Watermelons require consistent warmth and a long growing season, which challenges gardeners in many temperate regions. They can be successfully grown in a greenhouse, which is often the ideal way to ensure a successful harvest. Cultivating Citrullus lanatus indoors provides the necessary environmental control to extend the growing period significantly. This controlled approach is particularly beneficial for gardeners facing short summers or unpredictable weather patterns.
Why Watermelons Thrive in a Greenhouse Environment
Watermelons are native to the dry, subtropical regions of Africa and demand high temperatures and intense sunlight to develop sweetness and size. The enclosed structure of a greenhouse provides the prolonged heat unavailable in cooler climates. Optimal daytime temperatures for vegetative growth range from 77°F to 86°F (25°C to 30°C), while night temperatures should remain between 64°F and 68°F (18°C to 20°C).
The greenhouse environment buffers the plants from sudden temperature drops that can stunt growth or cause fruit abortion. Maintaining a consistent soil temperature, preferably above 70°F (21°C), is beneficial for root development. This warmth allows the plant to focus its energy on producing large, sweet fruit over an extended period.
The structure also protects the delicate vines and developing fruit from harsh elements like strong winds, heavy rain, and frost. Although watermelons require substantial water, the controlled environment prevents over-saturation of the foliage and soil common during outdoor rainstorms. Regulating the environment minimizes plant stress, promoting uninterrupted growth and higher yields.
Planning the Physical Setup and Growing Medium
The vigorous, sprawling nature of the watermelon vine requires proper physical setup to manage its size within a confined greenhouse space. Watermelons are typically grown vertically on a robust trellis system rather than allowed to sprawl across the floor. This method saves significant space and improves air circulation, which is necessary because the vines and mature fruit become heavy.
Vines should be trained upward using netting, twine, or cages, and side shoots can be pruned to manage density. Once a female flower is pollinated and the fruit begins to swell, individual support for the melon is necessary. This is accomplished by creating slings for each developing fruit. Slings can be made from materials like old pantyhose, mesh bags, or fabric strips tied to the overhead trellis.
Watermelons perform well in large containers, grow bags, or raised beds, provided the medium is well-draining and rich in organic matter. A soil pH between 6.0 and 6.8 is optimal for nutrient uptake. If using containers, they must be substantial, holding at least 10 gallons of medium per plant. This size supports the extensive root system and the high water demands of a maturing vine.
Crucial Steps for Successful Fruit Set and Development
The necessity of manual pollination is the most distinct difference between outdoor and greenhouse cultivation. Since the enclosed structure excludes natural pollinators like bees, the grower must transfer pollen between the male and female flowers by hand. Watermelon plants produce separate male and female flowers. The female flower is distinguished by a miniature, unpollinated fruit swelling at the base, while the male flower is attached directly to the vine by a thin stem.
Pollination should be performed in the morning when the flowers have freshly opened and the pollen is most viable. The technique involves gently collecting pollen from the male flower’s anthers using a soft brush or by removing the entire male flower. This pollen is then carefully dabbed onto the sticky stigma in the center of the female flower. Successful pollination is confirmed if the tiny fruit begins to swell within a day or two; otherwise, it will turn yellow and drop off.
Controlling the climate is important during the flowering and fruiting stages. The optimal temperature range for pollination and fruit setting is 68°F to 77°F (20°C to 25°C). High humidity, common in enclosed spaces, can make pollen sticky and less effective. Therefore, ventilation is necessary to reduce humidity during the morning pollination window.
Watering requires careful balance; vines need abundant moisture as fruit develops, but the soil must never become waterlogged. Allowing the top inch or two of the medium to dry slightly between thorough waterings encourages a healthy root system. Once the fruit is approaching maturity, slightly reducing the water amount can concentrate the sugars, resulting in a sweeter melon.
Managing Common Greenhouse Pests and Diseases
The enclosed, warm, and humid greenhouse environment creates ideal conditions for pests and fungal diseases. Common pests targeting watermelons include aphids, spider mites, and whiteflies, which feed on plant sap and multiply rapidly without natural predators. These pests are often found on the undersides of leaves, causing discoloration, stunting growth, and weakening the vine.
Prevention through sanitation and air movement is the first line of defense. Removing plant debris, disinfecting containers, and using screens on ventilation openings minimize pest entry. Good air circulation, achieved through fans and strategic ventilation, keeps air moving around the foliage. This makes the environment less hospitable for pests and fungal spores.
Fungal diseases like powdery mildew and downy mildew thrive in stagnant, humid conditions. Powdery mildew appears as a white, flour-like growth on the leaves, while downy mildew causes yellowish spots on the upper leaf surface. Improving air circulation and avoiding overhead watering are effective preventive measures. If an infestation occurs, biological controls, such as predatory mites or ladybugs, or organic controls like neem oil or insecticidal soap, can manage the problem.