Grafting Tomatoes for a Better Harvest
Heirloom tomatoes are celebrated for their flavor and variety, but are typically less vigorous and more disease-prone than many modern hybrid varieties. Grafting allows you to get the best of both worlds by using a vigorous, disease-resistant "rootstock" to power the growth of any variety you want on top. Grafting increases fruit production, often lengthens the harvest season, and solves many typical disease issues without requiring pesticides or fungicides. Grafting can even help with common issues such as tomato blossom-end rot, as the superior root system provided by the rootstock can help deliver more calcium to developing fruit. Once you've grown a grafted tomato plant, you'll never want to grow an un-grafted tomato again.
Grafting is an ancient practice dating back more than 4,000 years. A piece of one plant (called the "scion") is attached onto an existing plant (called the "rootstock") to grow. In the case of tomatoes, a particularly vigorous and disease-resistant rootstock plant with poor or even inedible fruit is used to power the growth of a variety that tastes good. Since many bacterial and fungal tomato diseases spread through the soil and enter the plant through the roots, using a disease-resistant rootstock can allow you to grow varieties which are highly susceptible to these diseases.
Grafting isn't limited to tomatoes- in the vegetable garden, eggplant, peppers, melons, squash, cucumbers and other plants can also be grafted. The technique covered here for tomatoes will work for most of these other vegetables as well. As you become more skilled at grafting, you can even use it to create vegetable plants that produce more than one variety of fruit-- it is even possible to grow tomatoes indoors onto a potato rootstock for a single plant which will produce both vegetables!
Selecting a Tomato Rootstock
There are several commercially-available rootstock varieties available, bred specifically for vigor and disease-resistance. The rootstock will typically be classified as either "generative" or "vegetative". Generative rootstocks encourage the scion to put most of it's energy into fruit production, meaning you'll get a plant that fruits quickly and profusely but then doesn't have enough leaves to support more fruit. Vegetative rootstocks encourage vigorous leafy growth of the scion, which leads to more fruit overall over a longer season, but may require you to prune your tomato plants to keep them under control. Vegetative rootstocks handle hot weather better than generative rootstocks. In general, for "determinate" tomatoes (those that fruit all at once and have a single crop) or those you don' intend to grow for a long time, the generative rootstock varieties may be a good choice, while "indeterminate" varieties (that keep fruiting over a long season) benefit most from a vegetative rootstock. Different scion varieties may behave differently, so it is worth some experimentation to find the right rootstock / scion combination.
- A supply of rootstock and scion seedlings of the same stem size.
- A grafting knife or sharp razor blade (the old-fashioned double-edged razor blades work particularly well). If you have difficulty with creating cuts at a specific angle, this grafting tool could help you out.
- Silicone grafting clips of the appropriate size to match the seedlings' stems.
- Humidity domes or a humidity-controlled chamber (aquariums work well).
- A spray bottle full of water.
To graft successfully, the rootstock and scion stems need to be the same diameter. In most cases when grafting tomatoes, the rootstock and scion seeds can be planted on the same day and will be roughly the same size when it's time to graft. If the rootstock grows faster than the scion (or vice-versa), sowing the faster-growing variety a few days later will result in matching stem diameters.
Plan on allowing an extra 1-2 weeks for grafted plants to be ready for transplanting, as the plants stall new growth while the graft is healing. Other than that, start the seeds as you would any other tomato seeds. You can play with the hanging height of the LED grow light and temperature a bit to control how long the stems are if the stems are too short for easy grafting-- hanging the light higher and keeping the environment warmer will cause the stems to stretch more, but if you over-do it the stems will be weak and floppy.
Seedlings are ready to graft when their stem diameter matches the grafting clips you have, typically when they have 2-3 true leaves. Test-fit a grafting clip on the stem just below the cotyledons (seed leaves) to see if the stems are the right size-- if it fits snugly, it is time. Depending on your growing conditions, this can be 14-21 days after germination. Younger, smaller plants are generally easier to graft and have a higher success rate, so don't wait too long to graft!
Fertilizing the seedlings before grafting isn't recommended, as it encourages rapid growth and may interfere with the healing process after grafting.
12 to 24 hours before you are ready to graft, water the rootstock and scion plants carefully so that the soil is moist but not saturated. If the rootstock or scion are too dry, the plant won't have enough water to heal the graft. If watered too soon before grafting, the water flowing through the stem will push the graft apart and it will fail.
Prepare a recovery area for the grafted plants. Newly-grafted plants need a warm (80 °F), low-light, high-humidity (90%-100%) area to heal in for a few days after grafting. Ideally this would be near where you're going to be performing the grafts, as you want to move the plants as little as possible after grafting to allow the stems to grow together.
Hygiene is critical when grafting; ensure your knife / razor blade is sterile, the grafting clips are sterile, and your work surface and hands are clean. If any of your equipment or hands become contaminated with a pathogen, it is easy to accidentally contaminate all of your grafted plants. A 10% bleach solution is ideal for sterilizing the blades, grafting clips, work surface and seedling recovery area. Wash your hands thoroughly before grafting, especially if you use tobacco. Never smoke while grafting or around newly-grafted plants, as Tobacco Mosaic Virus (TMV) is easily transmitted to tomato plants through open wounds, which you will be creating while grafting. Once the grafts have healed, the plants will be more resistant to TMV exposure.
Six Steps for Performing Tomato Grafting
- Select a rootstock and scion plant with stems of the same diameter.
- If you're grafting more than one variety, it is helpful to label the pot with the rootstock with the scion variety you are going to be grafting on. When you're mixing and matching plant parts with a knife, it is easy to get confused and lose track of what variety is grafted on top!
- Sever the top of the rootstock just below the cotyledons (seed leaves) at a 45° or greater angle. The steeper the angle, the more surface area the graft will have to heal on, and the more likely it is to succeed. Rather than just pushing the knife or blade into the stem, slide it while cutting to avoid crushing the delicate stem. You don't want to leave any nodes (meristematic tissue) on the rootstock at all, as it will grow from any dormant buds you leave behind. Throw the top of the rootstock away to avoid confusing it with a desired scion.
- Place a grafting clip around the top of the rootstock stem such that half the grafting clip is gripping the rootstock (the upper half will accept the scion); placing the highest point of the now-angled rootstock stem toward the open end of the grafting clip will help ensure the scion cannot slip out later.
- Sever the scion just below the cotyledons at the same angle you cut the rootstock. Throw the scion's rootball away to avoid confusing it with a rootstock. Slide the scion into the grafting clip so that the cut surfaces of the rootstock and scion match up. Any dirt or air between the two cut surfaces will prevent the graft from healing properly; the two cut surfaces should fit together snugly.
- Place the grafted plant in the recovery chamber and mist lightly with water. You don't want to spray enough water that any gets into the grafting clip, and certainly don't spray hard enough that it knocks the scion off the rootstock, but a light mist on the scion's leaves is helpful to allow it to absorb a little water after the trauma it just experienced.
- Repeat the process with all the plants you want to graft. Cleaning and re-sterilizing the knife or razor blade between grafts will help ensure the highest success rate.
It is important to keep the newly-grafted plants at 90%-100% humidity and at 80 °F for the first 4 days after grafting. Constant warmth helps speed the healing process. For the first 24 hours it is ideal to keep the plants in the dark to avoid the scion trying to bend toward the light and pull the healing graft apart. After 24 hours, give the plants dim light-- hanging an LED grow light at double the vegetative hanging height recommendations for our Black Dog LED lights (the hanging heights can be found below the footprint diagram on each product page here). The plants should not need watering at all during this time; if you see wilting of the scions, either the humidity is too low (a light misting can help restore humidity) or the graft has failed.
5 days after grafting, open the humidity chamber slightly to start decreasing the humidity. Check on the plants every 15 minutes for the first hour, and every hour or two after that; if you notice wilting, close the humidity chamber back up and increase the humidity inside, and try again in 6-12 hours. Once no wilting occurs, open the chamber more to decrease humidity further and repeat the process, until eventually the plants don't wilt at normal humidity levels. This entire process may take 2-4 days.
During the recovery process it is common for the scion to send out roots from just above the graft. Once the plants are acclimated to lower humidity levels, carefully trim these roots off with scissors; the entire point of grafting is to ensure that the scion is using the rootstock's roots exclusively; if any of the scion's roots are growing into the soil, it will not get the benefits of the rootstock's disease resistance.
After the plants are acclimated to lower humidity levels, start moving the LED grow light closer to the plants over a few days until it is at the recommended flowering hanging height. The grafts should be fully healed 2-3 weeks after grafting and will resume vigorous growth then, but for the first 2-3 weeks after grafting:
- Use only a weak liquid fertilizer; heavy fertilization can cause irregular growth which can even lead to the graft failing.
- Water carefully and don't use oscillating fans, to avoid knocking the scion off as it finishes healing.
- Avoid transplanting until the graft is fully healed.
- If the scion is continuing to grow roots above the graft, continue removing them. Rooting above the graft can indicate that humidity is too high, or that the graft isn't fully healed yet.
Once the graft has fully healed, treat your grafted tomato plant like you would any tomato seedling, with one important caveat: never bury the graft. While burying the stem of a non-grafted tomato plant is usually a good idea to get it to grow a larger root system, grafted tomatoes don't require this to develop superior roots as the rootstock does that for us. If the scion's stem is buried, it will grow its own roots into the soil and negate the disease resistance offered by the rootstock. As the grafted plant grows, take care to ensure that no leaves or stems from the scion contact the ground, as this will provide a vector for diseases to enter the plant.
Because grafted tomatoes are more vigorous than un-grafted plants, you may find yourself needing to raise the LED grow light more often and repot the plants more frequently as long as you are growing indoors.
It is not necessary to remove silicone grafting clips, as these will naturally fall off as the stem grows. If you collect them after they fall off, they can be re-used next year if you sterilize them with a thorough soaking in a 10% bleach solution.
Prune off any suckers that form below the grafting site, as these are from the rootstock and will not produce desirable fruit. If you grafted below the rootstock's cotyledons, this should never be a problem.
As your grafted tomato grows, don't be afraid to trim the plant when it starts getting large. The increased vigor offered by the rootstock means the plant will recover quickly from pruning, and it often becomes necessary to trim the plant to make picking all the fruit easier. Most commercially-available tomato cages will often be crushed by the sheer weight of a grafted tomato plant unless it is regularly pruned. Excellent, heavy-duty Texas Tomato Cages can be purchased here and typically hold up to an 8-foot-tall, 300-pound tomato plant without being crushed.