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Watermelon is a warm-season crop in the same family as cantaloupe, squash, cucumber, and pumpkin. Watermelon varieties are categorized into three main classes based on the development of their seeds: open-pollinated, F1 hybrid, and triploid (seedless). Watermelons come in various sizes and shapes, ranging from round to oblong. The rind can be light or dark green, with or without stripes, and the flesh can range in color from dark red to red or yellow.
Initially, seed germination of seedless watermelon was challenging, with low germination rates. One solution is to keep the seeds warm at around 90°F until they germinate and emerge from the planting media. However, this is difficult in cool climates where well water can be as low as 40°F, causing temperatures to drop each time the seeds are watered. On a small scale, warmth can be maintained by watering transplant flats, covering them, and letting them heat up in the sun within a greenhouse for a day or more before planting. On a larger scale, the seeds can be placed in dark rooms heated to 90°F with 95% relative humidity until the seedlings emerge, which typically takes 4-5 days. After emergence, the seedlings are grown in the greenhouse for three weeks and then transplanted to the field by late May or early June. These methods generally yield a germination rate above 90%.
Watermelon seed germinates at soil temperatures ranging from 68° to 95°F, but germination below 70°F is slow. At 77°F, seedlings should emerge in about 5 days. To ensure rapid germination, seeds should only be planted when soil temperatures are warm enough. Planting too early may delay germination, lead to uneven stands, and increase the risk of crop loss. While early planting can result in an earlier harvest and higher prices, balancing early plantings with later ones is the best strategy for consistent production.
Watermelons grow well in most well-drained soils, with clay or sandy soils being manageable for producing a good crop. However, sandy loams are considered the best soils for watermelon production, especially if they haven't been used for cucurbit crops (such as cantaloupe, cucumber, or squash) in at least 5 years. Raised beds are particularly beneficial for early-season production as they warm quickly and facilitate drainage in heavy soils. However, raised beds are more prone to drying, so extra attention should be paid to watering, especially during the first two weeks after emergence. Compacted soils can severely limit root growth, so it's important to avoid compaction when preparing the soil.
Transplanting watermelons offers several advantages, such as the ability to grow plants under controlled greenhouse conditions when outdoor conditions are unfavorable. This method increases seed-use efficiency, which is particularly important with expensive hybrid and triploid seeds. It can also reduce issues like soil crusting and damping off, both of which can hinder seedling growth, and it ensures more uniform planting depth, typically resulting in earlier harvests. For seedless watermelons, transplanting is the only cost-effective method.
However, there are also some disadvantages to transplanting. It can increase variable and labor costs, and seedlings can be fragile, making them prone to damage during the transplanting process. Additionally, newly transplanted seedlings may be killed by frost, further raising costs. Transplanting may also increase the risk of diseases like fruit blotch.
When transplanting, seeds should be planted about 1 inch deep. The number of seeds needed per acre typically ranges from 1 to 2 pounds, depending on the seed size, germination rate, and plant spacing. Watermelon transplants should be set slightly deeper than their growth in the greenhouse to protect the root/stem interface from wind damage. Peat pots should be planted so that no portion remains above ground, as the pot can act as a wick, drawing moisture from the soil and potentially causing desiccation or moisture stress. Transplants should be watered immediately after planting to remove air pockets around the roots and ensure adequate moisture for good root establishment.
24–48" in row and 72" between rows
Watermelons are considered optimal for eating when their flesh reaches a sweet flavor, crisp texture, and a deep red color, although some newer cultivars range from light red to yellow. Determining the perfect time for harvest is challenging, as external rind appearance doesn't always predict good internal quality and full maturity. Consumer demand for sweet, flavorful watermelons makes total sugar content, measured as soluble solids, an important quality factor. To check field maturity before harvest, a few melons can be cut and their sugar levels tested using a hand refractometer. High-quality watermelons should have a sugar content of 10% or more in the center of the flesh.
The time between harvest and consumption also plays a critical role in determining the ideal harvest time. Melons destined for distant markets are typically harvested when mature but not fully ripe to minimize handling damage and preserve texture during transit. Watermelons should be consumed within two to three weeks after harvest due to loss of crispness.
Identifying mature melons is more difficult early in the season. Immature melons are firm, not yielding to pressure, and have flesh colors ranging from white to pink. If harvested prematurely, the red color will develop, but the flesh will never reach acceptable sweetness because sugar content doesn't increase post-harvest. Melons should be harvested before vines wither, as overmaturity leads to mealy texture and reddish-orange color.
Several indicators can help determine ripeness. The tendrils or pigtails closest to the fruit should be wilting and have turned brown. The ground spot on the belly of the melon should transition from white to light yellow. The thumping sound should change from a metallic ring when immature to a soft, hollow sound when mature. The green bands of striped varieties should break up where they intersect at the blossom end. Ribbed indentations, felt with fingertips, should be present along the elongated body of certain varieties, such as Charleston Grays. Using a hand refractometer to measure sweetness can also confirm maturity. A brown tendril next to the fruit indicates maturity, while a white tendril indicates immaturity.
Watermelons should be cut from the vine rather than pulled, twisted, or broken off, as pulling can introduce bacteria and fungi that cause souring or decay. After cutting, the melons should be turned with their bottoms facing down to prevent sunscald.
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