WATERMELON – Agronomy

Back to > Major Fruits | Minor Fruits | Underutilized Fruits

Climate: Watermelons grow and produce fruits ideally during dry, sunny and long warm growing periods. Excessive rainfall and high humidity reduce productivity by affecting flowering and encouraging the development of leaf diseases. The plants are frost susceptible and low temperatures can cause growth abnormalities, poor setting and hollow fruit. Sunburn can be a problem for the fruit, particularly in the “ground spot” area if the fruit are placed upside down after harvest. Light green and grey-green melons are less likely to sunburn than the darker green or striped varieties.

Elevations up to 1000 m normally provide suitable conditions for growth although excessively high   temperatures of more than 300 °C may be harmful, reducing the degree of fertilization. Stable day – night temperatures promote a rapid growth rate.

Soils: Watermelons grow best on non-saline sandy loam or silt loam soils. Light-textured fields warm up faster in the spring and are therefore favored for early pro­duction. Very sandy soils have limited water-holding capacity and must be carefully irrigated and fertilized to allow for high yield potential. Clay soils are gener­ally avoided for watermelon culture, but they can be productive if irrigated with care to prevent prolonged saturation of the root zone (a condition that favors the development of root rot pathogens) and to allow good drainage between irrigations. Provided the soil is well prepared, watermelons can be grown on a wide range of soil types. Watermelons are well adapted to soils that are well drained, high in organic matter, with a good moisture retaining capacity. The main requirement is good drainage. Rigid irrigation practices must be adopted when growing on heavy soils otherwise plant losses from root rot may be high. Mud sticking to the harvested fruit can also make them unsightly and reduce market acceptance.

Watermelons can tolerate some degree of soil acidity. However, the pH of the soil should not be below 5.5 for good yields to be obtained. Liming at the recommended rate, should be done in the planting holes or mounds, if the soil pH is below 5.5.

Seed germinates best at soil temperatures of 20–35°C, and planted 2.5 cm deep. 0.45– 0.9 kg seed required per acre, depending on seed size, germination and plant spacing. Seeds in groups of 1-3 are sown 2- 4 cm deep in trenches, on mounds or prepared planting holes at 3-5 cm each way; seedlings are later thinned to 1 per station. Seedlings may also be raised in containers and transplanted when 10-14 cm high. Seed required per hectare is 2.5- 4 kg for a density of 5000 -10000 plants/ha.

In large-scale production in desert areas (of Southern California and in the San Joaquin Valley), USA, watermelons are plant­ed on flat beds 2 m wide and 20 to 30 cm high. In direct-seeded plantings, seed is placed 1.2 to 1.9 cm deep. After thinning and side-dressing, furrows are reformed prior to furrow irrigation. For transplants, a single drip irrigation tape is laid 15 to 20 cm below the bed surface. Black plastic film, 1.8 m wide, is then laid flat on 2-m beds to heat the soil and reduce weeds. Seeds or trans­plants are then planted directly through the plastic. Transplants are set 60 to 90 cm apart. About 15 cm of the plastic is covered with soil at the edges to hold the mulch in place.

In Australia, a cover crop is recommended before planting watermelons. If root-knot nematodes are a problem, a cereal crop such as forage sorghum should be used instead of legumes. All cover crop residues should be completely broken down before planting the watermelon crop. Ripping may be required to improve drainage on shallow or compacted soils. Laser leveling is recommended before planting. As watermelons are often used as a quick rotational crop, the cropping history of the ground they are to be planted on should be researched to ensure no carry over herbicides have been used. In sugar growing districts, it is a wise precaution to notify surrounding cane farmers of the crop’s presence, to avoid damage from herbicide spray drift.

Plastic mulch in conjunction with trickle irrigation and containerized seedlings is widely used in watermelon production. The plastic mulch used can vary from 0.9 to 1.2 m in width and is used to cover the plant row. Soil in the rows is usually raised about 10 cm in height and the trickle tape laid down the centre of the row or slightly to the side. The end is buried to about 5 cm deep. Both the bedding up and laying of the trickle tape can be carried out during or prior to the laying of the plastic.

Field planting is carried out by direct seed sowing in the field or by using containerized seedlings. As most commercial varieties are hybrids, the latter technique is preferred because of the expense of hybrid seed. Where plastic mulch is used, direct seeding machines on waterwheel planters are used, punching the seed or seedlings directly through the plastic mulch. For direct sowing approximately 1.5 to 2 kg of seed is required per hectare. Pre-germinated seed may be used to replant any which do not germinate.

Row spacing varies from 1.5 to 3.0 m and plant spacing ranges from 0.75 to 1.5 m. Seedless melons need a standard variety in close proximity to provide pollen for fertilization to occur. Without pollinators fruit will not set. Not more than two rows of the seedless variety should be planted to every one row of a standard variety for ease of identification at harvesting. Always ensure the seedless type is readily distinguishable from the standard variety. Seeds take 4 to 14 days to germinate and soil temperature should be at least 11 ° C at time of planting.

Icebox watermelon can be spaced more closely together in the field than picnic types, and in-row spacing is 60–90 cm while between-row spacing is 1.5–1.8 m.

For transplants production, allow 18.75–25 cm2 area per seedling. Seedlings should be between 3–5 weeks old with 3–4 leaves at transplanting. Seedless varieties may germinate very poorly, depending on variety and direct seeding is not recommended.

Seedless Watermelon: Common seedless watermelon planting configura­tions are one row of the seeded variety (pollinator) for every 2 to 3 rows of the seedless variety. However, the most popular practice is by mixing of seedless and pollinator vari­eties within the same row. In within-row plantings, the ratio of seedless to pollinator plants ranges from 2:1 to 5:1. As the seedless to pollinator ratio increases, the number of beehives should be increased to ensure pollination.

Although watermelon is a deep-rooted crop able that can tolerate a significant degree of soil moisture stress, peak production requires timely irrigation. After crop establishment (either by seed or transplant), irriga­tion may be withheld for a period of several weeks to encourage deep rooting. Watering should be regular throughout the growing period. Overwatering should be avoided when the plants are young but quantities should increase as the plant grows. Highest water requirements are during fruit set and fruit fill. However, irrigation should be managed to minimize water stress throughout the fruit set and fruit sizing periods. Water stress dur­ing early fruit development can result in small, mis­shapen fruit, and the occurrence of blossom end rot (a physiological disorder in which the blossom end of a fruit ceases to grow and becomes dark and leathery). As harvest approaches care must be taken to avoid large fluctuations in soil moisture content, as heavy irrigation (or rainfall) can result in fruit splitting.

In the past, watermelon was usually irrigated by the furrow method; irrigation was applied based on soil moisture status. In recent years, many growers have adopted drip irrigation. Drip irrigation lines are typically buried in the center of the soil beds. The irrigation system may be renovated each production season or left in place for a number of years, depend­ing on the grower’s management scheme and crop rotation. Drip irrigation scheduling is determined by potential evapotranspiration (ET_o) estimates and crop growth stage; frequency of irrigation can vary from once a week early in the season to daily during times of peak water demand. Some growers use drip irrigation lines placed in every other furrow after crop establishment. While this approach may not provide the full yield potential of a buried, in-row system, it does provide improved irrigation control compared with furrow irrigation, and the system is portable, which eliminates management issues, associated with crop rotation. Regardless of irrigation technique, care must be taken to minimize wetting of the bed tops. Fruit in contact with moist soil may develop unsightly ground spots and fruit rots. Water can be applied to the crop through overhead, furrow, or trickle irrigation. Trickle irrigation allows the most efficient use of available water and is compatible with fertigation (application of soluble fertilizers in the irrigation water).

The amount of water used to grow a crop will vary depending on the time of year and soil type but, the following figures are a useful guide:

Water quality is important, particularly if using overhead irrigation. EC (electrical conductivity) readings of much greater than 1300 to 1400 ms/cm could cause leaf burn and reduction in yield. With furrow or trickle systems, irrigation water with E.C readings of up to 2000 m s/cm can be used with care. Above this figure, losses from a disorder known as blossom end rot may occur. Avoid overhead irrigation between 7 – 11 am during flowering and fruit set, as this may deter bees from pollinating flowers.

Watermelon has moderate nutrient requirements com­pared with other vegetable crops, and due to its deep rooting it is efficient in extracting nutrients from the soil. However, a soil analysis is recommended. In the absence of a complete soil analysis, a complete fertilizer should be applied by spreading and incorporating (sandy soils) or drilling (red soils). NPK mixes at 700-1200 kg/ha of 5:6:4 or 300-500 kg/ha of 14:14:12 and nitrogen side-dressings can be used. Late applications or excessive nitrogen use in cold weather should be avoided as hollow melons may result. Excessive nitrogen application also favors the development of male flowers. A dressing of NPK should be applied to mounds or planting, followed by applications of a nitrogenous fertilizer at intervals up a flowering time. Alternatively liquid manure may be applied up to fruit set.

A high-yield watermelon crop will typically con­tain less than 180 kg/ha of nitrogen (N) in its biomass, therefore, the application of nitrogen beyond this amount is seldom warranted. Phosphorus (P) fertilizer requirement is a function of soil test phos­phorus level and soil temperature at the time of plant­ing. Soils with greater 20 PPM bicarbonate extractable phosphorus require little or no phosphorus fertiliza­tion. Soils below that level may require as much as 168 kg/ha of P₂O₅, with phos­phorus requirement increasing in colder soil and with lower soil test value. Soils with more than 120 PPM exchangeable potassium (K) can support high-yield watermelon production without fertilization, although a maintenance application of up to 168 kg/ha of K₂O can be used to maintain long-term soil fertility. Soils below 120 PPM exchangeable potassium should be fertilized with up to 168 kg per hectare of K₂O to ensure peak production. P should be applied preplant. Potassium applica­tion can be made preplant, after planting, as a sidedress application, or delivered in irrigation water (fertigation). Nitrogen application is typically split between a small preplant and one or more in-season applications. In drip-irrigated culture, small weekly nitrogen fertigations may be used to time nitrogen delivery to crop uptake rate. Where trickle irrigation is used, side dressings are applied through the trickle system. Depending on the results of soil or sap analysis, side dressings of calcium nitrate alternated with potassium nitrate can be applied every two weeks commencing at early running. Both these fertilizers are applied at between 20 and 40 kg per hectare. Magnesium sulphate at the rate of 20 kg per hectare should be applied after every 2 to 3 applications of the calcium or potassium nitrate. Plant tissue analysis can be used to gain an indication of the nutrient status of the crop during the season.

Pollination: Watermelons produce separate male and female flowers. Male flowers are produced initially, followed by production of both sexes usually at a ratio of 1 female to 7 males. Watermelons, pumpkins, cucumbers, and zucchinis can be planted side by side.

Cross-pollination can occur between cultivars of the same crop (e.g watermelons and watermelons) but not between different crops (e.g watermelons and pumpkins).

Bees are necessary for pollination. Bees need to be present and active in the crop. If bees are not plentiful, then import at least 2 hives/ha, spreading them around the field perimeter and check to ensure that male flowers are producing pollen. Poor pol­lination often causes misshapen fruit. A watermelon plant seldom produces more than 2 to 3 harvestable fruit. While it is too expensive to remove all excess fruit, misshapen and split fruit may be culled in the field to allow the plants to channel nutrients into mar­ketable fruit. Flowers are most receptive to pollination during the morning hours when bee activity is usually the highest. Bee activity is related to climate and is lower in cooler weather. Spraying and irrigation should be coordinated to occur when bees are least active. Care must be taken to prevent the bee colonies from being exposed to pesticides. If possible, use agrochemicals with minimal bee toxicity.