watermelon greenhouse

Watermelon (greenhouse cultivation)

Growing watermelons in greenhouses is an intensive, early production practice designed to supply the market before outdoor production begins. Watermelon (Citrullus lanatus), a member of the Cucurbitaceae family, grows rapidly and demands high levels of light, nutrients, and strict microclimatic control. Greenhouse cultivation typically begins in late January or early February, with harvesting occurring from May to June. Growers prefer hybrids with excellent fruit sets, earliness, and high flesh quality. They cultivate directly in the ground, utilizing black plastic and low internal tunnels to promote early growth. Proper management of pollination, pruning, and fertilization is essential for success.

Botanical characteristics

Watermelon is an annual, dicotyledonous plant of the Cucurbitaceae family, characterized by its creeping or climbing growth habit and rich foliage. It has monoecious flowers, with the male flowers appearing first and more frequently. In greenhouse cultivation, artificial pollination, the introduction of pollinators, or the use of parthenocarpy varieties is required. The fruit is a thick-skinned, water- and sugar-rich pod with red or yellow flesh, with or without seeds. The root system is shallow but extensive, requiring well-structured, aerated soil. Growers grow it vertically in greenhouses, controlling quality and size through topping and fruit handling.

Physiological characteristics

Greenhouse watermelon is a fast-growing plant with intense photosynthetic activity when there is sufficient sunlight and a suitable temperature. Vegetative growth begins immediately after transplanting, with rapid leaf expansion and shoot development. Flowering begins approximately 20–25 days after planting, and fruit set usually occurs in the 4th to 5th week. Light requirements are very high (ideally greater than 25,000 lux), while the optimal growth temperature ranges from 22°C to 30°C. Temperatures below 15°C slow down growth, while temperatures above 35°C cause stress. Normal ripening of the fruit takes about 35–40 days after successful fruit set. Controlling humidity inside the greenhouse is crucial to avoid physiological disorders and fungal problems.

Morphological characteristics

Watermelon is an annual herbaceous plant, creeping or prostrate, with long, hairy shoots. The leaves are large, lobed, and green, with a soft texture. The shoots multiply to a length of more than 2–3 meters in greenhouse conditions. Pollination is necessary because the flowers are solitary, yellow, and divided into male and female parts. The fruit is a large, fleshy berry (pseudofruit), spherical or elliptical, with a thick skin and smooth epidermis and red or pink flesh. The weight of the fruit in greenhouse varieties ranges from 3 to 10 kg, while seedless varieties require intercropping with a pollinating variety to ensure fruit production. The roots are shallow and sensitive to over-irrigation and salinity.

Reproductive characteristics

Reproduction is exclusively by transplanting ready seedlings, aged 25–30 days, which have been grown in cell trays. Growers mainly use hybrids that have strong growth, uniform fruit, and excellent resistance to viruses and downy mildew. Flowering begins within 3–4 weeks after planting, with the first female flowers appearing after the male flowers. Pollination is not adequate by natural means in the greenhouse, so manual pollination (with a brush) or the use of bumblebees (Bombus terrestris) is required. Fruit setting is a critical phase that depends on temperature, humidity, and nutrition. We recommend regulating the number of fruits (thinning) per plant (usually 1–2) for stable production.

Climate requirements

Watermelon requires specific climatic conditions to achieve high yields and quality fruit. You must meet the following climatic requirements:

Temperature

Growing watermelons in a greenhouse requires a stable and carefully regulated microclimate to support optimal growth and fruit quality. The daytime temperature should ideally range between 22°C and 30°C, while the nighttime temperature should be between 16°C and 20°C. Temperatures below 15°C inhibit shoot and root growth, while temperatures above 35°C, especially in combination with low humidity or high fruit load, lead to fruit set failure, cracking, or bitter taste due to stress.

Rainfall

In greenhouse cultivation, rainfall indirectly affects the microclimate by increasing humidity and reducing light penetration, particularly under conditions of poor ventilation. These conditions favor fungal diseases and hinder fruit sets and quality. When grown in soil, prolonged rain can lead to saturation, poor drainage, and root rot.

Sunlight

Watermelon has high solar radiation requirements, requiring at least 10–12 hours of sunshine per day for adequate photosynthesis and sugar production. Growers use transparent covers (made of PE or EVA) with high light transmission (>85% PAR) in greenhouses. We recommend using white ground cover films and, if necessary, artificial LED lighting during periods of reduced radiation. Maintaining the soil temperature above 16–18°C is crucial, as temperatures below this level reduce root activity.

Wind

Ventilation is a critical factor, not only for controlling temperature and humidity, but also for preventing air stagnation, which creates microclimates with a high risk of disease. Side and top ventilation, or the use of fans, is recommended in greenhouses without natural circulation. Continuous ventilation also aids in pollination, particularly when mechanical or insect pollination is not employed.

Moisture

Growers should maintain relative air humidity between 60% and 75%. If humidity exceeds 80%, it can lead to problems with fungi such as downy mildew (Pseudoperonospora cubensis) and botrytis (Botrytis cinerea), particularly in areas with flowers and young fruits. Conversely, low humidity (<50%) causes flower drying, pollination failure, and increased transpiration, leading to plant stress. For this reason, they must continuously monitor moisture levels and regulate them through ventilation or misting.

Soil requirements

Greenhouse melons require well-drained, light to medium soils rich in organic matter. The ideal pH is between 6 and 7.5. Cultivation in heavy or poorly aerated soils leads to suffocation of the root system and increases the risk of fungal infections (e.g., Fusarium). We recommend deep plowing before planting and adding compost or well-rotted manure to the soil. The use of black or two-color plastic mulch helps control weeds and maintain soil temperature. The conductivity (EC) of the soil should not exceed 2 dS/m, as the plant is moderately sensitive to salinity. Before planting, it is advisable to disinfect or sun-disinfect the soil.

Soil preparation

Soil preparation requires disinfection and soil care due to intensive cultivation and the risks posed by pathogens and salts. We remove plant residues and disinfect the structures. Growers follow this stage with deep plowing (30–40 cm) and the incorporation of organic matter (3–5 tons per hectare). We disinfect the soil through solarization, biological methods, or, if allowed, chemical methods.

Soil analysis is necessary to adjust the pH (6.0–6.8) and EC (<1.5 dS/m). In hydroponic or substrate-based systems, growers must follow specific preparation protocols to ensure optimal plant performance. We form raised beds with drainage, cover them with plastic mulch, and use drip irrigation. Planting begins when the soil temperature exceeds 16–18°C, with grafted plants to protect against soil-borne diseases.

Basic Fertilization

Before planting, greenhouse watermelons receive basic fertilization to boost their initial growth. Additionally, manure or compost (2–3 tons per acre) enriched with humic or marine extracts is applied to improve microbial activity. In phosphorus-deficient soils, emphasis is placed on P₂O₅ to enhance rooting. Growers apply basic fertilization to support the plants until establishment; afterward, they provide nutrition through drip irrigation.

  • Nitrogen (Ν)

    Nitrogen is essential for vegetative growth and yield, and it is mainly required as nitrate in the early stages and as ammonium nitrogen in the later stages.

  • Phosphorus (Ρ)

    Phosphorus promotes flowering, fruit sets, and root development. It is a critical element in the first 30 days after planting.

  • Potassium (Κ)

    Potassium determines the quality of the flesh’s color, taste, and consistency. It gradually increases as the dominant element before and during ripening.

  • Calcium (Ca)

    Calcium prevents cracking, enhances firmness, and improves post-harvest shelf life. You can also apply it foliarly.

  • Magnesium (Mg)

    Magnesium participates in photosynthesis. Its deficiency causes chlorosis in older leaves.

  • Sulfur (S)

    Sulfur participates in the synthesis of amino acids and improves the absorption of other nutrients.

  • Boron (Β)

    Boron is critical for fertilization, fruit set, and fruit elongation.

  • Zinc (Zn)

    Zinc is essential for initial growth. Its deficiency causes small leaves and delayed flowering.

  • Iron (Fe)

    Iron prevents chlorosis, especially in greenhouses with high soil pH.

  • Manganese (Mn)

    Manganese regulates enzymes related to growth. Overly aerated substrates often limit their availability.

  • Copper (Cu)

    Copper plays a vital role in pathogen resistance and cellular respiration.

  • Molybdenum (Μο)

    Molybdenum helps convert nitrates to ammonia and is particularly useful in cold or acidic conditions.

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Foliar applications

Foliar fertilization is a complementary strategy in greenhouse watermelon cultivation, especially during phases of increased nutrient demand (e.g., flowering, fruit set). Trace elements such as boron (B) are often applied before and during flowering to enhance fruit sets, and calcium (Ca) and magnesium (Mg) are used during fruiting to prevent cracking. Absorption is best under conditions of moderate humidity (60–70%) and temperatures <28°C. Applications should be made with low solution conductivity (<1.5 dS/m) and in combination with biostimulants or amino acids during periods of stress.

Drip irrigation

Drip irrigation is the primary means of nutrition in greenhouse watermelon cultivation. A drip system applies it, adjusting the daily water and fertilizer supply to the phenological stages. Growers prefer formulations high in phosphorus during the first to second week of establishment. Growers emphasize the importance of nitrogen for rapid growth and fruit sets. During fruiting, the focus shifts to potassium and calcium to ensure the production of high-quality fruit. The use of high-purity, water-soluble fertilizers is necessary to prevent dripping. The pH of the solution is adjusted to 5.5–6.2 for optimal absorption of the elements.

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The doses and growth stages mentioned are indicative and are always adjusted according to the recommendations of the agricultural consultant. Because rational fertilization is always assessed in relation to:

  • Expected yield
  • Soil characteristics
  • Planting density
  • Environmental conditions
  • Growth stage
  • Plant vigor
  • Cultivation duration
  • Hybrid

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