The ‘State’ of Vegetable Grafting in the U.S.

Published: December 30, 2015

The authors of this article had an opportunity to attend an exciting two-day workshop in Michigan, the National Vegetable Grafting Symposium and Extension In-Service Training , which brought together leading researchers and extension agents from across the US. It was recent (early December), so we thought this was a good opportunity to summarize ‘the latest’ in this newly developing area of vegetable production. The workshop was funded by the USDA.

Tomatoes received much of the attention, and are the source of most grafted vegetables in the US. Interestingly, in Asia watermelons and other cucurbits are most grafted. In Asia they have become highly efficient at producing the grafted transplants, thus the cost is much less. And because watermelon spacing is much lower per acre than tomatoes, the cost increase to plant an acre of grafted watermelons is much less as well. We’ll return to watermelons later.

Both in the US and around the world, resistance to soil borne pathogens is the leading reason to consider grafting. The economics are very compelling in this situation as crop yields decrease, sometimes dramatically, unless grafted plants with resistant rootstock are used. There is much research occurring in this area. An example is with tomatoes, where more than 50 tomato rootstock varieties are now described. They vary significantly in pathogen resistance.

In many regions and systems, grafted vegetable plants routinely outperform ungrafted counterparts in terms of vigor, stress tolerance and/or yield. But, is the increased expense of grafting paid for in these situations? Usually, but certain conditions make it more likely, such as:

• Techniques used to produce fruit with grafted plants often need to be different than in standard, ungrafted systems (in order to maximize the return on investment in grafted plants). Management of fertility, irrigation, pruning and trellising, and crop protection, as well as harvest regimens and plant populations, may need to be altered. For instance, grafted greenhouse tomatoes often use the twin leader system when trained up a string, which generally cuts plant population in half.
• More intensive production systems are more likely to benefit (e.g. greenhouse or high tunnel tomatoes vs field grown). The value of the crop is generally higher and the harvest season is longer, thus there is more time to recoup added production expenses. But there are glitches; for example, grafted tomato plants were noted as often set back about one week, and if first harvested fruit are very highly priced, this may have economic consequences.
• When the scion has lower vigor, such as heirloom tomato varieties. The fruit prices are also higher.

There remains much to be learned. For instance, researchers in NY documented that grafted tomatoes in high tunnels were MORE likely to get foliar diseases. Why? The plants grew so vigorously that the foliage was thicker, setting up conditions that made the plants more disease-prone. Note that the rootstock imparts resistance mostly to the ‘roots’, not up into the scion, or top part. This points out a challenge in ‘experimenting’ with just a row or two of grafted plants. If you put them on the fertility regimen of ungrafted plants, they may grow bullish, the phenomenon of plants that are overly vegetative to the point of not setting fruit.

Grafted watermelon plants have shown an odd, but beneficial characteristic, improved fruit quality. The flesh tends to be denser and redder (higher lycopene levels). The plants are so vigorous that spacing needs to be increased, which saves some cost but also impacts weed control tactics.

With all the work being done in this area by researchers, seed companies, growers, and extension specialists, a unified source of information was recognized to be beneficial. So a website was formed that will be revised to stay current- www.vegetablegrafting.org . Other crops receiving grafting interest are melons, cucumber, peppers and eggplant. There is a list of suppliers of grafted plants on the website. Additional suppliers can be added, but need to be willing to ship or deliver plants regionally.