Detailed Explanation of Key Techniques for Grafted Bitter Melon Cultivation in Greenhouses

Bitter melon, also known as balsam pear, is a plant in the Cucurbitaceae family and an annual climbing herb. The stems, branches, petioles and pedicels are covered with soft hairs, with axillary tendrils. The leaves reach 3 to 12 cm in diameter, with 5 to 7 palmate deep lobes, the lobes elliptical with serrated outer edges. It flowers at the turn of spring and summer, is monoecious, and yellow. The fruit is oblong-elliptical, with numerous irregular wart-like protrusions on the surface. The seeds are hidden within the fleshy fruit, wrapped in a red aril when mature. Bitter melon is a vegetable that people enjoy; its place of origin is unclear, but it is generally believed to be native to tropical regions. It is widely cultivated in South Asia, Southeast Asia, China, and the Caribbean islands.

The botanical history of bitter melon

Earlier medical texts did not record bitter melon; it was first included in the Ming Dynasty's “Herbs for Famine Relief” and “Compendium of Materia Medica”, and is suspected to have been transplanted from the Nanyang Islands when the Sanbao Eunuch made his voyages to the Western Seas. Wang Mengying of the Qing Dynasty said in “Recipes for Daily Living”: “Bitter melon when fresh is bitter and cold; it clears heat, brightens the eyes, and calms the heart. It can be pickled or preserved. Those with internal cold (cold constitution) should not eat it. When cooked it turns red in color, sweet in taste and neutral in nature, nourishing the blood and liver, moistening the spleen and tonifying the kidneys.” This means that when the melon is ripe and yellow, the bitterness decreases, the cold nature is reduced, and the nourishing effect becomes apparent; compared with when it is not yet ripe, it is used as a tonic when fresh. In fact, bitter melon is best eaten when the color is green and not yet yellow and ripe, and its heat-clearing and summer-heat-dispelling effects are even more valued.

Bitter melon tastes bitter, and southerners mostly eat it as a vegetable. Bitter melon can be eaten in summer and autumn; used as a side dish, it tastes delicious without the bitterness being noticeable. The Cantonese in China slice bitter melon, dry it in the sun and store it for medicinal use; this is the dried bitter melon used to treat summer colds.

Bitter melon is named for its taste; the word “bitter” sounds unpleasant, so the Cantonese also call it balsam pear (the Cantonese do not call it balsam pear but only bitter melon; it was only after people from other places or other provinces came to Guangdong that it was called balsam pear). Bitter melon has a shape like wart-like protrusions, so it is also called scabby melon; the melon surface is wrinkled, resembling lychee, so it is also called brocade lychee.

Key points for grafted bitter melon cultivation in greenhouses

To solve cultivation difficulties such as early-planted bitter melon not being tolerant of low temperatures, the first melons coming to market rather late, a short harvest period, and low yields, black-seeded squash can be used for grafted cultivation with bitter melon. Grafted bitter melon is planted in the greenhouse at the end of January, and fresh melons come to market in mid-March, 90 days earlier than conventional cultivation. Because it comes to market out of season, the price at the beginning is relatively high, and the economic benefit is greatly increased. The method is as follows:

1) Select suitable superior varieties

Bitter melon is a warm-loving crop, while black-seeded squash is extremely tolerant of low temperatures and weak light, and can still grow normally under low-temperature conditions of 5°C. Grafting bitter melon onto black-seeded squash means using the squash's root system, so that bitter melon can grow normally under low-temperature conditions of around 10°C, achieving earlier marketing, higher yields and increased benefits. The rootstocks and varieties selected for grafting are respectively: the main rootstock varieties are “90-l” and “Yunnan black-seeded squash”, both of which are relatively tolerant of low temperatures and have good grafting compatibility; the main scion varieties are “Lanshan long white bitter melon” and bitter melon varieties such as “89-1”, “89-2”, “90-1”, “90-2”, whose characteristics are long, straight melon strips and good market quality.

2) Soak seeds and sprout them, sow at the right time

① Soaking seeds to sprout them. Soak the selected bitter melon and black-seeded squash seeds in hot water at 55-60°C (bitter melon should be started 1-2 days earlier), stirring constantly so the seeds are heated evenly. After soaking for 10-15 minutes, add cold water to bring the water temperature down to around 30°C and continue soaking. After soaking for 8-12 hours, rub the seeds repeatedly, rinse them clean with warm water, wrap them in coarse gauze, and place them at 28-30°C to sprout. Rinse the seeds once a day with warm water at 30°C, drain off excess water, and turn the seeds to increase air permeability. The seeds begin to sprout in 2 days, and in 3 days about 85% of the seeds have white tips showing, so they can be sown.

② Sowing at the right time. When bitter melon seedlings emerge and grow, they need a relatively high temperature, so electric heating wire hotbeds can be used for raising seedlings at sowing time. Before raising seedlings, prepare the land as needed, lay the electric heating wire, and install the previously prepared nutrient soil before sowing. The sowing time should preferably be the first or second ten days of December. Although the temperature is relatively low at this time, because an electric heating seedling bed is used, the seedlings can emerge and grow normally. The temperature during the emergence period is generally controlled at 32°C during the day and 22°C at night. Black-seeded squash should be sown 1-2 days later than bitter melon, and the temperature required for emergence is 30°C during the day and 18°C at night.