Grafting has become a routine technique in continuous cropping systems in many parts of the world. It was first introduced in Korea and Japan during the late 1920s by grafting watermelon onto bottle gourd rootstocks to address the problems of declining yield due to soilborne disease in plants.
It is estimated that more than half of the world’s watermelons and cucumbers are produced in China and more than 90% of these are grafted. Use of rootstocks can enhance plant vigor through attaining soil nutrients, avoiding soil pathogens, and tolerating low soil temperatures, salinity, and wet-soil conditions.
The type of rootstock used has been shown to affect cucurbit plant growth, yield, and fruit quality. Cucurbit grafting is rare in the U.S., but with quality farmland becoming scarcer, and since the phaseout of methyl bromide, the U.S. cucurbit industry is looking at grafting as a viable option. In fact, some seed companies now offer watermelon transplants grafted onto squash or bottle gourd rootstocks.
Examining The Plant
In 2007 and 2008, four southern states joined forces to examine grafted watermelon plants. Two commercial cultivars and seven different rootstocks were grafted on each of these cultivars at each location.
Yields and quality were assessed. The results saw no difference in yields or fruit shape and size of melons produced. In addition, there were no differences in sugar levels and internal defects. However, there were differences in flesh firmness. All grafted plants produced firmer flesh. There also were differences in rootstocks.
In particular, the squash rootstock was found to be the firmest, followed by the gourd rootstock. Growers typically are interested in the firmer flesh because it lasts longer and doesn’t decay in the field. The fresh-cut industry also is interested in the firmest fruit because it has less juice. Processors don’t want juice to accumulate in the bottom of fruit containers.
One big factor in producing a grafted transplant is the labor cost. At present, the price tag on such a transplant is about $1. This is in comparison to 25¢ for a non-grafted transplant. Various grafting methods and procedures have been developed to reduce those costs and transplant growers must choose which method produces the highest quality transplants at the lowest price for their operation.
Grafting methods vary considerably with the crops being grown, rootstock material selected, and scion material chosen. Just as important a consideration is the growth stage needed for both the rootstock and scion material needed for the graft to be successful.
For example, the grafting method called “hole insertion graft” would be convenient for watermelons because of their small seedling size compared to the size of the rootstock seedling. This method, however, only works with the gourd rootstock. Four methods have been evaluated: approach graft, hole insertion graft, one cotyledon graft, and hypocotyl graft.
Each graft requires different materials as well as different procedures in order for it to be successful. Just as important as the grafting procedure is the post-grafting care. Growers must place their grafted plants into a healing chamber. They must remember to maintain the grafted plants at 77°F and 100% humidity in the chamber for five to seven days prior to moving them into the greenhouse.