Guide to Growing Greenhouse Watermelon

Open-field watermelon cultivation often fails because of insects, pests, weeds, and unpredictable weather. The low yields and poor quality make growers lose money. Greenhouses are a great alternative for watermelon cultivation. By minimizing the impact of the natural environment and providing the optimal growing environment in a greenhouse, growers are able to obtain higher yields and better quality. But improper practices of watermelon cultivation in a greenhouse may result in leggy and weak plants, poor fruits, and yield reduction. Here, we would like to show the guide to growing watermelon in a greenhouse and help you get a successful cultivation.

Knowing watermelons

Watermelons are warm-season crops like cucumber and pumpkin. According to whether it contains seeds, watermelon is classified as seeded and seedless. Watermelon plants have sprawling vines and tendrils. There are large and lobed leaves on the vines.

Both female and male flowers are found in a watermelon plant, and they are bright yellow. The mature fruits of watermelons have a round or oblong shape and green rind. Some of them have stripes on the rind while others do not. The red or yellow flesh is juicy and sweet.

Watermelons thrive in well-drained soil and warm temperatures. They also require sufficient lighting. For greenhouse cultivation, controlled temperature, and supplemental lighting help watermelons grow better regardless of winter or cloudy days.

When and how to start watermelon seeding?

In general, the right time for planting watermelon is from late spring to early summer. Germination highly depends on temperature and moisture. Therefore, ensure the soil temperature is 21°C or above when you plan to start seeding. If the warm temperature is not available, you can use a warm germination room for seeding. Here are some tips for watermelon seeding:

• Ensure the soil temperature reaches 21°C or above.
• Fill and wet the trays, and then let them stand for 24 hours to drain excess water.
• Place seeds at least one inch deep in the mix.
• Do not water the seeds. The excess water and high moisture will inhibit oxygen.  
• Remove the trays to the greenhouse after two days (48 hours). If the seedlings are visibly to emerge, it is too late to remove the trays.

Transplant and grow watermelon in greenhouses

Environment control is critical for healthy and strong plant growth in a greenhouse. Keep the night temperature at 18°C and the day temperature at 21-24°C for continuous germination of watermelon. Too warm temperature will cause spindly plants, while a cold temperature will slow the plants' development. Don’t water until seedling emergence appears. You can only fertilize after the first true leaf appears. Please note that it is the true leaf, not the cotyledon leaf.

It takes about 4-6 weeks from seedling to transplant. The actual period depends on variety, growing environment, and your locations. Watermelon can be transplanted into containers or the ground. Water the transplants as soon as possible to ensure the root establishment. For good quality and stocky plant growth, watermelon plants require high intensity light, adequate heat, and sufficient moisture. If natural light cannot meet the needs of plant development, LED grow lights are ideal for supplementing lighting.

How to care for watermelon plants?

Precise and regular care is needed for greenhouse watermelon. Growing environment including water, temperature, humidity, fertilizer, disease management, lighting, etc., should be taken into account when caring for watermelon plants. Here, we show you the guide of three main factors, water, temperature, and lighting.

Water

Water the plants only when necessary. Watermelon plants require uniform and sufficient water for germination and rapid growth. But excessive water will restrict root growth, resulting in damping off and high seeding mortality. Therefore, the best time to water the plants is when the surface of the soil is dry to touch. In addition, water the plants in the morning. Wet foliage will introduce disease. Watering the plants in the morning allows the foliage to dry faster.

Temperature

Temperature ranges from 20°C to 25°C is suitable for growing watermelons.

Growers should consider the soil temperature because the soil temperature is the true temperature for root development. And the strong root is the basement of vigorous and healthy watermelon plants. Soil temperature between 21°C and 23°C is perfect for germination. After the trays move to greenhouses, set the night temperature at 18°C and the day temperature at 21-24°C.

Lighting

The natural lighting may be inadequate for greenhouse watermelon. However, lighting affects the flavor and taste of watermelon significantly. With sufficient lighting, watermelons are sweet and delicious. Therefore, supplemental lighting is needed.

With the addition of LED grow lights, watermelon plants can get the duration and intensity of light they need. LED grow lights also contribute to better plant growth. LEDs can emit light wavelengths from 250nm to 1000nm. It means that LEDs have great spectral control and the ability to match plant photosynthesis and growth. The survey showed that supplementing LED grow lights with natural daylight with a combination of red light (630nm, 660nm) and blue light (450nm, 460nm) was effective in improving chlorophyll content, plant height, fruit size, fruit mass, and flesh thickness.

The use of LED grow lights with optimal spectrum has proved that they are beneficial for greenhouse watermelon. Why not try retrofitting or adding LED grow lights to your greenhouse?

For greenhouse watermelons, appropriate seeding is the basis of successful cultivation. Moving the trays and transplanting the seedlings in time helps to form stocky plants. By providing warm temperatures, sufficient water, and adequate lighting, thrive growth, higher yields, and better fruit quality are achieved. Moreover, it has been proved that supplementing LED grow lights is effective in improving greenhouse watermelon cultivation.

Interested in the supplemental lighting for greenhouse watermelon, contact us!