Grafted plant use is increasing rapidly but is still far below its estimated potential
Grafting makes one plant out of two, the root system coming from the rootstock and the above-ground fruiting portion from the scion. Grafted planting stock is the norm in many areas of the world, and although it is less common in the U.S., grafted plant use is rising rapidly in the U.S. Grafted plants are usually prepared using young seedlings. In 2012, few, if any, grafted plants were produced on a commercial scale in the U.S.; however, U.S. propagators now supply more than eight million plants annually, and many growers are preparing their own grafted plants, learning through programs and resources (e.g., https://u.osu.edu/vegprolab/grafting-guide/ and http://www.vegetablegrafting.org/resources/grafting-manual/). Still, that production is thought to be less than 1% of the potential market for or eventual use of grafted plants in the U.S. Demand for grafted plants by vegetable growers managing greenhouse, high tunnel, and open field plantings of different scales and approaches (e.g., organic, conventional) continues to increase sharply.
Industry-University-USDA teams are working to increase access to and success with grafted plants throughout the industry
Grafted vegetable plants are now more available and important to some U.S. growers than ever before, but much more is required to maximize their value throughout the industry. Experts in plant breeding, plant propagation, engineering, crop management, economics, and other fields – including at Purdue University — are testing ways of improving people’s experiences with grafted plants. They focus on improving grafting and grafted plant distribution methods and increasing access to grafted plants, including reducing their cost and increasing their availability to all growers, regardless of where or how they farm. Research and extension personnel also help growers unlock the full economic potential of grafted plants through adjusting cultural, nutrient, irrigation, harvesting, and other aspects of management, as needed. The overall goal of this collaborative work is to raise the value of grafted plants to those who prepare and/or use them. Globally, the technical literature includes more than two thousand articles and reports describing industry-focused efforts to advance the use of grafted plants as products and production tools, including in the U.S. See and download a database listing these resources at http://www.vegetablegrafting.org/resources/reference-database/. Similarly, consider utilizing the “vegetable grafting decision support tool” at http://graftingtool.ifas.ufl.edu/ to assess your potential experience. Growers of tomato and watermelon have been the first to gain from using grafted plants, and growers of pepper, eggplant, cucumber, and specialty melon have also been benefitting.
Rootstock (RS) traits, availability, and selection remain key
All farming outcomes result from the specific combination of crop genetics and nearby environments involved. Success requires selecting crops and varieties ideally suited for growing conditions they are likely to experience and then ensuring those above- and below-ground conditions are ideal as often as possible. Using grafted plants vastly increases the arsenal of traits available to most vegetable growers that could be useful in that process. Including all traits that may be important (e.g., vigor, soilborne disease resistances, tolerances to abiotic stresses) in all varieties has been impossible and is likely to remain so for many years. Grafting eliminates the need for compromises required when using single-variety nongrafted plants. ‘Physical hybrids’ created through grafting allow root and shoot traits to be included in separate varieties that are later combined through grafting. This is why RS varieties should interest many vegetable growers. RS varieties available in the U.S. are listed and described in tables available at http://www.vegetablegrafting.org/resources/rootstock-tables/.
From the tables, we can learn:
(a) sixty-two Solanaceous RS varieties can be used with tomato, thirty-two with eggplant, and sixteen with pepper, but only one can be used with all three crops.
(b) Solanaceous RS varieties carry reported resistances for various combinations of sixteen diseases and root-knot nematode.
(c) forty-one Cucurbit RS varieties can be used with watermelon, thirty-one with specialty melons, nineteen with cucumber, and seven can be used on all three crops.
(d) Cucurbit varieties carry reported resistances for various combinations of thirteen diseases, root-knot nematode, and tolerance to cold, heat, and/or drought stress.
A word about RS-scion (fruiting variety) compatibility: it is important. Rules of thumb apply. For example:
(a) company recommendations (e.g., reports of a RS being best for one or more crops) should be taken seriously.
(b) if a RS is listed as compatible with ‘tomato’ or ‘watermelon’, it will graft well to all scion varieties of those crops but its influence on them may differ. Therefore, RSs should be selected based on quality information and more than disease resistance, if possible.
Careful, deliberate on-farm testing is useful and help is available now
Experience from focused research and on-farm testing each season continues to reinforce the idea that getting the most from grafted plants often requires managing them differently, perhaps beginning before planting and continuing through harvest. The biology of some rootstocks and rootstock-scion combinations can require growers to alter various standard practices to gain fully from using grafted plants. Those practices are typically developed through years of experience with nongrafted plants, so it can be difficult to give them up. Getting the most from grafted plants may require reducing preplant or after-planting fertilizer rates and/or plant populations (using wider spacing) and/or altering irrigation programs or harvest schedules, since grafted plants can be more vigorous and efficient with nutrients and water. Reducing plant numbers and inputs can help offset the cost of grafted plants. Regardless, growers looking to maximize their returns on investments in grafted plants are encouraged to connect with experienced grafted plant users and testers, consult reports (e.g., https://u.osu.edu/vegnetnews/2021/08/21/grafted-watermelon-plants-under-what-conditions-and-practices-does-using-them-offer-the-best-return-on-investment/) and experiment carefully.
Growers rely heavily on traits available in their planting stock. Delivering all the traits growers require in single-variety nongrafted plants has been impossible and is likely to remain so for many years. Grafting speeds the delivery of traits to farms and, in combinations, often unavailable in nongrafted plants. Teams are working to overcome the challenges associated with relying on grafted planting stock much more heavily.