Detailed Explanation of Key Points of Off-Season Watermelon Cultivation Techniques

I. Planting Density

Watermelons grown in solar greenhouses are generally yellow-skinned gift watermelons and seedless watermelons. The planting density should be determined according to the maturity of the variety, the number of vines retained per plant and soil fertility. For early-maturing varieties, 1500-1800 plants per mu (two-vine pruning); for mid-maturing varieties, 1500 plants; for late-maturing varieties, 1200 plants.

II. Temperature Management

Regulate and control according to the temperature requirements of watermelon at different growth and development stages. The daytime temperature is generally controlled at 30°C-36°C, and the night temperature must not be lower than 15°C. During fruit setting and fruit development, the night temperature must not be lower than 18°C. The soil temperature should be maintained above 18°C.

III. Water and Fertilizer Management

The suitable soil moisture for watermelon is 65%-78% of the maximum field water-holding capacity. When the soil moisture is lower than 48%, drought symptoms will occur. Watermelon's demand for soil moisture is high during the germination stage, relatively low during the seedling stage, moderate during the vine elongation stage, and reaches a peak water demand during the mid-fruiting stage, when full irrigation is required; during the late fruiting stage, irrigation should be reduced again.

For watermelon fertilization, on the basis of applying sufficient base fertilizer, topdressing should be carried out. During the fruit swelling stage, apply 30 kg of potassium sulfate and 30 kg of diammonium phosphate per mu. Around the time the fruit “sets its size,” if conditions permit, foliar spraying of a mixed solution of 0.2% urea and 0.2% monopotassium phosphate can be carried out. To observe whether watermelon is deficient in fertilizer, look at the color of the leaves. If the basal leaves are dark green, the middle leaves are green, and the top leaves are light green, it indicates normal growth; if the leaves of the whole plant are yellow-green from top to bottom, it indicates fertilizer deficiency; if the leaf color of the plant is dark green from top to bottom, it indicates excessive nitrogen fertilizer, which will cause excessive vegetative growth, and attention should be paid to controlling it.

IV. Pruning

Solar greenhouse watermelons adopt two-vine pruning. The main vine serves as the fruiting vine and grows along the ground, while the secondary vine serves as the vegetative vine and is trained upward on a trellis. Pruning begins when the vines are 50-60 cm long. Side branches before the fruit-setting node should be removed in time. For varieties with weak growth, more vines may be retained or pruning may be omitted.

V. Fruit Setting and Pollination

The position where the fruit is retained has a direct impact on fruit size and yield level. The second and third female flowers on the main vine should be selected for fruit retention. When the fruit grows to the size of a chicken egg, fruit selection and fruit setting can be carried out. Select and retain young fruits with enlarged ovaries, normal fruit shape and fresh, shiny color. Eliminate the rest. Because there is little insect activity in solar greenhouses, artificial pollination is needed to ensure fruit setting. Every morning after 9 o'clock, when the female flowers open and the male flowers shed pollen, artificial pollination is carried out. In order for yellow-skinned watermelons to receive light evenly and have a uniform yellow rind, turning the fruit should begin about 20 days after flowering, so that the color of the rind on the bottom is bright and beautiful. Fruit turning should be carried out when the sun is in the west.