The University of California Cooperative Extension has a slice of sweet news for watermelon growers: Farmers may be able to produce stronger, higher-yield, more disease-resistant watermelon crops by grafting.
Grafting combines a scion, the above-ground section of a plant, with the sturdy rootstock of a related plant.
"Just as many tree crops are grafted, we are learning that other crops can be successfully grafted, too," David Jarrett, a San Joaquin Valley watermelon grower and field manager at Van Groningen & Sons, said in a statement.
Zheng Wang, vegetable crops and irrigation advisor at the University of California Cooperative Extension, has experimented with grafting watermelons for the past few years, and the results have been positive.
In his first few trials with commercial growers in 2019, he compared grafted to nongrafted watermelon plants, analyzing differences in yield and fruit quality when the crop was planted in different in-row spacings.
Wang found that grafted plants spaced about 4-1/2 to 5-1/2 feet apart could produce the same or higher yield than nongrafted plants spaced 3 feet apart.
In the trials, growers on average reported that their fields of grafted plants produced 15% to 20% more watermelons than nongrafted fields per acre, while using 30% fewer plants.
Grafted plants also have the potential to lengthen the harvest window.
Although production volume varies by variety, microclimate and other factors, on average, nongrafted plants produce only three picks of good-quality watermelons. After the third pick, the quality may rapidly decline.
In contrast, a grower may be able to harvest high-quality watermelons from a grafted plant up to eight times, though the number depends on labor availability, field conditions and market demand.
Wang's research also found that grafted watermelons tend to have stronger, more vigorous root and vine systems.
Grafted watermelons are also generally more resistant to soil-borne pathogens and diseases, although they are not more resistant to other diseases, such as air- or wind-borne diseases and foliage diseases.
One of Wang's trials illustrates the difference in disease resistance. He tested grafted versus nongrafted plants in a field he believes was pathogen-infested based on an initial diagnosis. The nongrafted plants wilted. The grafted plants performed well.
"The differences were remarkable," he said.
Experienced growers in California are gaining confidence in grafting watermelons, leading to a significant increase in planted acreage. In 2018, the state had fewer than 250 acres of grafted watermelon. In 2021, California had more than 1,500 acres.
Wang said he thinks grafting might prove a useful tool for growers in other states as well, including in Hermiston, Ore., known for its watermelon production.
"I think the grafting of watermelon could be helpful beyond California," he said.
However, Wang cautioned growers that different factors — scale, weather, climate, irrigation practices — may produce different outcomes.
He also advised growers who are new to grafting to experiment gradually — starting with 10 acres of grafted watermelons rather than 300, for instance — "because if something is wrong, you will probably cry."
Although there are downsides associated with grafting — including greenhouse production costs — many growers say the benefits outweigh the costs.
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