Commercial Watermelon Production

• Culture
• Soils and Fertilizer Management
• Watermelon Transplant Production
• Plastic Mulch
• Growing Seedless Watermelons
• Diseases
• Insect Management
• Pesticide Application
• Weed Control in Watermelons
• Harvest and Handling
• Production Costs
• Marketing

Culture

Watermelon is a warm-season crop related to cantaloupe, squash, cucumber, and pumpkin. Watermelons can be grown on any well-drained soil throughout Georgia but are particularly well adapted to the Coastal Plain soils of South Georgia. Yields of 20,000 to 40,000 lb per acre are common. More than 35,000 acres of watermelon are produced in Georgia, with more than 25% of this produced on plastic mulch. Watermelons will continue to be an important part of vegetable production in the state. Increases in average yield per acre will continue as more growers adopt plastic mulch, intensive management and new hybrid varieties.

Cultivars

Watermelons range in shape from round to oblong. Rind colors can be light to dark green with or without stripes. Flesh colors can be dark red, red or yellow. Watermelon varieties fall into three broad classes based on how the seed were developed: open-pollinated, F₁ hybrid, and triploid or seedless. 

Open-pollinated varieties are developed through several generations of selection. The selection can be based upon yield, quality characteristics, and disease resistance. Open-pollinated varieties have true-to-type seed (seed saved from one generation to the next will maintain the same characteristics) and are less expensive then F₁ hybrid varieties.

F₁ hybrids are developed from two inbred lines that have been selfed for several generations and then crossed, with the subsequent seed sold to growers. F₁ hybrid seed will exhibit increased uniformity of type and time of harvest compared with open-pollinated seed and can exhibit as much as a 20% to 40% increase in yields over open-pollinated varieties grown under similar conditions. The disadvantages of F₁ hybrid seed are cost and availability. F₁ hybrid seed will be as much as five to 10 times as costly as open-pollinated seed, and available F₁ hybrid varieties will change from year to year.

The third type is triploid or seedless watermelon. These are developed by creating watermelon plants with double the usual chromosome number and crossing them with normal watermelon plants. The resulting plants have one-and-a-half times the normal chromosome number. Because they have an odd number of chromosomes, they cannot form viable seed. In addition, they produce very little pollen; therefore, normal watermelon must be planted with triploid watermelon as a source of pollen. Although triploid watermelons are referred to as seedless, they are not truly seedless but rather have undeveloped seeds that are soft and edible. Triploid seeds will be even more expensive than F₁ hybrid seeds, and the melons should command a premium in the marketplace (see Growing Seedless Watermelons).

Watermelons are also grouped according to fruit shape, rind color or pattern, and size. These groups are often named for a popular variety with those characteristics. For example, oblong melons with dark stripes on a light background in the 25 to 35-lb range are called Jubilee types after the popular 'Jubilee' variety. Melons of similar shape and size as Jubilee but with a light green rind are called Charleston Gray types, again for a popular cultivar, 'Charleston Gray'. Round melons in the range of 20 to 30 lb with a striped rind are called Crimson Sweet types. Small oblong melons (15 to 25 lb) with a dark green rind and light yellow stripe with dark red flesh are called Allsweet types. Watermelons with a blocky shape (between a Jubilee and Crimson Sweet type) are referred to as Royal Sweet or Mirage types. Finally, round watermelons of 10 lb or less are referred to as icebox types to denote their ability to fit into a refrigerator. Because varieties are constantly being changed and market trends are also changing, selecting varieties acceptable for your market is important. Consult your seed dealer, buyers, brokers, or your county Extension office for the latest information on available varieties.

Planting and Spacing

Watermelon seed germinates at soil temperatures of 68 to 95 °F; however, germination below 70 °F is very slow. At a soil temperature of 77 °F, watermelon plants should emerge in about 5 days.

Watermelon seed should not be planted until soil temperatures are warm enough to ensure rapid germination. Planting seed too early will delay germination, can result in uneven stands  and will increase the likelihood of crop loss. Early seeding can, however, result in an early harvest, which generally commands better prices. These contradictory elements in deciding when to plant watermelon seed are best resolved by successive plantings that attempt to produce for the early market while ensuring a crop by planting when soils are warmer.

Seed should be planted approximately 1 in. deep. The amount of seed required (usually 1 to 2 lb per acre) depends upon seed size, germination, and plant spacing. Correctly labeled, uniform, disease-free, certified seed with 85% to 90% germination is preferred.

There are several methods of planting watermelon. With the widespread use of more expensive hybrid seed, equipment that can plant to stand or come close to this is best. Precision seeding equipment, plug mix planting, and transplants reduce or eliminate the need to thin stands after planting.

Plug mix planting consists of blending watermelon seeds, fertilizer, and water with a growing medium of approximately one-third vermiculite and two-thirds peat. Prepared in cement mixers, the mix often is allowed to remain in bags for 24 to 48 hr prior to planting to allow seed to imbibe water and begin the germination process. Precision plug mix planters dispense the mix in the field by injecting 1/8 to 1/2 cup of mixture (plug) per hill. The mix should have enough seed to dispense from three to five seeds per hill. Plug mix planting is especially advantageous when planting watermelon seeds in plastic mulch: these planters punch or burn holes in the plastic to insert the mix. Growers who have little or no experience with plug mix planting should contact a county Extension office for additional information before using this specialized procedure.

Watermelons traditionally have been spaced 6 to 8 ft between hills on bare ground without irrigation. With irrigation, use a spacing of 5 to 6 ft between hills. With plastic mulch and trickle irrigation, use an in-row spacing of 3 ft and between-row spacing of 6 to 8 ft. Icebox watermelons can be spaced even more closely, with in-row spacing of 2 ft and between-row spacing of 5 ft.

Pollination

Watermelons produce two types of flowers. Most varieties generally produce imperfect female and male flowers (Figure 1). When flowering begins in watermelon, male flowers will be produced at every node while female flowers will be produced approximately every seventh node.

Watermelon flowers are viable for only 1 day; therefore, an adequate population of pollinating insects must be available every day during the flowering period. Even with sufficient pollinators, it is not uncommon for watermelons to abort flowers. Under average conditions, two to three fruit should set per plant. The actual number of fruit set will depend on variety, cultural practices, environmental conditions, irrigation, and number of pollinating insects.

Figure 1. Watermelon flowers: female (left) and male.

Watermelons require insects for proper pollination and fruit growth. Research has shown that each female flower must be visited, on average, seven times by a pollinating insect to ensure proper fruit set. Insufficient pollination results in misshapen melons, which must be culled (Figure 2).

Figure 2. Immature watermelon. The poor shape is due to insufficient pollination.

Individually, honeybees are not as efficient at pollinating as wild bees, but their large numbers make them very good at ensuring proper pollination. If an insufficient number of pollinators are present, supplement them with domestic hives. One strong hive (30,000 bees in a two-story hive) for every 1 to 2 acres is recommended. Ideally, hives should be spaced evenly throughout the field. Hives should have adequate clean water. Hives often are clustered along the edge of the field, which results in bees foraging further into a field because of competition between the hives. Apply pesticides when bee activity is low to minimize impact on the hives. This will occur late in the day, around dusk, and on overcast days. Check pesticide labels for additional precautions concerning bees.

Watermelon flowers are not nutritionally attractive to honeybees; therefore blooming weeds or other crops can outcompete watermelons in attracting honeybees. Destroy nearby flowering plants that may be attractive to honeybees. This will ensure that the bees work the watermelon flowers exclusively.

Monitor hives and honeybee activity during flowering. Early to mid-morning is the best time to monitor bee activity. If numerous bees are not vigorously working watermelon flowers, corrective action must be taken immediately to prevent poor or delayed set.

Soils and Fertilizer Management

Most well-drained soil, whether clay or sandy can be managed to produce a good crop of watermelon. The best soils, however, are sandy loams that have not been in cucurbit (cantaloupe, cucumber, squash, etc.) production for a minimum of 5 years.

Soils with a history of watermelon diseases should be avoided or fumigated to avoid problems (please see the chapter on diseases and consult the current edition of the Georgia Pest Control Handbook; look for information on curcurbits under vegetables). Your local county Extension agent can help with determining potential disease problems.

Land preparation involves one or more tillage operations performed: 

• to make the soil more suitable for seeding and seedling (or transplant) establishment,
• to enhance productivity by providing the best soil structure for subsequent root growth and development, and
• to help control some disease problems.

Several operations may be required to prepare land for planting. This is partially determined by previous cropping history. Land that has been under cultivation for several years may develop a hardpan several inches below the surface. This is particularly problematic on clay soils. To penetrate and break up this hardpan, a subsoiler should be used.

Litter from previous crops should be disked and deep turned with a moldboard plow 2 to 4 weeks prior to planting to insure its decomposition. Broadcast fertilizer should be applied at this time (if no other soil preparation is anticipated) or just before final bedding.

Watermelons respond favorably to warm soils. Raised beds tend to warm quickly and are particularly desirable for early season production. Raised beds will facilitate drainage in heavy soils but are more prone to drying; therefore, particular care should be taken with watering, especially during the first 2 weeks after emergence.

Root growth can be severely restricted by compacted soil. Proper land preparation should eliminate or significantly reduce soil compaction. Recent studies have determined that watermelon root growth is primarily confined to noncompacted soil. Disking fields after they have been plowed tends to recompact the soil and should be avoided. Tillage systems utilizing the moldboard plow without subsequent recompacting operations consistently produce the highest watermelon yields. Basically, this superior performance results from more extensive root systems that are more efficient at extracting nutrients and water from the soil.

Cover Crops and Green Manure

Winter cover crops help protect the soil from excessive water and wind erosion. When incorporated into the soil as green manure, cover crops add organic matter (OM) to Coastal Plain soils, which are naturally low (often less than 1%) in OM.

Soil organic matter consists of plant and animal residues in various stages of decay. Adding OM improves soil structure, which in turn enhances soil tilth (helps to reduce compaction and crusting), increases water infiltration, and decreases both water and wind erosion. Also of importance, OM serves as a storehouse of many plant nutrients. Furthermore, OM improves the efficiency of applied fertilizers by increasing the soil’s ability to retain plant nutrients under leaching conditions.

Georgia watermelon growers frequently plant wheat, oats, rye, or ryegrass as winter cover crops. Whenever these non-nitrogen-fixing cover crops are to be incorporated as green manure, they should be provided with adequate nutrients (especially nitrogen) during their growth. This increases the quantity of OM produced and helps provide a carbon to nitrogen (C:N) ratio less likely to tie-up (immobilize) nitrogen during decomposition. As a general rule, when nonleguminous OM having a C:N ratio greater than 30:1 is incorporated into the soil, it is usually beneficial to broadcast supplemental nitrogen before incorporation. The amount of nitrogen to add varies, depending on the C:N ratio, soil type, and amount of any residual nitrogen in the soil. Typically, green manure crops should be plowed under as deeply as possible with a moldboard plow so that large amounts of crop residue will not be in the immediate vicinity of germinating watermelon seed. Lush cover crops should be turned under at least 2 weeks prior to planting the succeeding crop.

If small grains are grown as a cover crop, strips of grain (2 ft to 6 ft wide) left in spray or harvest lanes provide windbreaks that help reduce damage and sandblasting of small plants during early spring. To minimize the possibility of insect migration to the watermelon crop, grain strips should be turned under before the onset of senescence.

Lime and Fertilizer Management

The only way to accurately manage soil fertility and pH is to have the soil tested. Soil sampling must be conducted in such a manner that it is representative of the field being sampled. This is essential to ensure accurate results and recommendations. Your county Extension agent can help you with the proper method for collecting a soil sample. The University of Georgia Agricultural and Environmental Services Laboratories can analyze your soil and make recommendations.

A good fertilizer management program for watermelon production answers four basic questions:

1. What fertilizer materials (including lime) are to be applied?
2. In what quantities will they be applied?
3. How frequently will they be applied?
4. By which methods (broadcasted, banded, etc.) will they be applied?

In addition, the most successful management programs include frequent evaluations and modifications, if needed, to deal with unanticipated problems such as floods, droughts, and other factors that affect the plants’ ability to utilize nutrients.

Soil pH measures the acidity or alkalinity of the soil. A pH of 7 is considered neutral, with values above 7 being alkaline and values below 7 acid. Most soils in Georgia are slightly to strongly acid. Soil pH will have a profound effect on plant growth, development, and yield. Soil pH affects the availability of nutrients for plant growth. A slightly acid soil with a pH of 6.0 to 6.5 is ideal for watermelons.

The only accurate way to determine the soil pH is to have the soil tested. This analysis can determine if lime is required to raise the pH. Lime is relatively slow-acting in raising soil pH and is relatively immobile in soils. For this reason lime should be added 2 to 3 months before planting and completely incorporated into the top 6 to 8 in. Soils that are also deficient in magnesium should receive dolomitic lime instead of calcitic lime.

For watermelon production, the maximum recommended amount of nitrogen (N), phosphorus (P₂O₅), and potassium (K₂O) is 120 lb per acre. Watermelons are a relatively long-season crop; therefore, applying fertilizer in small amounts several times throughout the season will maximize production. Rain and overhead irrigation can leach nutrients from the soil,  particularly N and K. All required phosphorus can be applied preplant and should remain available throughout the growing season, because it is relatively immobile in the soil.

Many different methods exist for applying the recommended fertilizer. A simple method would be to broadcast and incorporate all of the P and K and apply half the N preplant, then apply half the N 4 to 6 weeks after seeding.

More complex application methods generally result in maintaining optimum nutrient levels throughout the growing season. In one such method, a modified broadcast concentrates the fertilizer in the area of the roots compared with broadcasting. With the modified broadcast method, apply the fertilizer in bands 2 to 3 ft wide in the row prior to planting. This method will also eliminate the potential for burning emerging plants if fertilizer were banded near the emerging seedlings. In this method, all the P is applied preplant with any micronutrients. One-third to half the recommended N and K are also applied in this modified broadcast. At approximately 3 weeks after seedling emergence, apply one-fourth the remaining N and K on the sides of the beds just past vine tips. At approximately 6 weeks after emergence, apply the remaining N and K.

Apply 1 lb of boron per acre and 10 lb of sulfur per acre. If the soil tests shows that the zinc level is low, apply 5 lb of zinc per acre.

Leaching rains or insufficient applications may result in nitrogen and/or magnesium deficiencies after vines have covered the soil surface. If under center pivot, symptoms may be alleviated by fertigating 20 to 30 lb of nitrogen per acre or 10 to 15 lb magnesium per acre. If fertigation is not practical, 10 to 15 lb of magnesium sulfate in approximately 100 gallons of water can be applied as a foliar spray to correct magnesium deficiency. To alleviate nitrogen deficiencies after full vine cover, sodium nitrate may be broadcast over the top (when vines are dry) at 135 to 175 lb (22 to 28 lb N) per acre. Granular calcium nitrate should not be used over the top, because it tends to result in a significant incidence of leaf burn. Any time granular fertilizer is applied over the top, leaf burn may be reduced by thoroughly washing the fertilizer from the leaves with irrigation water.

Watermelon growers have occasionally experienced unsatisfactory fruit set even with sufficient bee activity. Two to three foliar applications of water-soluble boron (approximately 1 oz by weight of actual boron per application) at weekly intervals coinciding with opening of the first female flowers can enhance pollination and improve fruit set. Many growers routinely use a commercial formulation that also contains calcium (2 to 3 oz by weight of actual calcium per application) to help prevent blossom-end rot. A good fertilizer management program includes frequent observations of plants for any nutrient deficiency symptoms. Frequent (8 to 12 days) tissue analyses may be used to monitor nutrient levels in plant tissues. These tests provide a sound basis for fertilizer applications prior to plant stress and symptom development. For optimal yield and quality, monitor watermelon fields frequently and apply supplemental applications of fertilizer promptly if needed.

Melon Defects

Blossom-end rot (BER) is a physiological or nonparasitic disorder related to calcium deficiency, moisture stress or both. Prevention recommendations include adequate amounts of calcium, proper soil pH (6 to 6.5), and a uniform and sufficient supply of moisture. The incidence of BER usually is quite variable from season to season and tends to occur more readily in oblong melons. Watermelons having BER are considered unmarketable (Figure 3).

Figure 3. Blossom-end rot appears as black dead tissue where the blossom was attached.

Hollow heart (HH) and white heart (WH) are two physiological disorders influenced by genetics, environment, and probably a number of nutritional factors. To decrease the incidence of these two problems, only cultivars that have not shown unusually high incidences of HH or WH should be planted. In addition, the crop should be grown under optimal (as close as possible) nutritional and moisture conditions. HH and WH harm watermelon quality and may be severe enough to cause potential buyers to reject melons (Figure 4).

Figure 4. Hollow heart and white heart are generally avoided by planting appropriate varieties.

Sunscald is damage to the melons caused by intense sunlight. Sunscald can be particularly severe on dark-colored melons. Developing and maintaining adequate canopy cover to afford protection (shade) to the melons may prevent sunscald. Sunscald reduces quality by making melons less attractive and may predispose the melon to rot.

Stem splitting can occur in seedlings grown for transplanting. This problem seems to be associated with high humidity and moisture that can occur under greenhouse conditions. Watering evenly to maintain soil moisture, avoiding wet-dry cycles in the media, and good air circulation may help alleviate these problems (Figure 5).

Figure 5. Stem splitting. Longitudinal splits can occur in greenhouse-grown transplants. Theses transplants are still suitable for planting.

Sandblasting occurs when wind and blowing sand damage seedlings when first planted. This appears as dead or dying tissue, usually on the side of the prevailing winds (Figure 6).

Figure 6. Sandblasting. White dead tissue on the stem or leaves is usually indicative of this problem.

Transplant handling damage may result at the soil line because of handling. Tops will flop around and may wilt more readily. In addition, brown or callused tissue may appear at the soil line. Transplants with this damage should be planted slightly deeper to prevent any further damage.

Watermelon Transplant Production

Transplanting watermelons offers several advantages: 

• Plants can be produced under greenhouse conditions when outdoor conditions are not conducive to plant growth.
• Seed-use efficiency increases, which is especially important with costly hybrid and triploid seed.
• Soil crusting and damping off, detrimental to seedling growth, can be eliminated or reduced.
• Planting depth is more uniform. 
• It usually results in earlier harvests.
• It is the only cost-effective way to grow seedless watermelons.

The disadvantages of transplanting include:

• higher variable costs,
• increased labor costs,
• holding plants if weather delays planting,
• fragile watermelon seedlings are easily broken during transplanting,
• higher costs than direct-seeded watermelons if newly transplanted seedlings are killed by frost, and
• possible increased incidence of diseases such as fruit blotch. 

Purchased transplants should be inspected carefully.

Figure 7. Transplant production. These Georgia Department of Agriculture inspected watermelon transplants are being produced in a greenhouse.

Yellowed or flowering transplants should not be accepted because they may be too old to grow properly. Transplants of standard varieties more than 7 weeks old may never perform well in the field. Purchased watermelon transplants should be pathogen- and insect-free (Figure 7). If plants must be held for several days because of bad weather, they may elongate, making transplanting difficult. 

Growers who raise their own transplants can control growing conditions to produce suitable plants and to reduce the risk of importing diseases that can be a problem with purchased transplants. Successful transplant production depends on four basic requirements: 

• a weed-, insect-, and disease-free medium;
• adequate heat and moisture;
• high-intensity light of good quality for stocky plant growth (avoid yellowed fiberglass structures); and
• a hardening-off period when plants are subjected to lower temperatures and/or less water prior to transplanting to the field. 

The time for watermelon transplanting will depend on frost-free dates, but plants generally will take 3 to 5 weeks to be field ready (Table 1) depending on variety and growing conditions. Plants grown under less than ideal conditions will take longer to produce.

Containers

Watermelons suffer transplant shock if the roots are even minimally disturbed. Watermelons must therefore be sown directly in the container that will transfer them to the field. Generally, the size of the transplant container is more important than the type of container. Research has shown that 1-, 1½-, and 2-in. containers, if properly scheduled, can be used successfully without reducing plant vigor or production. The cost of the container may determine the choice of size. Larger containers (1½ in.) are better designed to allow continued root growth and avoid the development of rootbound transplants if the weather prevents timely planting. Root-bound transplants may never grow properly.

Media

Transplants should be grown in a commercially prepared media suitable for vegetable plants. Many commercial mixes (Fafard Mixes, Jiffy Mix, Metro-Mix, Pro-Mix, Redi-earth, Terra-Lite, etc.) are readily available. Commercial mixes are preferred due to consistency of performance.

Sowing

Sow one to two seeds per container for open-pollinated varieties and one seed per container for hybrids to reduce seed costs. Pinch off or cut seedlings to avoid disturbing the roots. Do not pull seedlings out of the container to thin.

Growing Conditions

Cultural conditions under which watermelons grow best are described in Table 1. High temperatures and low light will produce spindly plants. Conversely, low temperatures will delay plant development. Low temperatures can be used when trying to slow plant growth as field planting approaches.

Status and Revision History
Published on Nov 01, 1999
Unpublished/Removed on Feb 24, 2009
Published with Full Review on Mar 13, 2013
Published with Full Review on Jan 04, 2014
Published with Full Review on Aug 01, 2017