GRAFTING GREENHOUSE TOMATOES
By Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

Grafting of vegetable seedlings is a common practice – in Asian countries. Here in North America, the practice is just getting a foothold, mainly for greenhouse production.

Why graft?  The idea behind grafting is to take a variety with desirable above-ground horticultural characteristics (like fruit size, flavor, etc.) and connect it to the roots of a variety with desirable under-ground characteristics (like resistance to soil-borne diseases and vigorous root growth.) Grafting vegetables follows the same principle as grafting fruit trees, which has been done for a long time. In both cases the crop-producing shoot is called the scion, which is removed from its original roots and placed onto a new plant, called the rootstock.

Among greenhouse tomato growers, grafting is quickly being adopted as a way to manage root diseases and increase fruit production. Organic growers in particular can gain from grafting because growing tomatoes in soil and compost rather than in sterile media often leads to problems with weak roots, as a result of soil-borne pathogens.

Varieties for grafting.  In my neck of the woods, growers have a wide range of preferences for varieties of greenhouse tomatoes, depending on their markets and production systems. Popular greenhouse varieties include Trust, Cobra, Buffalo, as well as cherry, cluster and even heirloom-types. All of these have been used as scions. There are far fewer tomato varieties that make sense to use for rootstock. The two most common are ‘Maxifort’ and ‘Beaufort’. Both of these have tolerance to some common soil-borne diseases. ‘Maxifort’ also results in very vigorous growth while ‘Beaufort’ leads to a more moderate increase in plant vigor. These increases in vigor can be seen in both above ground-ground and below-ground growth.

Types of grafts.  Two common grating techniques are top grafting and side grafting. With top grafting the scion is completely cut off from its roots and placed on top of the rootstock stem. Side grafting involves making a partial cut into the stem of the scion plant and then inserting the cut-off stem of the rootstock into that cut. The seedling is then allowed to retain both set of roots until the graft with the new rootstock heals.

Top grafting relies on a tiny plastic tube or sleeve to hold the scion and rootstock together until the graft heals. Top grafting is quicker and bit less complicated to do than side grafting because it requires only a single complete cut through both the root and shoot portions of the graft. This technique can be used on very small seedlings.

Side grafting takes a little longer but is preferred by some tomato growers because it is a bit more forgiving. If greenhouse conditions for graft healing are less than ideal, the grafted seedling still has its original set of roots to help during the transition. Side grafting can also be done with seedlings that have become larger than is ideal for top grafting. And, side grafting seems to accommodate a bit less uniformity between scion and rootstock stem diameters than is the case with top grafting, where the two stems must be almost exactly the same size. A small clip, much like an office binder clip, is used to hold side-grafted plants together until they heal.

Starting the plants.  Rootstock varieties tend to have long thin stems so they are often sown a few days earlier than seed for the scions. Some growers will make sequential seedlings of the rootstock over several days to assure that they have the right selection of plants size to choose from when grafting. There is a narrow range of plant size for grafting. The ideal is when the stems are about 2 mm in diameter for top grafting, and 2 to 3 mm for side grafting. (Depending on conditions, plants this size usually have 4 or 5 true leaves showing.) It’s physically difficult to graft plants that are thinner than this, and the success rate declines rapidly as the stem diameter increases.

Top grafting.  Start by selecting healthy scion and rootstock plants that have the same stem diameter. Then remove the first set of true leaves on the scion to reduce transpiration during the healing process, leaving only the uppermost couple of leaves. Leave the cotyledons in place. Next, cut off the scion from its roots just below the cotyledons, at about a 60 degree angle.

Now take the rootstock, and cut off the top just below the cotyledons, at exactly the same angle as the scion was cut. (If the plants are too small the cut can be made above the cotyledon but be sure to prevent these suckers from growing later.) Slip the grafting sleeve onto the rootstock stem and gently push in the scion stem so that the cut surfaces make full contact. Wiggle gently if needed. Place transplant into a large cell tray (72s) or small individual pot and water promptly.

Side grafting. With this method, larger seedlings are used, typically two to three weeks old, since the stem diameter must be large enough to perform the graft. First, select a pair of healthy plants with similar stem diameter. Then remove the cotyledons and the first true leaves on the scion.  Make an upward cut that goes two-thirds of the way through the scion stem at a 60 degree angle, about an inch below where the cotyledons were.

Then take the rootstock, and cut off the top just below the cotyledons. About an inch below that make a downward cut in the remaining stem that matches the cut in the scion: two-thirds of the way through, at a 60 degree angle.

After the matching cuts are made insert the rootstock stem up into the scion stem, and clip the plants together using the small side grafting clip. Hold the two root balls together and transplant into a small pot, watering promptly.

The side-grafted roots of the scion are left intact for 4 to 5 days while the graft union heals. Then it’s time to sever the stem connecting to the original scion roots, so that the plant will rely only on the desired rootstock. An initial cut should be made part way through the stem to ‘wean’ the plant and reduce the shock of removing the original roots all at once. At this time it helps to stabilize the plants by attaching them to small stakes. After 2 more days, the stem to the scion root is cut all the way through. Leaving both root systems in place, rather than removing the scion roots, is not recommended as that may enhance the population of root pathogens which can thrive on the more susceptible roots.

Post-operative care.  After grafting keep the plants where it’s warm (80-85 degrees F) and at least 95% relative humidity while the grafts heal. They should be held in a heavily shaded area, like under a bench, and misted enough to maintain the humidity, but not more. (The leaves don’t need to be wet all the time, this will reduce success.)

It takes about 4 to 5 days for top grafts to heal, and 6 to 7 days for side grafts. Placing plastic domes over trays of top-grafted plants appears to enhance success. For a couple of days before setting the grafted plants out, gradually increase their exposure to direct light by pulling them out from under benches or removing any covering for a few hours early or late in the day. If using plastic domes, prop them open during this time.

When you eventually move the plants completely out of cover mist them if needed to prevent wilting. Finally, when setting plants in the production house be sure that the graft union is above the soil line. If the scion roots into the soil, the plant will be susceptible to soil-borne diseases.

Managing plant vigor.  Because grafted plants are more vigorous, they will produce a lot of vegetative growth at the expense of reproductive growth, in other words, too much foliage and not enough fruit. So, you have to take steps to reduce plant vigor. This may take some getting used to, especially if you’ve previously been trying to promote vigor in your un-grafted plants.

Leaf removal is one way to reduce plant vigor. Removing leaves may feel counter-intuitive, but apparently only 10 to 12 fully expanded leaves are needed to do the job of capturing sunlight to feed a grafted greenhouse tomato plant, at least in the Northeast. Allowing more leaves than that sure looks nice and lush, but it seems to suppress fruiting.

Another way to suppress the vigor of grafted plants is to let them develop two leaders, or main stems, rather than the single stem that’s customary in greenhouse production. This also reduces the number of grafted plants that you’ll need by half. The double-leader system adds a ‘load’ to the roots, and that suppresses vigor sufficiently so that only ‘normal’ leaf removal is needed. By that I mean taking off all the leaves below the lowest cluster with maturing fruit.

There are a couple of ways to get a double leader. One is to cut off the top of the plant soon after the graft heals so that two equal-sized leaders will be produced from the buds at the base of the cotyledons. This technique works well with top grafted plants. Of course, it will set back the plant, delaying first fruit harvest date by about a week. Alternatively, you can allow the main stem to grow normally, but then let the sucker below the first fruit cluster develop to become the second leader. Since this leader develops later, it will be shorter than the main stem, which can lead to shading. To prevent that from happening, when trellising the crop the main stem can be angled to the side to allow the sucker to grow straight up and eventually catch up, resulting in double leaders of equal size and competitiveness.

With ‘Maxifort’ rootstock a double leader is recommended. ‘Beaufort’ is a good rootstock for a single leader plant because it is not as vigorous. When using the double leader system don’t forget to adjust plant spacing accordingly, so that each grafted plant has the growing area of two ungrafted or single leader plants.

Tips for success.  Expose seedlings to full sun and some water stress before grafting to keep the plants short and increase tolerance to water stress. Avoid excess fertility in your potting mix so that plants are not too lush. Shortly before grafting, make sure plants have been watered and are not wilted. Make grafts early or late in the day to avoid water stress. If you can, do your grafting on cloudy days. Graft in a location that’s protected from direct sunlight and away from greenhouse heater discharge. Don’t cut more plants than you can graft together in a few minutes, so the cut surfaces do not dry out. Always match scions and rootstocks of equal stem diameter, cut them at exactly the same angle, and make sure the cut surfaces make good contact when the plants are clipped together so that they have the best chance of successfully connecting to each other.

For more information you can order a CD with a 20-minute video featuring Mike Collins, an organic grower who’s been grafting for 15 years. Send your request with $10 payable to UVM to: University of Vermont Extension, 11 University Way, Brattleboro VT 05301. Also see articles on grafting greenhouse tomatoes by Richard McAvoy, Univ. of Connecticut at:http://www.hort.uconn.edu/ipm/veg/croptalk/croptalkpdfs/croptalk1.3.pdf and by Jack Manix, of Walker Farm at: www.nevbc.org/sessions_03/tomato/tomato_grafting.pdf.

Grafting clips and rootstock varieties are available from: Johnny’s Selected Seeds www.johnnyseeds.com and Hydrogardens www.hydro-gardens.com.

RETURN TO VERMONT VEGETABLE AND BERRY PAGE

          1/07