Vegetable grafting is a centuries-old technique first used in Asia to improve plant production, reduce disease susceptibility, and increase plant vigor. Commercial production of and demand for grafted vegetable plants continues to increase across Asia and Europe, although it is relatively new in the U.S. Beginning in the 1990s, commercial growers and home gardeners in the U.S. have gradually become more aware of the advantages of using grafted vegetable plants.
This Extension fact sheet provides a summary of how to prepare watermelon seedlings for grafting, how to graft, how to heal grafted plants, and how to transplant and maintain grafted plants in the field. For grafting techniques that require high humidity, construct a healing chamber following the instructions provided in the Washington State University Extension Publication FS051E titled Vegetable Grafting: The Healing Chamber.
Watermelon plants are grafted when they are 14–21 days old. In order for a successful graft union to form, the scion and rootstock plants must have similar stem diameters at the time of grafting so their vascular bundles can be aligned and in complete contact with one another. (See Figure 1 for an illustration of plant parts, including the vascular bundles in a watermelon stem.) The scion and rootstock seedlings may not germinate or grow at the same rates, so it is important to conduct a preliminary test to determine their growth rates in your growing environment. Seed more plants than necessary so you have a greater selection when matching stem diameters. Also, it is rare to get 100% graft survival, so graft at least 20% more plants than needed.
Water both rootstock and scion plants 12–24 hours before grafting. Do not water plants immediately before grafting, unless they are wilted. If reusing grafting clips, wash them in warm, soapy water, sterilize them by soaking for 1 minute in a 10% bleach solution, and rinse them under tap water. Allow the clips to air-dry before reuse. Use only clean, sharp razor blades for grafting, and wash your hands with antibacterial soap or hand gel, or use latex-type surgical gloves. While there are many tools that can be used for cutting vegetable grafts (Figure 2), the double-edged razor blade snapped in half is most commonly used. Fill a spray bottle with tap water in order to mist plants frequently during grafting. If you are using a healing chamber, spray the inner surfaces of the chamber
Grafting Techniques Commonly Used for Watermelon
There are four methods commonly used to graft watermelon: 1) approach (tongue), 2) hole insertion, 3) one cotyledon (splice), and 4) side grafting.
1. Approach (Tongue) Grafting
Both rootstock and scion should have one or two true leaves. Cut a 45° downward slit halfway through the rootstock stem below the cotyledons, and cut an identically angled upward slit in the scion stem (Figures 3A and 3B). The angle and location of the cuts must be relatively precise so the scion can be placed on top of the rootstock. Bring the two cut stems together so they overlap (Figure 3C), then attach a clip or securely wrap the joined stems in plastic wrap, foil, or parafilm (Figure 3D). Figure 4 shows this grafting technique with actual plants. Place the joined plant in a transplant tray or small pot. Mist the plant with water and place it on a greenhouse bench. Water the plant as needed. Cut off the top of the rootstock 5 days after grafting. Wait 7 days, and then cut off the bottom portion of the scion.