or try our hand at home grafting.
Rootstock seed costs about 50¢ a seed and clips are 15-50¢ each, depending on type. For a small home gardener, the self-grafting investment is more in time than cost. Grafted plants cost 3 or 4 times what a typical non-grafted transplant costs, but again for a small number of plants, the total outlay is reasonable given the increased yield expectations. The growth of grafted vegetable cultivation speaks to its legitimacy, for commercial and home growers, for tomatoes, pepper, eggplant, squash, cucumber and melons. Check your seed catalog and favorite garden centers and decide which part of the wave you want to ride.
“Graft is Good,” Iowa Gardener Magazine (Carol Michel, 10/16/13), statebystategardening.com/state.php/ia/print/graft_is_good
“Grafting for Disease Resistance in Heirloom Tomatoes,” North Carolina Coop. Ext., ces.ncsu.edu/grafting-for-disease-resistance-in-heirloom-tomatoes
“Grafting Vegetables,” Royal Horticultural Society, www.rhs.org.uk/advice/profile?PID=825
“Vegetable Grafting: History, Use, and Current Technology Status in North America,” HortScience hortsci.ashspublications.org/content/43/6/1664.full (October 2008. vol. 43 no. 6 1664-1669).