Watermelon Cultivar Trial: Data to Help You Decide Which Watermelon to Grow

by Dr. Matt Bertucci - July 23, 2020

New Horticulture Assistant Professor Dr. Bertucci shares the results from a watermelon cultivar trial he conducted last summer and reveals the mysteries behind seedless watermelons!

Fruit breeding and the development of new cultivars provide growers new options for production: plants that ripen more rapidly, produce more fruit, or harbor resistance to pests or diseases in the field. New cultivars also offer benefits for consumers such as different fruit sizes or sweetness. As new cultivars are generated by public and private breeding programs, it is important to evaluate their relative performance and suitability to our region of the country. Thus, we conducted this trial to evaluate yield, fruit production, and relative sweetness of six watermelon varieties.

Figure 1: Overhead image of plots in Hope, AR.

In the summer of 2019, we planted six watermelon varieties at the Southwest Research and Extension Center in Hope, AR and at the Vegetable Research Station in Alma, AR. Cultivars in this trial included 2 seeded watermelon types: Jubilee and Charleston Gray, and 4 seedless watermelon varieties: Exclamation, Excursion, Fascination, and WDL4410. Watermelon seeds were started in the greenhouse and transplanted into the field when seedlings reached the 3- to 5-leaf stage. All watermelon were grown under black plastic with drip irrigation (Figure 1-2).

One issue with seedless watermelon is that they cannot properly pollenate and set fruit on their own. They require dedicated pollenizer vines nearby to provide pollen to generate fruit. To ensure seedless varieties were properly pollinated, pollenizer vines SP-7 (‘Super pollenizer 7) vines were transplanted within each seedless watermelon plot. One pollenizer vine was planted for every 3 seedless watermelon vines. Fortunately, seeded watermelon shed sufficient pollen and will set fruit without the aid of pollenizer vines. In fact, growers commonly use a regular seeded watermelon variety to serve as the pollenizer vine for their seedless watermelon vines. If you choose to do so, be sure to pick a distinct rind pattern, so you know which fruit is seedless!

Figure 2: Overhead image of plots in Alma, AR.

In this trial, fertility and pest management were maintained in accordance with state and regional recommendations, as outlined in the 2020 Southeastern US Vegetable Crop Handbook and Arkansas Common Cucurbit Problems. Our goal was to provide a representative environment that accurately reflects the production conditions of commercial growers around the state. And to hopefully generate some information to help growers determine if any of these watermelon varieties are suitable for their production.

We harvested all fruit as they ripened and recorded the weight for each melon. Fruit were graded into culls or marketable based on fruit weight, with a threshold of 9 lb. as the cutoff for a marketable fruit, using the grading standard by Schultheis and Stark (2019). Anything weighing less than 9 lb. was scored as a cull.

Results

Total fruit production of each watermelon cultivar differed between Hope and Alma. For total fruit in Hope, the seedless Exclamation produced the greatest number with over 3,700 fruit per acre (Figure 3). More interesting is the marketable fruit (≥ 9 lb.) where Excursion and WDL4441 exhibited the highest number of marketable fruit. Marketable fruit production was similar in Hope and Alma, so the data are presented with sites averaged (Figure 3). Interestingly, Exclamation and Fascination had the lowest number of marketable fruit, despite producing the highest number of total fruit. This indicates that these cultivars produced many fruit which were too small to be considered marketable (i.e. <9 lb.).

Figure 3 – Comparison of total fruit production and marketable fruit production (≥9 lb) from watermelon cultivar trials in Hope, AR and Alma, AR. 

Total yields did not differ among fruit types. All watermelon produced between 30,000 and 38,000 lbs. per acre of watermelon in these trials (Figure 4). However, marketable yields indicate that Jubilee produced the largest amount of marketable yield. Our findings indicate that Charleston Gray, Excursion, and WDL4441 produced similar marketable yields to Jubilee. Only Exclamation and Fascination produced significantly reduced marketable yields, relative to Jubilee.

Figure 4 – Comparison of total fruit yield and marketable yield (fruit ≥9 lb) from watermelon cultivar trials in Hope, AR and Alma, AR. 

Sugar content, measured as degrees Brix (^oBrix), was highest in Fascination and in Excursion watermelon cultivars. However, all watermelon measured between 14 and 15 ^oBrix, which is acceptably sweet for consumers. Thus, sweetness was not greatly impacted by cultivar selection (Figure 5).

Figure 5– Comparison of sugar content from watermelon cultivar trials in Hope, AR and Alma, AR. 

Discussion 

This trial offers some good news for growers who love the older, seeded watermelon varieties. With the highest “marketable yield”, Jubilee seemed to be the top performer in the environments tested. But there is one major consideration to remember: watermelon are not typically sold by the pound! So long as the fruit are meeting the proper bin-count or size threshold for your customer or contractor, the number of marketable fruit may be a better estimation of how much revenue you will be able to generate at market.  

As you consider your selection of watermelon cultivars for planting, be sure to check the fruit characteristics, disease resistances, and suitability to our growing region. If planting seedless cultivars, another consideration is that seedless watermelon have more expensive seed, but they may also command a market premium. Producers are encouraged to evaluate their local markets and keep these factors in mind when selecting which watermelon to plant! Also, seedless watermelons require pollinizer vines which could be an additional cost.

Food for thought: If they are seedless, where do the seeds come from!? 

Have you ever thought about the source of the seeds for seedless watermelon? “Seedless watermelon seeds” can sound like an oxymoron at first! But it is relatively easy to understand once explained. Seedless watermelon, or triploid watermelon, are hybrids derived from crossing two watermelon varieties. By crossing two specific types of watermelon (diploid and tetraploid), a sterile “triploid” watermelon is produced. This sterile offspring is incapable of producing viable seeds, which is why seeds never develop in the fruit! Though sometimes a small seed coat may still develop. If you are familiar with animal husbandry, a fitting analogy is the mule: if a horse is crossed with a donkey, a sterile mule results. Seedless watermelon are similar in that way, though the underlying genetics differ.