Yield Response of Watermelon to Planting Density, Planting Pattern, and Polyethylene Mulch

This article by Douglas C. Sanders, Jennifer D. Cure, and Jonathan R. Schultheis, Department of Horticultural Science, North Carolina State University, Raleigh, appeared in "HortScience" 34(7):1221-1223. 1999.

Although costly, the use of black polyethylene mulch enhances earliness, raises yields, improves fruit quality, and increases the profitability of growing many crops. Currently, watermelon growers may profit by using polyethylene mulch, but they may be failing to maximize yields if they are following recommendations for spacing based on earlier work with non-mulched plantings.

In general, experimental data fit models in which crop yields increase with planting density to a maximum, and then plateau or decrease at some threshold density. This threshold density should produce higher yields as environmental conditions are optimized, as when polyethylene mulch and drip fertigation are used. Optimum plant density may also shift as growing conditions improve. Without mulch, linear increases in watermelon yields were obtained as area-per-plant decreased. Watermelon yield-per-hectare also increased as area-per-plant decreased, and mulch increased yields at all spacings when irrigation was adequate.

In Georgia, the recommended population density for watermelons has been 1,012 to 1,219 plants/A; however, in North Carolina, it was 870 to 1,742 plants/A. Thus, there is a need to evaluate the response of watermelon to high planting density when polyethylene mulch and drip fertigation are used.

Another technique for enhancing earliness is the use of transplants. Like the use of polyethylene mulch, this is more expensive, but the extra cost can usually be more than repaid with premium prices in the early market. One possibility for lowering the cost of transplanting would be the use of two plants/hill. For instance, if the in-row spacing were doubled to two transplant/hill (thus maintaining average area-per-plant), the number of transplant operations per hectare would be halved. This work was undertaken to evaluate the effects of in-row spacing and of planting pattern (one vs. two plants/hill) on yield variables of watermelon, using black polyethylene mulch and drip fertigation.

Watermelon hybrids ‘Prince Charles’ (Charleston Grey type) and ‘Royal Jubilee’ (Jubilee type) were used in experiments in four environments in 1988, 1989, and 1990. The effects of in-row spacing and planting pattern on yield variables were evaluated, with and without polyethylene mulch. In-row plant spacings of 1.5, 2, 3, 4, and 5 feet on 5-foot-wide (center-to-center) beds were used to evaluate the effect of 1 versus 2 plants/hill. All beds were mulched with black polyethylene, and plant water needs were supplied by drip irrigation.

In all locations and years, ‘Royal Jubilee’ yielded significantly (P < 0.05) more than ‘Prince Charles’. “Royal Jubilee’ had higher numbers of marketable fruit per hectare (significant in all comparisons), and tended to have higher individual weights (statistically significant in only two of the four comparisons). There was no varietal difference in percentage of culls except in the 1990 study at one location.

Polyethylene mulch increased marketable yield at nearly all in-row spacings of either one or two plants/hill. Also, the number of marketable fruits increased 30 to 60 percent with polyethylene mulch at close spacings. Weight-per-melon was increased by polyethylene mulch at nearly all in-row spacings and both planting patterns; this increase effectively decreased the percentage of culls from 40 to 25 percent in the very high density (close spacing) treatments, where many melons were borderline in size. The mulch significantly increased the number of larger melons at all plant spacings, but had little effect on the numbers of medium-sized melons, except for an increase at the closest spacing.

These data provide a basis for new, closer spacing recommendations for watermelons (9.7 to 10.7 ft.2/plant) as long as water and nutrients are not limiting. They also support the option of planting double-seeded transplants at half the sites in a row, at a significant savings in labor and transplant cost per hectare.