1. Selection of Grafted Rootstock Choosing the right rootstock is crucial for successful grafting. The most commonly used rootstocks include melons, pumpkins, winter melons, and wild watermelons, with melon and pumpkin varieties being the most popular. Melon types such as long melons, round melons, aphrodisiac melons, and Japanese gourds are often selected, while pumpkin varieties like New Tuzuo are also widely used. The main selection criteria involve choosing rootstocks that have strong compatibility, good disease resistance, and do not negatively affect the flavor or quality of the watermelon. Before using a new variety as a rootstock, it should be tested to ensure its effectiveness and safety in production.
2. Preparation of Seeding Substrates For the rootstock preparation, a mix of peat, perlite, and vermiculite is typically used in a ratio of 3:1:1. In winter, the ratio can be adjusted to 2:1:1, or alternatively, a 3:1 ratio of peat to vermiculite may be used. When selecting peat, imported brands like Hair, Earl, and Sunlight are preferred. Domestic options such as Panda and Chinese peat can also be considered, but unfamiliar brands must be tested before large-scale use. Matrix disinfection is essential; imported substrates are usually pre-disinfected, while domestic ones can be treated by adding 200g of chlorothalonil per cubic meter or spraying 800 times diluted thiophanate methyl (45-60kg per m³). A high-quality 15-15-15 NPK compound fertilizer (1.0-1.2kg per m³) or a specialized watermelon nutrition solution is added to the mix. If needed, industrial calcium carbonate is used to adjust the pH. All fertilizers must be fully dissolved in water before mixing into the substrate.
3. Sowing and Germination The timing of sowing depends on the planting date. Rootstock seeds are typically soaked and germinated 25–40 days before planting, while scion seeds are soaked and germinated 7 days later. If temperatures are low, the interval between rootstock and scion can be extended to ensure the rootstock reaches an appropriate size for grafting. Soaking seeds in warm water (55°C) for about three times their volume, then cooling to 30°C and stirring until the temperature drops, followed by a 10-minute soak in 0.1% bleach solution. After rinsing, seeds are soaked for different durations—12 hours for pumpkins, 24 hours for melons, and 6–8 hours for watermelons. After rinsing thoroughly, they are ready for germination.
4. Seedling Management Rootstock seeds are placed in square trays and covered with a damp cloth, germinating at 28°C. Once the seeds show signs of sprouting, they are transplanted into 50-cell plugs at a depth of about 1cm. The radicles are positioned diagonally to reduce seedling capping. After sowing, watering is done evenly, keeping the moisture at around 80% of saturation. Seedlings are then moved to a germination room with daytime temperatures of 28°C and nighttime temperatures of 18°C. For scions, seeds are sown 6–7 days after the rootstock, with a 4cm layer of substrate in the bottom of the tray. Seeds are spread evenly, covered with 1cm of substrate or vermiculite, and watered from the bottom. A light source of about 5000 lx is required in the germination room to support growth.
5. Grafting Process Grafting is performed when seedlings are 8–12 days old, just after the first true leaf appears. Rootstock leaves are removed, and the scion is cut at a 60° angle below the cotyledons. The scion is then inserted into the rootstock. Alternatively, a 30° cut is made at the base of the cotyledons, and the scion is matched with the rootstock. After grafting, the plants are kept in a humid environment (over 95% RH), shaded during the day, and gradually ventilated after 3–4 days. Temperature is maintained at 26–28°C during the day and 20–22°C at night. Water management keeps the substrate at 75–80% of its maximum water-holding capacity. During winter, a week of low-temperature acclimation is recommended before transplanting.
6. Pest and Disease Control Prevention is key in managing pests and diseases. After seedlings emerge, green hen No. 2 is sprayed to prevent damping-off. Chlorothalonil or quick-clone is applied 2 hours before grafting to protect against downy mildew. During the survival period, high humidity increases disease risk, so chlorothalonil aerosol is used. Post-grafting, mancozeb and chitosan-based products are applied to prevent infections. For damping-off, 72.2% propamocarb is used for irrigation or foliar spray. Anthracnose is controlled with chlorothalonil or anthrax fungicides. Regular monitoring and timely treatment are essential for healthy seedlings.
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