Watermelon (Pakwan) Cultivation Guide: All You Need to Know in Planting, Growing and Harvesting Watermelon

Watermelon (Citrullus lanatus) known locally as pakwan or sandia is a highly profitable crop in the Philippines, with increasing demand from both the domestic and international markets. It is a warm-season crop that requires high temperature and adequate water supply for optimal growth and fruit development. 

Nationwide, watermelon is cultivated in about 6,500 hectares of land, with major production areas in Iloilo (27%), Pangasinan (16%), and Nueva Ecija (11%). While the majority of the area is planted during the regular season from October to January, some commercial growers also cultivate the crop during the off-season. In 2005, Ilocos Norte only accounted for 3% of the total harvested area, according to FAO crop statistics.

The crop is typically grown in the lowlands during the dry season from November to May and in the uplands during the wet season from June to October. Watermelon can be grown in a variety of soil types but prefers well-drained loamy or sandy loam soil. With proper cultural management, watermelon production can provide substantial income for farmers.

Varieties

Select varieties which are high-yielding, early-maturing, tolerant to pests and diseases, adapted to local climatic conditions, and are easy to sell. The following varieties were tested in Currimao, Ilocos Norte.

Variety	Fruit Characteristics			Maturity (Days after transplanting)
	Weight/ fruit (kg)	Rind color	Flesh color
Sugar Baby	4-5	dark green	red	65
Sweet Senorita F1	3-4	light green- striped	deep red	56-60
Sweet Ruby F1	4-5	light green- striped	deep red	60 -65
Potenza F1	4-5	light green- striped	deep red	60-65
Sweet Gold F1	3-4	dark green- striped	yellow orange	56-60

Land Preparation

Plow clayey and weedy fields at least twice to help eliminate weeds, hibernating insect pests, and soil-borne diseases. The best time to plow the field is when a ball does not form when the soil is squeezed by hand and only a thin film of the soil sticks to the fingers and palm. Plow at a depth of 15-20 cm.

Harrow twice to break the clods and level the field.

A well-pulverized soil promotes good soil aeration and enhances root formation.

Raising Seedlings

Watermelon is usually direct-seeded, but raising seedlings in trays is also recommended because it uses less seeds, promotes uniform growth of quality seedlings, reduces transplanting shock, and minimizes seedling mortality. It also saves on labor for thinning, weeding, watering, and pest management.

Follow these steps in raising seedlings in plastic trays or bags.

• Preparing the sowing materials:

1. Prepare the sowing medium by mixing thoroughly 2 parts rice straw compost, 4 parts carbonized rice hull (CRH), and 1 part processed chicken manure (PCM).
   
   
2. When using seedling trays, fill the holes with the medium and slightly compact it with your palm. Use a seedling tray with 100 or 104 holes. The amount of medium in each hole contains enough nutrients to sustain the seedling until transplanting time.
   
   When using black polyethylene bags (6.5 cm x 9.0 cm), fill the bags with the sowing medium and slightly compact it with your fingers.

• Pre-germinating the seeds:

1. Soak the seeds in tap water for 30 minutes. A 1000 m2 area (tenth of a hectare) requires 30 grams (g) of seeds.
2. Remove the seeds from the water.
3. Spread the seeds in a wet, clean cotton cloth, and roll.
4. Incubate in a safe dark place until the radicles come out.

Watermelon seeds germinate in 2-3 days at 250C to 300C. In January and February when the temperature is colder, germination can extend to five days or more.

When the radicles come out, the seeds are ready for sowing.

• Sowing:
  

1. Sow one seed per hole of the plastic tray, or per bag at a depth of 1.5 cm.
2. Cover the seed with enough medium.
3. Water the seeds using sprinkler with fine droplets.
4. Cover the tray or bags with old newspaper, plastic sack, or rice straw to maintain soil moisture and temperature.
5. Remove the cover as soon as 1 or 2 seeds have sprouted.

• Caring for and maintaining the seedlings:

1. Place the trays on the platforms of a simple nursery. Roof the nursery with transparent plastic sheets during the rainy season; plastic nets or chicken wire during the dry season, for the seedlings to take full advantage of the sunlight. Seedlings elongate abnormally when put under the shade.
2. Water early in the morning and afternoon. Apply less water during rainy and cloudy days. The seedlings weaken and elongate with too much water.

Construction of Planting Beds and Furrows

• When using plastic mulch construct 1.0 m wide beds raised to 20 cm height. Space the adjacent beds at 0.5 m (refer to Fig.1) to serve as path for manual watering and furrow irrigation. Provide a 4.0 m crawling space for the vines after every two adjacent beds.
  
  
  
• For the modified zero tillage technology, plow and harrow only the areas, about 1.0 m wide, where the seedlings are to be transplanted. Make furrows for the planting rows. Space the two adjacent furrows 0.6 m apart. Provide a 4.0 m crawling space for the vines after every two adjacent furrows (refer to Fig. 2).
  
  
  

Fertilizer Application

Collect soil samples for analysis at the nearest soils laboratory. Apply fertilizers based on the results of the analysis. Otherwise, use the following recommended rates:

• With plastic mulch

Basal

Mix the fertilizers thoroughly and distribute the mixture evenly in a shallow furrow per 10 linear meters, or incorporate it in a 10 m2 planting area:

• 1 kg PCM or any commercial organic fertilizer
• 1 kg ammonium phosphate (16-20-0)
• 0.4 kg ammonium sulfate (21-0-0)
• 0.2 kg muriate of potash (0-0-60)

Cover the fertilizers applied in furrows with at least 3 kg of CRH
per 10 linear meters.

Side dress

Drench 170 ml (1 small can of sardines) of fertilizer solution per hill. Follow this recommended schedule, and source and amount of fertilizer to apply.

Time of Application                           Drenching Solution
3 weeks after transplanting (WAT)	4 tbsp urea (46-0-0) per 15 L water
5 WAT (onset of flowering) and 7 WAT (onset of fruit setting)	4 tbsp muriate of potash per (0- 0-60)/15L water

• Modified zero tillage

Basal

Apply 1 kg PCM or any commercial organic fertilizer and 1 kg ammonium phosphate (16-20-0) per 10 linear meters of planting furrows. Cover the fertilizers with at least 3 kg of CRH per 10 linear meters.

Side dress

Dibble the 1st side dress application. Dibble 2nd side dress at the midpoint (30 cm) between plants along the rows, or distribute it evenly on a shallow furrow along the planting rows. Follow this recommended schedule, and source and amount of fertilizers:

Time of Application                           Amount of Fertilizer
3 weeks after transplanting (WAT)    1 tbsp (15g) urea/hill
6 WAT	1.3 tbsp (15 g) muriate of potash /hill or 250 g (21.7  tbsp)/10 linear meters

Mulching

Mulching is recommended in growing watermelon to conserve soil moisture and control weeds. Using rice straw is a common practice among farmers. The use of plastic mulch is an improved technology for watermelon production. It controls weeds, preserves soil moisture, prevents soil erosion and leaching of fertilizers, and reflects sunlight to repel insect pests hiding under the leaves.

Placing the plastic mulch:

1. Stretch the plastic mulch over the planting bed, with the silver-colored side on top.
   
   
2. Fasten the edges of the plastic mulch to the soil with bamboo slats spaced 1.0 m apart.
   
   If bamboo slats are not available, cover the edges of the plastic sheet with soil using a rake, or by passing a moldboard plow near the edges.
   

Making the planting holes:

1. Measure 0.6 m planting distance 10 cm from the edge of the plastic mulch and mark the spots. Make sure that the markings are on the farther side of the bed from the crawling space.
2. Make planting holes following any of these three ways:
   
   
   • Using a serrated-lipped tin can: cut the lip of a 7-10 cm diameter tin can with metal scissors to form 1-cm long saw- like edges. Push the can on the marked planting spots of the plastic sheet, serrated side down.
     
     
   • Using a hot tin can: punch small holes using a nail at the sides and bottom of a 7-10 cm diameter tin can. Attach a wooden or bamboo handle. Fill the can with burning charcoal. The small holes ventilate the charcoal to keep it burning.
     
     
   • Using a cutter: cut intersecting lines 7-10 cm long to form “+”or “x” at the marked planting spots. During transplanting, fold up the4 flaps to form square planting holes.

Transplanting

1. For beds with plastic mulch flood the beds two days before transplanting to cool down the soil under the plastic mulch, and to dissolve the fertilizers applied.
   
   For the unmulched beds irrigate the furrows to dissolve the fertilizers applied and to minimize direct contact of the seedling roots the fertilizers.
   
   
2. Transplant 12 to 15 day-old watermelon seedlings, which should now have 2-3 true leaves. If plastic bags were used, transplant seedlings when they are 21-30 days old.
   
   
3. Transplant only one seedling per hole preferably in the afternoon. A 1000 m2 area (10th of a hectare) requires 740 seedlings in mulched areas; 724 seedlings in unmulched areas.
   
   
4. Water the transplants immediately with a sprinkler.

Irrigation

Water should be adequate throughout the vegetative stage for better growth of the plants. When the fruits start to develop, furrow-irrigate weekly to produce bigger fruits. Stop irrigating when the fruits reach full size (about 2 weeks before harvesting). Watermelon is sweeter when it does not absorb excessive water while ripening. Irrigate after each fertilizer application.


Pruning, Vine Training, and Fruit Thinning

1. Pinch off the main shoot at the 4th node. Allow three major vines to develop.
2. Remove all side shoots that appear before the first fruit to be maintained, to avoid overcrowding, and to control vegetative growth. Allow succeeding side shoots to grow. They will help nourish the developing fruits.
3. Train the vines toward the crawling area by clipping them with fine bamboo slats. Mulch the crawling area with rice straw for the tendrils to cling to.
4. Remove as early as possible all fruit buds before the 10th node of the major vines. Fruits too near the crown will not develop well. If the early fruits are allowed to grow, the succeeding fruits will be small and they sometimes fall off.
5. Maintain only two fruits per plant, preferably the 2nd fruit that develops on a vine. Pinch off all other fruits.

Pollination

Watermelon belongs to the family of cucurbits, which produce separate male and female flowers on the same plant. Hence, manual pollination is very important to ensure better fruit setting and development.

Manually pollinate the 2nd and 3rd female flowers. Detach a male flower and rub it on the female flower. Do this not later than 8:30 AM before the pollens dry up.

The 2nd and 3rd female flowers bloom only within 10 days, hence be alert when 2nd female flowers start to open.


Pests and their  Management

1. Fruitfly, Bactrocera cucurbitae (Coquillett), is the most damaging insect pest in watermelon causing 67% fruit damage during DS 2006 in Currimao, Ilocos Norte. The adult fruitfly is a good flyer and can travel long distances, hence, hard to manage.
   
   The fruitfly lays its eggs on young fruits. The eggs later hatch into small worms and bore into the fruits to feed on the flesh. Microorganisms follow the worms, making the fruits rot.
   
   Shrinking, deformed, and rotting fruits are symptoms of fruitfly damage.
   
   Manage fruitfly by collecting regularly all damaged fruits and burying or decomposing them in sealed plastic bags.
   
   Cover the female flower with a 9 cm x 12 cm paper packet or envelope after manual pollination to protect the developing fruit.
   
   
2. Squash or cucurbit beetle, Aulacophora sp., feeds on the leaves of new transplants that may result to total defoliation.
   
   Dust wood ash on the watermelon leaves to repel the beetle.
   
   
3. Aphids, Aphis gossypii Glover, suck the sap of leaves causing them to curl and turn yellow.
   
   Monitor the plants regularly for initial infestation of aphids. Ants carry aphids to the plants so spray the ants with any green-label insecticide. Aphids can be controlled by spraying water or chilli- soap solution at high pressure to dislodge them from the plant, consequently disrupting their life cycle. Remove infested leaves or uproot the whole plant if necessary.
   
   
4. The leafroller, Diaphania indica (Saunders), feeds on and rolls young leaves and shoots. Too much nitrogen fertilizer results to very dense and green foliage, a condition very favorable to the pest.
   
   The larvae become destructive and feed on the rind of the fruits when the population of the pest is not managed properly. Careless chemical spraying could increase pest population. Two wasp species (Cotesia sp and Opius sp) that parasitize up to 92% of the larvae could also be killed by the insecticides applied.
   
   In small production areas, leaf rollers can be managed by removing rolled leaves. Then crush the larvae and pupae which are usually found at the shoots and underneath fruit. In larger areas, spray the larvae up to the 2nd instar with “halt” (a biological insecticide).
   
   
5. Downy mildew is a fungal leaf disease which usually occurs when the watermelon fruits are about 10-15 cm in diameter. Symptoms are small irregularly shaped yellow spots turning bigger and dark brown as the disease progresses. The leaves curl and die.
   
   A susceptible variety, high humidity, and windy weather could hasten the spread of the disease. Management includes close monitoring and spraying of systemic fungicide immediately upon observation of symptoms. Unnecessary flooding triggers development of the disease.
   
   The OP var Sugar Baby is more tolerant to the disease than some hybrid varieties.

Turning of Fruits

Turn the fruit when it has almost reached full size to expose the underside to the sun, and prevent an uneven color of the rind. Uneven color lowers the attractiveness and marketability of the fruit.


Harvest and Post Harvest Handling

Harvesting starts 56-65 DAT, depending on variety.  Harvest early in the morning and protect the fruits from the sun, rain, and mechanical damage. Remove damaged fruits. Sort according to market standards.

Pack the fruits in bamboo crates lined with old newspaper or rice straw when transporting.

Line platforms with rice straw when displaying and storing watermelons. The fruits of some varieties can be stored for a month under this condition.

Checklist and Calendar for Cultivating Watermelon (Pakwan)