Watermelon

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Suitable varieties for organic production

Best market varieties of watermelon (Citrullus lanatis) fulfill characteristics such as strong rind for better transportation, small mesocarp (the white part between the rind and the fruit flesh) and sweet and juicy flesh. Seedless varieties also exist, but cause higher seed costs (and needs to be explained to the consumer as fruits are not 100% free of seeds). In general, the selection of the right variety depends on different factors such as site conditions, local availability, market demands and resistances (in case of watermelons some varieties are resistant against Fusarium, Anthracnose,etc.).

Propagation and Nursery Management

Direct sowing is possible but not recommendable when dealing with seedless varieties or less than ideal site conditions. The latter would increase the germination time and delayed development in the plant. Watermelon vines require space; therefore, the following practices are suggested:

  • Plant seeds 3cm deep in hills spaced 2m apart.
  • Allow 2-3m between rows.
  • After seedlings are established, plant single transplants 60-90cm apart in the rows, with an optimum maximum of three plants per hill.
  • To replace failures, it is best to prepare seedlings inside. Start the seeds 3 weeks before they are to be set out in the field. Plant 2 or 3 seeds in pellets, pots or cell packs and thin to the best one or two plants.

For expensive seedless types, plant one seed to a pot or cell and discard those that do not germinate. Do not start too early - large watermelon seedlings transplant poorly. If you grow seedless melons, you must plant a standard seeded variety alongside. The seedless melon varieties do not have the fertile pollen necessary to pollinate and set the fruit.

Harvest and post-harvest handling

Handling

Harvest generally begins about 30 days after full bloom and continues for several weeks with 3 to 4 cuttings at 3-to-5-day intervals. Ripeness in watermelon is difficult to determine because the fruit remains attached to the vine, rather than “slipping” off. The flesh of a typical red-fleshed watermelon changes from immature pink to red-ripe, and then to overripe within a 10-to-14 days harvest window. Overripe fruits have a watery, mushy texture and lower sugars. Rind color changes indicating maturity, if any, are specific to cultivars. The “Golden Midget” variety turns yellow as it ripens, and “Sugar Baby” becomes dark green and loses its stripes. Generally, however, the only indication of ripeness is that the tendrils of the leaf closest to the fruit attachment become dry.

Additional ripeness indicators include a change in ground spot color from greenish-white to pale yellow. The rind becomes hard to pierce with the fingernail and the blossom end “fills out”. When ripe, there is also a “bloom” or powdery coating giving the fruit a duller appearance and a rough feel. Although researchers are experimenting with various non-destructive gauges of fruit soluble sugars, the usual method for timing the start of watermelon harvest is to cut open a few representative melons in the field.

If the field has received abundant water, the watermelons may crack open, especially if harvested in the morning when they are full of water (turgid). The risk of cracking can be reduced by harvesting in the afternoon and by cutting the stem rather than pulling the fruit off. Stacking watermelons on their side, rather than on their end, also reduces the risk of cracking.

Cut watermelons must be shaded to minimize additional heat build-up and because direct sunlight after harvest (especially on the ground-spot) reduces watermelon quality. If plants are not too turgid, field heat can also be minimized by harvesting in the morning. Watermelons should be dry when loaded, however, rather than dew-covered. They are either bulk loaded into straw-padded trucks, or placed in multi-walled corrugated fibreboard bins holding 60 to 80 melons and weighing 500 to 550kg when fully loaded (which corresponds to an average weight of 7.5kg per melon). Transit temperatures should be 7 to 10°C. Watermelons are stored at higher temperatures and lower humidity than muskmelons (10° to 13°C, 90 percent relative humidity). Storage for prolonged periods below 10°C can lead to chilling injury; one week at 0°C can cause pitting, color loss and off flavors. At 10° to 13°C, they can be kept 2 to 3 weeks after harvest. Even within this range, however, the red color is gradually lost.

Although watermelons do not ripen off the vine, flavor and color in seeded watermelons will improve over a 7 day holding period at room temperature.


Sources and References

Bibliography

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Soil Fertility Management. [Accessed 19 March 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Conversion to Organic Farming. [Accessed 19 March 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Crop Management. [Accessed 21 March 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2005). Organic Cotton Crop Guide. Authors: Frank Eyhorn (FiBL), Saro G. Ratter (BioSim), Mahesh Ramakrishnan (ICCOA). [Accessed 19 March 2012]

FiBL (2011): African Organic Agriculture Training Manual. Version 1.0 June 2011. Edited by Gilles Weidmann and Lukas Kilcher. Research Institute of Organic Agriculture FiBL, Frick. [Accessed 12 July 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Soil Fertility Management. [Accessed 19 March 2012]

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