When to Plant Watermelon in a Greenhouse? A Detailed Guide to Growing Techniques

Watermelon greenhouse cultivation belongs to intensive and meticulous cultivation, and it is highly technical. Only by adopting comprehensive supporting measures can the goals of early maturity, high yield, and high efficiency be achieved. So when should greenhouse watermelons be planted? What are the techniques and methods for growing watermelons in a greenhouse?

1. When to plant watermelons in a greenhouse?

If using four-layer film covering, you can start on December 20; for three-layer film seedling raising, it is not advisable to raise seedlings too early, so start raising seedlings on January 10; for two-layer film (plastic film + small arch film), raise seedlings around February 20; for open-field watermelon seedling raising, raise seedlings around March 10 and transplant around Qingming. Generally, the seedling age is required to be 35 days—45 days, and transplanting is done at the 2-leaf 1-heart—3-leaf 1-heart stage. Everyone should make reasonable arrangements according to the crop rotation, soil, and facilities.

2. Techniques and methods for growing watermelons in a greenhouse

1. Land preparation and fertilization

Greenhouse watermelons are generally planted at a relatively high density, so land preparation must be meticulous. If using a greenhouse that lies idle in winter, deep plow 25 cm before winter and carry out freeze-thawing of the soil to make it loose. Generally, per mu apply 4000-5000 kg of high-quality barnyard manure or 3000-4000 kg of fully decomposed chicken manure, 50 kg of superphosphate, 15-20 kg of potassium sulfate, and 100 kg of fully decomposed cake fertilizer as base fertilizer. Spread half of the base fertilizer evenly over the whole area and then plow it into the soil. After leveling the land, open furrows and apply the other half of the base fertilizer. Generally, small high ridges and high beds are used for cultivation, and beds are made at a row spacing of 1-1.2 m. Under the conditions of dense planting with trellises, grafted cultivation, two-vine pruning, and leaving 1 melon per plant, make small high ridges at a row spacing of 1 m. When using creeping ground cultivation, beds can be made in a north-south direction (parallel to the longitudinal direction of the greenhouse).

2. Transplanting and planting

Under the condition of covering with 3 layers of film, greenhouse watermelons should be planted about 10 days earlier than double-cover arch shed cultivation, and the density should be somewhat greater than that of double-cover arch shed cultivation, and careful pruning is required. Cultivation with dense planting on trellises should be denser than creeping ground cultivation.

3. Temperature management

After planting greenhouse watermelons, pay attention to increasing the soil temperature for 5-7 days. If the daytime temperature inside the greenhouse is higher than 35°C, measures should be taken to shade and cool it down. After the seedlings recover, ventilation can begin to regulate the temperature inside the greenhouse. Generally at this time, the daytime temperature inside the greenhouse should not be higher than 32°C, and the nighttime temperature should not be lower than 15°C. When the melon vines grow to about 31 cm, the small arch shed inside the greenhouse can be removed. During the full flowering period of greenhouse watermelons, sufficient light and relatively high nighttime temperatures should be maintained, because if the nighttime temperature is relatively low after artificial pollination, it will affect watermelon expansion and cause melon drop. When the outside temperature exceeds 18°C, ventilation should be increased, opening the skylight and both sides of the greenhouse at the same time for ventilation, so that the daytime temperature inside the greenhouse is not higher than 30°C, preventing excessive day-night temperature difference from causing the watermelon flesh to deteriorate and quality to decline.

4. Light management

(1) Increase light intake. Watermelons require relatively strong light. However, due to condensation or dirt on the surface of the greenhouse film, the light intensity inside the greenhouse is often reduced, especially under multi-layer covering. Therefore, attention should be paid to keeping the greenhouse film clean, and old films with poor light transmittance should not be used.

(2) Regulation of gas composition inside the greenhouse. Under sealed greenhouse conditions, the carbon dioxide content in the air is seriously insufficient, affecting the normal progress of photosynthesis and the accumulation of assimilates. Applying carbon dioxide gas fertilizer can increase the carbon dioxide concentration inside the greenhouse and supplement the carbon dioxide content inside the greenhouse.

5. Pruning and vine tying

(1) Pruning. Under dense planting conditions, greenhouse watermelons must be strictly pruned. When the main vine is 30-50 cm long, the side vines have also clearly extended out. When the side vines grow to 20 cm, select and retain one robust side vine, and remove all the rest. After that, the side vines growing on the main and side vines should be removed in time, and the top should be pinched after leaving 10-15 leaves above the fruit-setting node. Pruning work should be carried out before the fruit is set. When using dense planting cultivation on trellises, the work of removing side vines should continue until the melon vines fill the trellis and the top is pinched. While removing side vines, tendrils should also be removed.

(2) Building trellises and tying vines. Bamboo poles and hanging ropes, etc., can be used when building trellises, but bamboo poles are better because bamboo poles are less likely to cause watermelon drop. 20 days after planting greenhouse watermelons, when the main vine is about 30 cm long, immediately after removing the small arch shed inside the greenhouse, insert the trellis. Insert two bamboo poles per melon vine, on both sides of the plant, 10 cm from the root of the plant. The bamboo poles should be inserted firmly and upright. After inserting the trellis, begin guiding and tying the vines.

6. Artificial pollination

Pollination should be carried out from 8-9 a.m. On cloudy days, male flowers shed pollen later, so it can be appropriately delayed. To prevent male flowers from shedding pollen late on cloudy days, you can collect the male flowers that will open the next day on the afternoon of the previous day, place them indoors in a dry, warm place, so that they bloom and shed pollen on time the next morning, and then use these flowers to pollinate female flowers. Pollination should start from the second female flower in order to retain the fruit.

7. Selecting and hanging melons

To increase the weight of a single melon and make the melon shape proper, the melon set on the second female flower should be selected and retained. If the melon is retained too early, the melon will be small and the shape improper; if too late, it is not conducive to early marketing. Generally, 3-5 days after pollination, the young fruit will obviously grow larger. Priority should be given to retaining melons on the main vine. If they cannot be retained on the main vine, melons can be retained on side vines. When using dense planting cultivation on trellises, when the melon grows to the size of a bowl mouth, it should be hung in time to prevent the young melon from falling after it enlarges.

8. Topdressing and watering

In the early stage of greenhouse watermelons, the amount of watering should not be too large. After the seedlings recover, if the soil is not dry, there is no need to water; if it is too dry, water thoroughly once along the furrow.

In addition, note that no watering should be done during the flowering and fruit-setting period, to prevent excessive vegetative growth and promote fruit setting.