How and Why to Graft Heirloom Tomatoes

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Photo by Getty Images/bacillux.

Interest in tomato grafting is high among gardeners these days, and grafting isn’t limited to modern hybrid plants. Heirloom tomato cultivars can benefit from being grafted onto sturdier rootstock than they naturally possess — the rootstock won’t alter the growth habit, fruit, or foliage of the plant, but it will offer more disease resistance and productivity. However, grafted plants can be expensive, and sources of grafted tomatoes are few. You can save money by learning how to graft yourself, and with a little practice, you’ll be skilled enough to graft any cultivars you wish.

There are several styles of grafting, all of which involve carefully cutting a shoot from the desired plant (called a “scion”), inserting it into a cut or slice in the desired rootstock plant, and caring for the wound until the plants grow together and make a strong join. I’ll focus on splice grafting, which is very similar to the grafting used on fruit trees. Because the technique was first developed in Japan, it’s also known as Japanese top grafting.

Materials for Grafting Tomatoes

To successfully graft tomatoes, you’ll need one specialty item: silicone grafting clips in sizes that will fit your plants’ stems (1.5 and 2.0 millimeter are the most commonly used sizes). The clips look like short tubes with a slit down the length, and you’ll use them to hold the scion and rootstock together while the graft heals.

Transparent grafting clips make aligning the cut stems of the scion and rootstock plants much easier, which makes the graft more likely to take. Photo by Adobe Stock/vallefrias.

You’ll also need a new, straight-edged razor blade, a spray bottle filled with sterile water, and a lidded, clear plastic bin that’s large enough to hold a tray of grafted plants.

Germination Station

Of course, before you can start grafting, you’ll need to choose rootstock and scion seeds, plant them, and allow them to grow until their stems are large enough in diameter to graft. When the scion plants have 2 or 3 true leaves, they’ll be ready for grafting. Rootstock plants only need to have cotyledons, though they’re likely to have a few true leaves by the time their stems are thick enough to graft. The first time around, allow about 21 days from seed-planting to grafting. Your rootstock and scion plugs may grow at varying rates, but, through trial and error, you’ll find the ideal timing for scion and rootstock stems to match in diameter and fit in the grafting clips.

Choose rootstock you know will grow vigorously in your soil and that offers resistance to whatever soilborne diseases you might have. Scion seeds can be any cultivars you please, and heirloom cultivars are excellent choices for grafting because you’ll gain the benefits of a larger harvest with less care needed for disease prevention.

Your grafts won’t all be successful; in fact, I recommend that you plant 2 or 3 times as many scion and rootstock plants as you want successful grafts.

Grafting for Success

When you’re ready to begin, prepare a clean working surface with all your materials at hand. Make a mental note of where you’ll put the scion tray and the rootstock tray, and put them in the same spots whenever you graft — you don’t want to confuse them when you begin cutting. Finally, wash your hands to avoid inadvertently introducing harmful organisms to the cut surfaces.

Remove one rootstock plant from the tray. Look it over carefully before you cut, and discard any that are sickly or damaged. Always cut just below the cotyledons (the small set of leaves first from the bottom) and at least 1/2 inch above the soil line to discourage the scion from sending out roots because of its proximity to the soil. Use your razor blade to remove the cotyledons, and then cut just below them at an angle between 30 and 45 degrees. After you cut, slide a silicone grafting clip over the cut end of the rootstock. The clip should fit firmly for the best results.

To speed up the grafting process, you can cut several scions at once. Photo by Getty Images/glebchik.

Next, examine your scion plants and select a healthy one with a stem diameter that matches your rootstock. Remove all but the topmost leaves to reduce transpiration while the graft is healing. Cut at an angle matching the angle of the rootstock — about 1/3 inch above or below the cotyledons. You’ll want an exact fit between the scion and the rootstock, so keep your cutting angle as consistent as possible.

Align the scion with the rootstock and slide the scion into the grafting clip so the two cut surfaces meet cleanly. The grafting clip should hold the scion and rootstock firmly together.

Spray the grafted surface with a fine mist after completing each graft to prevent it from drying out, and then move the grafted plant to a clean tray. Keep newly grafted plants in the shade and away from strong winds while you continue grafting.

Experienced grafters might remove the foliage from an entire flat of rootstock plants at once, but then must work quickly to keep the cut stems from drying out. Photo by Getty Images/burapa.

Healing the Grafts

Next comes healing, a critical part of the process. If all goes well, your grafted plants will heal in 4 to 7 days. Maintain high humidity — near 100 percent — and steady warmth, with temperatures from 82 to 84 degrees Fahrenheit for the best results. Keep the plants in total darkness for the first day or two to prevent transpiration from the scion and allow time for the scion and rootstock cells to seal together. Thereafter, grafted plants will need moderate light that’s roughly equivalent to the intensity of two side-by-side fluorescent tubes.

If you have a greenhouse, you can create a healing chamber by placing new grafts on a table beneath a bench and covering the bench with plastic to retain humidity. Or, you can put your grafts in a propagation chamber for the first two days, and then beneath a shaded bench and misters for the duration. For gardeners without greenhouse access, a clear plastic storage bin with a lid is the easiest way to provide optimal humidity and soil moisture for newly grafted plants.

While your plants are healing, too much water applied to the soil can create a thin layer of water on the grafted surface where the water drawn up by the rootstock’s veins emerges from cut cells that haven’t yet sealed to scion cells. This can create a barrier that will interfere with the union between the scion and rootstock. The best way to water during the healing period is to mist the graft union at regular intervals, which will help keep the scion hydrated while the cut heals. If you’re using a clear storage bin, fill the bottom with about 1/2 inch of water before you put your grafted plants inside. This should provide enough moisture and create a high level of humidity inside the container as long as the lid has a good seal. If in doubt, tape over any openings.

A spray bottle full of sterile water will help to moisten the graft just enough to encourage healing, but not so much that the water impedes the graft. Photo by Getty Images/koromelena.

Approximately one week after making your grafts, you should begin to expose your plants to more light and increased airflow by gradually opening the lid. The entire hardening-off process will take about two weeks.

The grafting clips will fall away from the graft as your tomato plants grow. To prevent the introduction of disease, don’t reuse your grafting clips, and keep the graft line at least 1/2 inch above the soil surface to keep soil microbes away from the scion plant.

David Baldwin is the founder of The Natural Gardening Company in Placerville, California. He recently retired, but the company continues to sell organic vegetable, herb, and flower seeds, as well as organically grown seedlings.

Mother Earth Gardener
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