Last revised February 11, 2010
Many new varieties of watermelons have been developed in recent years. Yellow and seedless types are finding an increasing share of the specialty watermelon market. It is estimated that seedless red and yellow varieties that were virtually unknown ten years ago, and represent about 5% of the market today will increase their market share substantially in the near future. Small excellent quality "icebox" melons are also becoming increasingly popular.
For additional production information see Washington State University's Vegetable Research and Extension information for Icebox Watermelons.
Washington State University
Watermelons are best adapted to the warmer areas of Oregon, but may be grown in western Oregon if plastic ground mulches and row covers are used. They are grown commercially primarily in the Hermiston area, but may also be grown in the Ontario and Medford areas.
Maturity is approximately 80 days for small ice box types to 95 days for large-fruited melons from transplants in the Willamette Valley, or from seed in the lower Columbia River and Snake River areas.
See the Vegetable Variety Selection Resources page to find varieties that have been shown to perform well in Oregon. For an extensive list of varieties that have been trialed in the Pacific Northwest by Washington State University, see Icebox Watermelon Variety Trials.
SEED AND SEED TREATMENT
Watermelon seed numbers approximately 8,000/lb. Use fungicide-treated seed. Watermelons are subject to damping off and decay in cool wet soils. Seedless varieties may germinate very poorly, depending on variety. Expect to require 30% extra seed. Some seed companies offer primed seed, which can substantially improve germination.
The minimum soil temperature at the 2-inch depth required for germination of these crops is 60 F, with the optimum range between 70 to 95 F. Seedless varieties should not be direct seeded (see "Transplant Production" section, below).
SEEDING AND TRANSPLANTING
Watermelons are normally direct seeded in eastern Oregon and transplanted in western Oregon. All seedless watermelons are transplanted (see "Transplant Production" and "Flowering and Pollination" sections below). Use approximately 1.5-2.5 lb of seed/acre depending on seed size and variety.
Under normal conditions, about 15-25 square feet per plant is optimal. This would result in a plant population of 1200-1700 per acre. Where wind damage may be a factor, plant 6 seeds per foot and thin to an in-row spacing of 12-36 inches with 6-12 feet between rows. The closer spacings help tie the vines together and reduce wind damage.
Begin seeding April 10 - May 5 in the Columbia Basin and mid-May in the Willamette Valley. Minimum soil temperatures between 55 and 60 F at the 4-inch depth are desirable for first 48 hours of planting. Plant the seed 1 inch deep.
Use windbreaks as necessary especially in eastern Oregon. Grain windbreaks are effective when grain rows are used for each melon row. Establish windbreaks in the fall, spacing windbreak rows at intervals the width of the melon rows. Winter wheat varieties, rye, or oats can be used. Spring barley may be used for February plantings. Seed grain thickly, 2-3 seeds/inch. This requires about 10 lb of barley, 9 lb of wheat, or 8 lb of rye to seed grain rows 12 feet apart.
Windbreaks may be cultivated out after the melon plants are well established. If they are not, windbreaks should not be allowed to touch the plants because abrasion of the enlarging fruit can cause that fruit to be misshapen. Windbreaks may be cut off or rototilled around mid June before melon vines develop long runners that may be damaged by tractor tires.
Direct seeding is not recommended for seedless watermelons due to the high cost of seed and its slow and erratic germination under all but ideal conditions. To produce transplants, seed in modular trays in the greenhouse, allowing 3-4 square inches per plant. Research in Florida and elsewhere has shown the following for seedless watermelon germination:
Seedless watermelon seed germination requires temperatures of 85-90 F.
Excessive water during germination must be avoided. Water transplant growing medium well and allow excess moisture to drain and moisture to stabilize for 24-48 hours. Bring medium temperature to 85-90 F before seeding. Seedcoat adherence to cotyledons may be virtually eliminated by orienting the seed in transplant trays with the pointed end up at a 45 to 90 degree angle.
After seeding, place transplant trays in a germination room where temperature is held at 85-90 F and humidity is not limiting. When seedlings begin to emerge, move the trays to a greenhouse held at 80 F until emergence is complete.
Water only if necessary during the first week. Do not overwater. After germination is complete, vary greenhouse temperature as needed to produce a sturdy transplant.
Note: As a general rule direct field seeding of the pollenizer variety should be done on the same day the triploid seed is planted in the greenhouse. Small-fruited, icebox varieties usually flower earlier than standard watermelon varieties. If icebox varieties are to be used as the pollenizer, then direct seeding should be delayed a week to ten days. The diploid icebox pollenizer variety will frequently set fruit and stop producing male blossoms while the triploid variety is still producing female blossoms. Growers may make a second planting of a pollenizer 2 to 3 weeks after the initial planting to provide pollen for the late-developing female blossoms on the triploid variety. No consistent differences among any standard and icebox types in effectiveness of pollination have been noted. Icebox varieties used as pollenizers result in high early yields; standard varieties used as pollenizers result in high total yields.
Depending on transplant container size, seedlings should be between 3 and 5 weeks old (with 3-4 leaves per plant) at transplanting. Older plants establish very slowly in the field. Transplant to the field after all danger of frost has passed and soil temperatures are, and are likely to remain, above 70 F. Once established, seedless varieties are quite vigorous, and are resistant or tolerant to several soil-borne and foliar diseases.
FLOWERING AND POLLINATION
Watermelons bear separate male and female flowers on the same plant (monoecious). Only the female flowers set fruit. Bees transfer pollen from male flowers to female flowers, making fruit set possible.
It is recommended that at least one honey bee colony be introduced for every acre during the blooming period since native bee populations may not be adequate, or may not coincide properly with the blooming period. Research in California indicates that "a higher quality marketable crop results. Furthermore, "the harvest period was advanced a week or more and shortened by one week, reducing the pickings necessary by 33%".
Placement of colonies in the field has an effect on the number of bee visits per flower. Visits per flower were more than doubled with colonies spaced no more than 175 yards apart in comparison with colonies placed at only one spot in a (40 acre) field. At no time should any portion of a field be more than 250 yards from a bee hive. Avoid using insecticides injurious to bees and manage application of pesticides in a manner to protect bees and apiaries from injury.
For successful seedless watermelon production, an adequate bee population is especially important to transfer the pollen from the pollenizer variety to the seedless variety (seedless watermelons do not produce pollen). The pollenizer variety is normally planted in alternate, or every third row to insure adequate pollen movement by the bees. At least eight visits to an individual flower of the seedless variety are necessary for adequate hormonal stimulation for normal fruit development.
When first flowers occur under periods of temperature or moisture stress, first fruits are often seedy, rough, small and of poor quality. To reduce this risk, growers often remove first fruit by hand when it is small or delay placement of bees for one or two weeks or until the first female flowers have dropped off in the seedless variety.
Use a distinctly different variety for the normally seeded pollinator in order to easily distinguish the seedless fruit from the pollinator for marketing purposes.
Questions come up about cucumbers, melons, gourds, and summer and winter squash, crossing and affecting the eating quality of one vine crop or another. This is NOT a problem. Intercrossing is only a problem when seed is saved for replanting, in which case squashes of the SAME species need to be isolated for crop purity. Cucurbits of different species do not intercross sufficiently to create problems for seed producers.
Gibberellic acid (GA) is labelled for stimulation of fruit set in watermelon (except in California) during periods of cool temperatures. The rate is 2 g ai/acre in sufficient water to obtain thorough foliage coverage. The label calls for one application before bloom followed by two more applications at intervals of 10-14 days. Caution: The efficacy of GA applications for stimulating fruit set in melons has not been confirmed by research in the Pacific Northwest.
For the most current advice, see Nutrient Management for Sustainable Vegetable Cropping Systems in Western Oregon, available as a free download from the OSU Extension Catalog
The following are general recommendations. A soil test is recommended for each field to be planted. The following are recommendations for eastern Oregon production.
Nitrogen: 90-120 (N) lb/acre. Apply l/2 to 3/4 of the N as a side dressing as the vines begin to send out runners. Account for residual N levels in the top 24 inches of soil, as you plan your fertilizer requirements. Excess N is detrimental to watermelon flavor.
Phosphorus: 50-80 (P2O5) lb/acre. Band all the phosphorus 2 inches to the side and 2 inches below the seed row.
Potassium: 60-120 (K2O) lb/acre. Broadcast before planting.
Sulfur: 30-50 (S) lb/acre. Broadcast before planting.
Micronutrients: Apply as determined by soil test. Micronutrients that should be tested for are zinc, manganese and boron.
Watermelon is known to be sensitive to manganese toxicity, a common problem in low pH soils in western Oregon. Data from Mississippi indicates an association between high leaf manganese concentration and poor growth and yield of watermelon. Seedling watermelons react to manganese toxicity with stunted growth and yellowish crinkled leaves. Older plants generally exhibit brown spots on older leaves that may be mistaken for symptoms of gummy stem blight.
Manganese toxicity is usually associated with soils having a pH below 5.5. However, in wet seasons the condition may occur at higher pH levels when the soil has been saturated for a period of several days. This condition has been noted in several watermelon fields with pH ranges at 5.8 or slightly higher when the crop was planted flat. Planting watermelons and other cucurbits on a bed is good insurance against manganese toxicity during a wet season.
The best solution to manganese toxicity is to apply lime in the fall at rates based on the results of a soil test. A pH of 6.0 should be maintained for maximum yields, according to the Mississippi study.
GROUND MULCHES AND ROW COVERS
The use of black plastic or clear plastic ground mulches is recommended, especially in western Oregon, to enhance yield and earliness. Black plastic mulch controls weeds, may increase soil temperature, conserves moisture, and protects fruit from ground rots. For black plastic mulch to increase soil temperature, it is critical that the soil surface be smooth and that the plastic be in close contact with the soil. This can only be achieved by laying the plastic with a machine designed and properly adjusted for this task. Clear plastic mulch is superior for heat transfer to the soil but does not control weeds.
A new generation of plastic mulch films allows for good weed control together with soil warming that is intermediate between black plastic and clear film. These films are called IRT (infrared-transmitting) or wavelength-selective and are brown or green in color. They are more expensive than black or clear films, but may be cost effective where soil warming is important.
Plastic, spunbonded, and non-woven materials have been developed as crop covers for use as windbreaks, for frost protection, and to enhance yield and earliness. They complement the use of plastic mulch and drip irrigation in many crops.
Non-woven or spunbonded polyester and perforated polyethylene row covers may be used for 4 to 8 weeks immediately after seeding or transplanting. Covers should be removed when plants begin to flower to allow proper pollination. Row covers increase heat unit accumulation by 2 to 3 times over ambient. Two to four degrees of frost protection may also be obtained at night. Soil temperatures and root growth are also increased under row covers as are early yields, and in some cases total yields.
Watermelons are deep rooted in sandy soils when growth is vigorous. They require uniform irrigation for optimum growth and yield. Reduce irrigations as fruit reach harvest stage. A total of 12-15 inches of water may be needed in western Oregon and 20-25 inches in eastern Oregon, depending on seasonal variation, area, irrigation method (drip or sprinklers) and variety. Approximate summer irrigation needs for the Hermiston area have been found to be: 3.5 inches in May, 5.0 in June, 7.5 in July, and 7.0 in August. Research has shown that the use of drip irrigation under black plastic mulch is superior to sprinkler irrigation with black plastic mulch. Yields usually increase dramatically. Drip irrigation under plastic mulch is an effective way of applying water efficiently and may reduce total water requirements by as much as 30%.
Soil type does not affect the amount of total water needed, but does dictate frequency of water application. Lighter soils need more frequent water applications, but less water applied per application.
Watermelon is often grown with furrow irrigation in eastern Oregon. Water soluble polyacrylamide (PAM) is useful for flocculating soil particles in irrigation furrows and reducing erosion of soil from the furrow.
HARVESTING, HANDLING, AND STORAGE
Yields have ranged from 2,000-3,000 fruit per acre for standard varieties and ground culture (more for icebox types). This equals about 15 to 30 tons/acre. With the adoption of new production practices, yields of 45 to 70 tons/acre have been produced when plastic mulch, row covers and drip irrigation are used in conjunction with disease-resistant hybrids. Under normal conditions, harvest in the Hermiston area begins around August 1.
Watermelons are hand harvested into bins or trucks for shed packing. Use every sixth or eighth row as a heap row. Shortly before the harvesting season begins, the vines are turned out of the alleys. Harvest only melons that are ripe. Ripeness is indicated by a creamish to slight yellowing of the white background color of the part of the melon that rests on the ground. Drying of the stem tendril nearest the attachment point of the watermelon and green color tone of the rind are also indicators of ripeness but these vary with cultivar. Melons should be cut from the vine rather than pulled, twisted, or broken off.
Never stand melons on end, and handle the melons carefully at all times. For highest sugar levels, vines must be maintained in a healthy state throughout harvest.
Seedless varieties have thicker, more durable rinds than standard varieties, and hold better (do not become overripe as quickly) in the field, and in storage.
STORAGE (Quoted or modified from USDA Ag. Handbook 66 and other sources)
Hold watermelon at 50 to 60 F and a relative humidity of 90 %. Watermelons are not adapted to long storage. At low temperatures the are subject to various symptoms of chilling injury and loss of quality, and at high temperatures they are subject to decay. Between 50 and 60 F is a good compromise. Watermelons should keep at this temperature range for 2 to 3 weeks; some will keep longer. Melons held 6 weeks at room temperature will have poor flavor.
Watermelons should be consumed within 2 to 3 weeks after harvest, primarily because of the gradual loss of crispness. Quality in watermelons is determined largely by high sugar content, a deep red fresh color, and a pleasant crisp texture of the edible flesh. These factors are dependent on maturity, cultivar, and handling methods. Commercial melons for distant market are usually harvested when mature, but before full ripeness, to minimize handling damage and flesh breakdown. They are at their best for eating when mature . Immature melons have a pink flesh, mature melons are red to dark red, and over-mature ones have orange flesh. Actually, the red color and flavor of watermelons improve during storage for 7 days at or above room temperature, while at 50 F or below color fades. The decrease in redness may be due to chilling, since these melons develop other symptoms of chilling, such as pitting and loss of flavor, at low temperatures.
In tests with Florida watermelons stored at 43, 50, and 60 F for 2 weeks, chilling injury was observed during and after storage at 43 and 50 F but not at 60 F. Decay, mainly black rot, was always higher on melons previously stored at 43 or 50 F than on those held at 60 F, and it developed mainly after storage. Although decay is usually not a major form of deterioration, extended storage at warm temperature 75 F will result in more decay than at cooler temperatures. Seedless watermelons retain their quality for 2-3 weeks at temperatures of 50-60 F and 90% relative humidity.
Watermelons are sensitive to high levels of ethylene gas during storage, as it hastens loss of firmness. Melons exposed to 30 or 60 ppm ethylene for 7 days at 65 F were unacceptable for eating. Even at the relatively low concentrations of 5 ppm ethylene, watermelons will become less firm and less acceptable. Watermelons should not be stored or shipped with fruits that emit substantial amounts of ethylene.
Rough handling will result in serious losses. Watermelons should not be dropped, thrown, or walked on, as internal bruising and flesh breakdown will occur. Cartons holding three to five melons and bulk bins with pallets, if used, can speed handling and minimize melon damage.
Storage and marketing diseases are black rot, phytophthora rot, rhizopus rot, and stem-end rot.
Watermelons are packaged in 55 to 80-lb cartons holding 3-5 melons; 80-lb 2WGA crates; 800 to 1000-lb small bins; 1400 to1800-lb medium bins; or, shipped in bulk truck loads weighing 34,000-45,000 lb.