Agronomy

Description

As a cucurbit crop, watermelons are related to squash, pumpkins, cucumbers, muskmelons and gourds. They grow as a native crop in Africa (Jett), where watermelons have served as a water source. Watermelon fruits also supply lycopene, an antioxidant thought to affect cancer risk.

Growers throughout the world raise watermelons in warm areas. Consumers demand watermelons that are sweet. They’ve also shown interest in smaller watermelons that they can easily store in their refrigerators, pre-cut sections of whole watermelons and watermelons with unique flesh colors or rind patterns. Operations may sell their watermelon crop at produce auctions, farmers markets, local retailers or roadside stands. Large-scale operations may sell watermelons one field at a time to large-scale buyers, or brokers can help to move significant volumes of product.

Site Selection

When producing watermelons, the selected site should not have grown cucurbit crops, including cucumbers, squash and pumpkins; tomatoes; or peppers during recent years. If a field has had previous watermelon disease pressure, then it may not be a suitable site, or it may require fumigation before growing watermelons.

With respect to soil needs, watermelons prefer sandy or sandy loam soils. The soil should have limited compaction concerns because compaction will likely constrict watermelon root growth. The soil should also support good water and air infiltration. If growing watermelons in a sandy area, then installing windbreaks may be necessary. Young seedlings can be harmed by blowing sand. A wheat, rye or Sudan stand can serve as a windbreak. The growing site’s soil pH level should range from roughly 6.0 to 6.5. Taking a soil test that’s representative of the entire growing area can inform whether adjusting soil pH is necessary.

Fertility

To gauge nutrient needs, watermelon operations should conduct a soil test before the season begins and make fertilizer applications according to the soil test results. During the growing season, operations may pull plant tissue samples to check whether nutrient deficiencies arise in the crop.

In watermelon production, macronutrient fertility is important. At the maximum, growers may apply 120 pounds of nitrogen, phosphorus and potassium per acre. From a micronutrient perspective, key nutrients to monitor are boron, calcium, sulfur, zinc and magnesium. Boron applications can support watermelon pollination and fruit set. Applying calcium is a prevention strategy that some growers use to limit blossom-end rot incidence in watermelon fruit. Magnesium is a nutrient that may leach from the soil during the growing season and require additional supplementation.

Operations may choose whether to apply all fertilizer at preplanting or stagger applications throughout the growing season. Yields may be maximized if an operation applies fertilizer doses during the season rather than just at the season’s start. One option is applying all phosphorus and potassium and half of the nitrogen before planting. Then, the other half of the nitrogen application can be timed four weeks to six weeks post-seeding.

Within-season fertilizer applications can be administered through drip irrigation if water-soluble nutrients are available. Sidedress applications are also possible. If using granular fertilizer during the growing season, then fertilizer that reaches the leaf surfaces may cause burning. To limit this effect, operations may wash leaves following the fertilizer application.

Variety Selection

Watermelon varieties are diverse. Marketplace preferences and fit with the given growing environment should dictate an operation’s variety selection. Watermelon varieties can be distinguished by their fruit size, fruit shape, rind pattern and flesh color. Size can vary from 5 pounds to 35 pounds. For shape, watermelons may grow into round or oblong fruit. Rind patterns are described as crimson type, jubilee type, allsweet type and black or dark green. In terms of flesh color, many options are available: red, dark red, pink, orange, yellow and white. Varieties may also have different extents of disease resistance.

Varieties can also be designated in one of three categories: open-pollinated; F1 hybrid; or seedless, which is otherwise known as triploid. To produce open-pollinated watermelon varieties, seeds are selected from generation to generation given desirable traits, including yield and disease resistance. Growers may save seed from one year, plant it in another year and produce fruit with traits identical to those yielded in the first year.

Conversely, F1 seeds are hybrids created by crossing two inbred lines. Relative to selecting open-pollinated varieties, choosing F1 hybrid seed will enable operators to produce more uniform melons and harvest on a more predictable schedule.

Seedless watermelon represents the greatest share of watermelon acreage today. The seedless, or triploid, varieties are developed by crossing a variety with two chromosome sets and a variety with four chromosome sets. Crossing these varieties means that triploid watermelons have an unusual chromosome count. Despite the name “seedless,” it’s not that the triploid watermelons produce no seed, but the seed that they produce is undeveloped. The undeveloped seeds tend to be soft and edible.

Growing triploid watermelons has some differences relative to growing open-pollinated or hybrid varieties. For example, triploid watermelon seedcoats are thicker, which can challenge their germination. The seed also tends to be more expensive, which means that overseeding and thinning aren’t cost-effective. Achieving a solid first stand is critical.

Additionally, triploid varieties produce less pollen than other types of watermelons. To ensure that the pollen supply is adequate in a seedless watermelon field, growers must add seeded watermelon varieties to their planting plans. Alternatively, they can raise plants of a nonharvestable pollenizer watermelon variety to ensure that the triploid plants have an adequate supply of viable pollen available. Regardless of the other variety that’s selected, it — the more significant pollen producer — should bloom at the same time as the triploid plants. Producers often plant the pollenizer variety in a 1:3 ratio with the seedless watermelon variety, or one pollenizer row for every two triploid rows is another possible planting site configuration. Planting a pollenizer row along the field perimeter may also help to supply adequate pollen. Given that two varieties — seedless and nonharvestable pollenizer or seeded — would be raised in one field, operators must implement a system to sort seedless watermelons from the other melons.

Farms in the U.S. and Mexico grow roughly 200 to 300 watermelon varieties. Of those, 50 varieties tend to be most popular. The table lists common seeded and seedless varieties.

Common Watermelon Varieties

Planting

Producers may choose from planting watermelon as seed or transplants. Seed should be uniform, have no disease exposure and range from 85 percent to 90 percent germination. Additionally, it should be certified. With seed, the desired planting depth is roughly 1 inch. Deeper planting would be necessary if planting in sandy or dry soil. The desired planting population varies based on factors such as seed size, germination and spacing. However, plan to allocate 1 pound to 2 pounds per acre.

With transplants, growers can more efficiently make use of watermelon seed, and the crop may mature earlier, which enables the operation to harvest and sell watermelons sooner. The planting population with watermelon transplants typically ranges from 2,800 plants to 4,400 plants per acre. Often, rows are 5 feet to 8 feet apart, and spacing within the row tends to range from 24 inches to 36 inches in a plastic mulch system. Row and within row spacing may vary depending on whether a particular operation uses irrigation. As a general guide, provide 24 square feet to each plant.

To grow transplants, choose a container size that allows plants to grow without being root-bound. Each container can have one to two seeds. Hybrid and triploid seeds are expensive, so consider just using one seed per container for those watermelon types. For successful watermelon transplants, consider sourcing a growing medium that lacks weed, insect and disease contamination; providing high-quality light exposure; and supplying a growing environment with warm temperatures and sufficient moisture. Avoid watering too much, however. Generally, during the afternoon before planting, operations should water the growing medium. No other moisture should be applied until post-emergence.

Transplants should also have a hardening-off period before being planted in the field. The hardening-off period’s purpose is to transition watermelon plants to their outdoor growing environments. During hardening-off, which can range from three days to four days, young plants should be exposed to cooler temperatures and less moisture. “Hardened” plants can better tolerate the environmental conditions that they’ll face in the field, but note that plants that are “overhardened” may experience their own challenges, including slow growth.

Watermelon seedlings are ready for transplanting when they have grown to have two to four true leaves. Their stems should be short and thick, and they should have strong, healthy roots (Jett). The planter should place transplants slightly deeper than the plants had been growing. None of a peat pot should be exposed, and the roots shouldn’t be disturbed or seedlings could experience “transplant shock.” After transplanting, water the plants. The water not only supplies moisture for the plant to use, but it also helps to eliminate soil air pockets near seedling root systems.

Regardless of whether operations start watermelons in the field from seed or transplants, planting should be timed well. Watermelon plants thrive when temperatures are warm. To germinate, seed requires soil temperatures that are 68 degrees F to 95 degrees F. Temperatures being too cool will extend the germination process. As the germination process lengthens, the risk of uneven stands and crop loss may rise. Later, as watermelons are developing, temperatures that are too cool will cause fruit to ripen more slowly. Watermelon crops prefer temperatures that range from 50 degrees F to 95 degrees F, or their growth will lag.

Depending on a grower’s preferences, some specific site preparation operations may be necessary before planting. As an example, producers may choose to build raised beds, which support watermelons maturing earlier and enable the soil to drain better.

Before planting, producers have the option to lay plastic mulch and drip tape. The plastic mulch enables the soil to warm quickly, supports soil moisture retention, deters nutrient leaching and reduces weed competition. Some resources suggest that producing watermelons in a plastic mulch system may lead to planting and harvesting watermelons earlier and boosting fruit yields. Plastic mulch may also discourage fruit rot. Unless an operation lays degradable plastic, the plastic must be removed after the growing season.

As an alternative to growing watermelons in an open field, operations may raise watermelons in high tunnels, which are structures that serve as passive solar greenhouses. By raising watermelons in a high tunnel, operations may grow the crop during a longer period of time; remove drought, rain, wind and other environmental stresses from being production factors; and reduce a crop’s exposure to insect pressure and disease. Prior to planting, site preparation within the high tunnel is necessary. An ideal site will have undergone tillage and had a fertilizer application. Raised beds should be built to boost the soil temperature and enhance soil aeration and drainage. Plastic mulch can cover the raised beds and accompany drip irrigation lines. To provide extra protection, operations can add row covers after they transplant watermelon seedlings in a high tunnel. The row covers can supply added protection until watermelon plants flower. Pollinators must work within the high tunnel to pollinate blossoms. To support pollination, operators may add honey bee colonies near the high tunnel entrance or introduce bumblebees within a high tunnel.

Cultural Management

Good pollination is necessary for watermelon operations. Pollinating a watermelon crop by hand may work for some operations. However, the pollination window each day is relatively narrow — 6 a.m. to 9 a.m. — and hand pollination tends to be effective just half of the time.

Instead of hand pollination, pollinators are more frequently used to support pollination, and they should be available every day as watermelon flowers bloom. A watermelon blossom has a one-day viable life. Within that one day, a pollinator typically needs to visit a female flower roughly eight times in seeded melon varieties and 21 times in seedless melon varieties to provide enough pollen for sufficient fertilization. Seedless varieties require more bee visits because pollinators may expose the female flowers to pollen from both the seedless and seeded or pollenizer plant. To ensure that watermelon fields have an adequate pollinator population available, add honey bee hives. For every one or two acres planted to watermelons, supply one 30,000-bee hive.

Because honey bees tend to prefer other blooms relative to watermelon flowers, watermelon operations should reduce the competition by removing nearby flowering weeds and positioning other flowers a distance from watermelon fields. Additionally, bees require a clean water source, so provide one if clean water otherwise isn’t available nearby.

Watermelon growers can adopt practices to preserve the health of honey bee pollinators. In particular, insecticides should only be applied during off-peak hours for bees when they aren’t typically pollinating crops. Given that honey bees tend to move through fields from dawn to dusk, insecticide applications are best timed at some point after the sun sets.

Fruit thinning is another cultural management need associated with watermelon production. When vines are dry, fruit thinning involves identifying and removing watermelons that have an odd shape or some other defect. By removing damaged fruits, the remaining fruit may grow larger, and the plant may continue to set new healthy fruit. Growers may also choose to thin the crop to two to three fruit per plant — by removing even healthy fruit — to encourage larger melon growth.

Last, depending on the variety and growing environment, producers may train their watermelon plants to grow on a trellis. This may be viable when raising mini watermelon varieties, which are those that grow to be no heavier than 6 pounds, in a high tunnel. Built from materials such as string, wire and netting, using a trellis has several advantages in watermelon production: enhanced light interception, simplified harvest, boosted pollination and decreased harvest-related vine injury. A watermelon trellis system should use mesh bags, such as those used to package fruit or vegetables, to hold melons until harvest time.

Water Management

More than 90 percent of a ripe watermelon is water. For a watermelon crop to grow and yield well, consistent moisture availability is important. Water needs vary by development stage. Moisture stress is particularly detrimental as watermelon plants flower and support developing melons. Watering well within just the top 12 inches of soil, which is where most roots grow, will ensure that water is efficiently used.

Although sufficient moisture is important, too much water can damage the crop. Melons may crack if moisture levels are too high. Additionally, too much moisture near harvest time can reduce the fruit’s sugar level and cause the fruit to develop white heart. With irrigation, the time of day is important. Late afternoon, evening or night irrigation may stimulate foliage diseases. If irrigating early in the day with a sprinkler, then don’t disrupt pollinators.

As mentioned earlier, drip irrigation is one option available to watermelon producers. The drip irrigation line can not only provide a slow and steady moisture source — this enables operations to use water efficiently — but it can also be used to deliver water-soluble nutrients to plants. Drip tubes should be covered with an inch of soil to discourage rodents from harming the lines and avoid having the lines expand and contract. Sprinkler irrigation is an alternative to drip irrigation.

Weed Control

In watermelon fields, weeds present a problem as they can use light, space, nutrients and water that would otherwise be available to the watermelon crop itself. Additionally, insects and diseases can live within weed cover, and weeds can make the watermelon harvest more challenging.

For effective control, weed management can begin before watermelon plants emerge. Creating a stale seedbed through chemical applications is one strategy. If operations lay plastic mulch, then recall from earlier that weed control is one benefit associated with this practice. For best results, the mulch should be black or made from a plastic material that prevents light from penetrating the mulch barrier.

Weed control strategies include hand weeding, mechanical operations before the plants start to vine and chemical applications. Also note that when watermelon plants develop a canopy that shades the ground, the crop itself can limit weed pressure.

Insects and Diseases

Several diseases are harmful to watermelon crops. Among them are anthracnose, gummy stem blight, Phytophthora fruit rot, Fusarium wilt, Alternaria leaf spot, Cercospora leaf spot and yellow vine decline. Others include damping-off, downy mildew, rind necrosis, fruit blotch and watermelon mosaic virus. Disease prevention starts by sourcing disease-free seed, selecting disease-resistant varieties, removing all crop litter that may be infected from production sites, appropriately rotating crops and choosing a site with the air and water infiltration. Because rain can spread diseases, disease control strategies may precede and follow rain showers. Regular scouting, particularly in areas prone to disease outbreaks, may help with early disease detection. Fungicide applications tend to produce better results if they’re initiated prior to diseases spreading.

From an insect pressure perspective, insects have the potential to target watermelon leaves, stems, roots and fruit. Watermelon crops may be threatened by aphids, cucumber beetles and two-spotted spider mites. Other pests that may harm watermelon crops are wireworms, whitefringed beetle larvae, root maggots, thrips, cutworms, pickleworms, melonworms and rindworms. As a strategy to minimize insect-related harm to young seedlings, operators should give watermelon crops the best chance at a strong start. Seedlings that establish themselves quickly are more likely to bypass insect-related damage. To monitor pest populations during the growing season, fields should be scouted on a weekly basis. Insect traps may assist with detecting insect pressure in fields.

Although raccoons, coyotes, dogs and deer aren’t insects, they are pests that can harm watermelon crops. Rodents, such as field mice and rats, may eat seeds that haven’t germinated. Also, crows may poke holes in watermelon fruit.

Watermelon are also susceptible to certain defects. As mentioned earlier, a calcium deficiency may trigger blossom-end rot, which is characterized by tissue at the blossom end dying and turning black. Poor moisture availability is another factor that may contribute to blossom-end rot. Hollow heart and white heart are fruit development issues that result in the watermelon having a hollow or white interior. To prevent these conditions, choose varieties that tend to be less susceptible, and supply adequate moisture and nutrients. Sunscald is a sunburn-related condition that may lead to watermelons rotting more easily. It can be avoided by ensuring that a good vine canopy protects the melons. Stem splitting, often exhibited in conditions with high humidity and moisture, and sandblasting, which refers to wind or sand damage to young plants, are other potential problems.

Harvest and Storage

Watermelon harvest timing depends on the extent of the transportation distance. Fully mature watermelons have the most desirable taste and texture. If shipping watermelons, however, then the melons should be mature at harvest, but they shouldn’t be completely ripe. Avoid harvesting watermelons that are immature because such melons won’t develop more sweetness post-harvest.

To know when watermelons are ready for harvest, operators can watch for several clues. Among those are dried tendrils found on vines close to the fruit, the rind yellowing where it sat on the ground and the fruit having a soft hollow sound if thumped. Another hint is the melon having grown to the appropriate size. Waiting for the vines to wither will mean that the watermelons have become overmature. A watermelon ready for consumption has several characteristics: sweetness, crispness and good color. Based on soluble solids tested from the middle of a watermelon, sugar levels should reach at least 10 percent.

Operations rely on hand labor to harvest their watermelon crops. As a result, harvesting and handling tend to represent significant expenses associated with watermelon production. Note that multiple harvests may be necessary depending on whether plants were pollinated evenly over time.

During harvest, the crew should cut fruit from vines and place the fruit with their bottoms facing the ground. Cutting fruit from vines is preferable to pulling, twisting or breaking it from vines, and positioning the melons with the bottom down discourages sunscald. To further prevent sunburn, watermelons shouldn’t sit in the sun for an extended time. Additionally, excessive heat — considered to be temperatures warmer than 90 degrees F — may cause watermelon quality to deteriorate and the fruit to develop decay. During transportation, good ventilation can help to manage temperatures. Protect melons by placing them in trucks or wagons covered with padding. Also, don’t allow harvest or handling crews to ride on a watermelon load because their weight may cause fruit damage.

To store watermelons, the facility’s temperature should be set to 50 degrees F to 59 degrees F. If temperatures dip to be too cool, then the fruit may be subject to chilling injury. In storage, relative humidity levels should range from 85 percent to 90 percent. The humidity helps to prevent desiccation and maintain a fruit’s glossy finish. Storage facilities should separate watermelons from crops that produce ethylene, which can have an adverse effect on watermelon quality.

Sources

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Jett, Lewis W. 2006. High Tunnel Melon and Watermelon Production . University of Missouri Extension. Columbia, MO 65201.

Miller, Gilbert and Justin Ballew. 1999. Watermelons . Clemson Cooperative Extension. Clemson, SC 29634.

Orzolek, Michael D., William J. Lamont, Lynn F. Kime, Steven M. Bogash and Jayson K. Harper. 2010. Watermelon Production . The Pennsylvania State University. State College, PA 16801.

Saha, Shubin and Matt Ernst. 2014. Watermelon . University of Kentucky. Lexington, KY 40506.

Shrefler, Jim, Lynn Brandenberger, Eric Rebek, John Damicone and Merritt Taylor. Watermelon Production . Oklahoma Cooperative Extension Service. Stillwater, OK 74074.

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