Field-grown watermelon

Field-grown watermelon is a heat-loving, annual crop of great economic importance for many regions of Greece. Growers cultivate watermelon, a member of the Cucurbitaceae family, for its fruit, which contains high levels of water, sugars, and lycopene. Suitable for light to medium soils, watermelon yields its best in areas with high sunshine and temperatures. The central growing regions are Trifilia, Argolis, Ilia, Xanthi, and Rodopi. The sowing season varies by region, with the aim of harvesting from June to August. Farmers commonly use hybrids that exhibit vigorous growth, tolerance to viruses, and uniform fruit size.

Botanical characteristics

Watermelon (Citrullus lanatus) is an annual, creeping, or climbing plant of the Cucurbitaceae family, originating in Africa. It has an extensive, shallow root system and a large leaf area, which supports photosynthesis. It is monoecious, with more male flowers, while the female flowers appear later on the nodes. Bees primarily perform pollination and play a crucial role in the setting of fruit. The fruit is a fleshy pod, rich in water and sugars, with or without seeds, depending on the variety.

Physiological characteristics

Watermelon is a C4 plant with intense photosynthetic activity and high efficiency under conditions of full sunlight and high temperatures. Vegetative growth is rapid, characterized by extensive lateral shoots and large leaves that shield the fruits from the sun. Flowering begins approximately 4–5 weeks after sowing or transplanting, followed by fruit set, which is sensitive to weather conditions. Water requirements increase significantly during fruiting, especially from the stage of kernel hardening onward. The period from fruit set to ripening lasts 30–40 days, depending on the variety and temperature. Temperatures below 15°C delay growth, while temperatures above 38°C negatively affect fruit quality.

Morphological characteristics

The watermelon develops creeping, multi-branched shoots with a hairy surface, reaching a length of up to 4 meters. Lobed, large, and deep green, the leaves boast a rich photosynthetic surface. The male and female parts of the solitary flowers emerge a few days later. The fruit is spherical or ellipsoidal, with green or ribbed skin, red or pink flesh, and a weight that usually ranges from 6 to 12 kg. The seeds are black or brown, while sterile hybrids (seedless) require the presence of a pollinating variety. The root system is rich but superficial, making it sensitive to over-irrigation and low oxygen levels.

Reproductive characteristics

Watermelon cultivation begins with sowing in a nursery and transplanting to the field after 15 March, when the soil temperature exceeds 16°C. Flowering begins 25–30 days later, with the first female flowers appearing on the 6th or 7th leaf. Bees and bumblebees naturally assist in pollination, and we recommend installing bee colonies in the field (2–3 hives per acre). Fruit sets are susceptible to sudden temperature fluctuations or low sunlight. Growers limit the number of fruits per plant to 1–2 to ensure better quality and uniformity of the crop. Harvesting occurs when the skin of the fruit hardens, the base turns yellow, and tapping it produces a heavy, hollow sound.

Climate requirements

Watermelon is a thermophilic and photophobic plant, which means that its successful open-field cultivation depends entirely on suitable climatic conditions.

Temperature

The ideal temperature for germination, growth, and fruit set of watermelon ranges between 22°C and 35°C. Temperatures <16°C slow down vegetative growth, while temperatures <12°C limit root activity. Conversely, temperatures >38–40°C with dry air cause fruit set failure, leaf burns, and quality deterioration. Frost damages the plant at temperatures below 2–3°C.

Rainfall

Rainfall is beneficial in the early stages of cultivation but becomes problematic from the fruit-setting stage onward. Rainfall during ripening, in humid and hot conditions, increases the risk of fungal diseases and fruit cracking. The total water requirement is 500–600 mm, with increased demand during the rapid fruit growth phase.

Sunlight

Watermelon requires intense sunlight, with at least 10 hours of sunshine per day, for photosynthesis and sugar production to take place unimpeded, which determines the taste and commercial value of the fruit. Shading or prolonged cloud cover results in bland flesh, poor coloration, and reduced soluble solids (Brix) content.

Wind

Moderate winds do not significantly affect the crop, but strong winds (>40 km/h) can cause mechanical damage to shoots and flowers, as well as disrupt pollination. We recommend planting in a direction parallel to the wind or using windbreaks in areas with frequent winds.

Moisture

Maintain a relative humidity of 50–70% during flowering and fruit set. Too much humidity (over 80%) while the fruit is ripening increases the chances of botrytis (Botrytis cinerea) or anthracnose. If it rains, such conditions can cause the fruit to split because the roots absorb water too quickly, leading to excessive moisture.

Soil requirements

Watermelon grows best in sandy to medium-textured, well-drained soils rich in organic matter and with a depth of more than 60–80 cm. The pH should be between 6 and 7.5. Avoid cultivating in heavy, clayey, or cold soils, as they increase the risk of root rot. Farmers should apply fertilizer and manure before sowing or transplanting seeds or plants until the soil is fully incorporated. The use of black plastic for soil cover reduces evaporation, limits weeds, and increases soil temperature. In cases of soil pathogens or monoculture, we recommend using hybrids with resistant rootstocks.

Soil preparation

Soil preparation is critical for outdoor watermelon cultivation, as the plant is sensitive to compaction and poor drainage. Conduct soil analysis in advance to adjust organic matter, pH (6.0–7.0), and nutrient levels. Prefer light, well-drained soils; avoid heavy, wet soils due to disease risk.

Basic cultivation includes plowing (30–40 cm), harrowing, leveling, and ridge formation. Incorporate manure or compost (2–4 t/ha) and basic fertilization. Install plastic mulch and drip irrigation before planting. In areas contaminated with pathogens, we use soil disinfection. Planting begins when the soil temperature exceeds 16–18°C.

 

Basic Fertilization

We apply basic fertilization during soil preparation and adjust it according to the analysis results. We give special attention to phosphorus and potassium to improve rooting and fruit quality. Growers can supplement the application with microbial inoculants or organic extracts to enhance soil life. They then continue fertilization through hydroponic methods via drip irrigation, depending on the plant’s developmental stage.

  • Nitrogen (Ν)

    Nitrogen is essential for germination and early development. Excess leads to excessive foliage and reduced fruit quality.

  • Phosphorus (P)

    Phosphorus is essential for a strong root system, flowering, and fruit set.

  • Potassium (Κ)

    Potassium enhances color, sweetness, and the consistency of the flesh. It is crucial for the transportation of sugar.

  • Calcium (Ca)

    Calcium helps reduce cracking and strengthens the fruit’s structure. It also facilitates the transport of other essential nutrients.

  • Magnesium (Mg)

    Magnesium aids photosynthesis. A deficiency causes the older leaves to yellow.

  • Sulfur (S)

    Sulfur is essential for the production of aromatic compounds and the health of plants.

  • Boron (B)

    Boron is essential for the development of fruit and helps prevent hollow hearts in them.

  • Zinc (Zn)

    Zinc regulates auxins and promotes shoot growth.

  • Iron (Fe)

    Iron prevents chlorosis and is essential for the production of chlorophyll.

  • Manganese (Mg)

    Manganese plays a prominent role in photosynthesis and antioxidant defense.

  • Copper (Cu)

    Copper is associated with metabolism and plant resistance to pathogens.

  • Molybdenum (Mo)

    Molybdenum aids nitrogen utilization and is helpful in acidic soils.

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Foliar applications

Growers use foliar applications as a supplementary measure, particularly in cases of nutrient deficiencies or increased requirements. They apply boron pre-flowering to enhance the fruit-setting process, and adding calcium after fruit-setting enhances stability and prevents cracking. Apply iron and magnesium during periods of low sunlight or when chlorosis is present. Applications are made early in the morning or in the afternoon, at temperatures below 28°C and humidity below 70%. Experts recommend using low-conductivity, highly soluble foliar preparations. Growers should avoid combining them with plant protection products unless they first check for phytotoxicity.

Drip irrigation

The drip irrigation system is a key tool in the nutrition of outdoor watermelons, ensuring accurate and gradual delivery of nutrients. At the beginning of cultivation (the first 3–4 weeks after transplanting), emphasis is placed on nitrogen (N) and phosphorus (P) to promote growth and root development. From the beginning of fruit set until ripening, the need for potassium (K) increases, which should be the dominant element in fertilization, as it determines the sweetness, size, and consistency of the flesh.

Growers apply fertilizer daily or every other day in small dosages (fertigation) to minimize losses and maintain a stable nutrient level. They use nitrate, phosphate, potash, and sulfate fertilizers alternately to control the pH and improve the absorption of the elements.

The total volume of irrigation water varies depending on the soil type and season, but during full fruiting, it ranges from 3 to 6 cubic meters per acre per week. In sandy soil or during periods of high temperatures, requirements can reach 1,000 liters per acre per day. It is essential to continuously monitor the electrical conductivity (EC) and pH of the solution, with ideal values of EC ranging from 1.5 to 2.2 dS/m and pH from 5.8 to 6.5.

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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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