Watermelons (Citrullus lunatus) are in the family Cucurbitaceae and are divided into types based on their weight: personal (<6 lbs), icebox (6–15 lbs), and picnic (>15 lbs). Believed to have originated in Africa, evidence of watermelon cultivation has been found in ancient hieroglyphics in Egypt and India, dating back to 2500 BC. David Livingston found wild watermelons in the Kalahari Desert in 1850. The watermelon arrived in the Americas in the early 1600s with traders and was first cultivated in Massachusetts in 1629. By the middle of the 17th century, the watermelon had made its way down to Florida.
There are over 1,200 varieties of watermelon worldwide, and 200–300 varieties are grown in the U.S. and Mexico. Until 1980s, watermelons were considered a seasonal fruit, but today imports combined with local production ensure a year-round supply. Icebox watermelons are gaining in popularity as they are ideal for small families and fit easily into a refrigerator.
China ranks first in worldwide watermelon production (126,832 million pounds in 2002) and the United States currently ranks fourth (3,920 million pounds in 2002). The top five producing watermelon states in 2003 were Texas, Florida, California, Georgia and Indiana, with Texas harvesting 770 million pounds on 35,000 acres.
Seed germinates best at soil temperatures of 68–95°F, and planted 1 inch deep. 1–2 pounds seed required per acre, depending on seed size, germination and plant spacing. Icebox watermelon can be spaced more closely together in the field than picnic types, and in-row spacing is 2–3 feet while between-row spacing is 5–6 feet.
For transplants production, allow 3–4 square inches per seedling. Seedlings should be between 3–5 weeks old with 3–4 leaves at transplanting. Seedless varieties may germinate very poorly, depending on variety and direct seeding is not recommended.
Watermelons bear separate male and female flowers on the same plant (monoecious). Only female flowers set fruit. Bees are necessary for pollen transfer pollen. For successful seedless watermelon production, bees are especially important as seedless varieties do not produce pollen. The pollenizer variety is planted in alternate or every third row, or as every third plant in the row. Use a distinctly different variety as pollenizer in order to easily distinguish seedless fruit. Icebox varieties used as pollenizers result in early yields; picnic varieties used as pollenizers result in greater total yields. Icebox varieties usually flower 7–10 days earlier than picnic varieties, so delay icebox pollenizer planting.
Seeds for producing seedless watermelon are made by crossing a female parent that has the normal two sets of chromosomes (2N, also called diploid) with a male parent that has twice the normal sets of chromosomes (4N, referred to as tetraploid). The tetraploid parent is formed by treatment with cholchicine, a plant-derived chemical. The female parent of this cross sets fruit, and the fruit will contain seed that is 3N, which is referred to as triploid. These triploid seeds will grow and produce seedless watermelon plants, but the triploid plants are sterile and can’t reproduce.
Triploid watermelon seeds that result from this process generally have poor germination compared with normal diploid seeds, and are slow to emerge and become established. Specific temperature and moisture conditions are required for triploid watermelon seed germination to be successful. It is generally recommended to grow triploid watermelon as transplants so they receive special care and attention during this critical growth stage, rather than seeding directly in the field.
While triploid watermelon vines will bear female flowers that will set fruit, the male flowers are sterile, that is, they do not produce viable pollen. Therefore it is necessary to plant a diploid watermelon variety next to the triploid plants in order to provide a source of pollen. Most garden seed packs of triploid watermelon will contain a smaller pack of diploid seeds to be used as the pollinator variety. The general rule for seedless watermelon production is to plant in the garden or field 1 diploid plant for every 2–3 triploid plants.
When the diploid plant pollinates the triploid plant, fruit will be formed. But because the female parent has an uneven number of chromosomes (3N), it is unable to sexually divide (sexual cell division is referred to as meiosis), and so no seeds can be formed.
More information on growing seedless watermelon
Egel, D.S. 1999. Seed germination and health for triploid (seedless) watermelons. Purdue University Cooperative Extension Service, BP-62.
Icebox Watermelon Seed Sources. This list includes varieties that we have included in our variety trials. This list is designed to help readers find seed of icebox watermelon. It is not meant to endorse any of these businesses or detract from any businesses not listed.
Most of the U.S. watermelon production is consumed fresh. Per capita watermelon consumption in 2003 was 13.7 pounds and watermelon is about 1/3 more popular in the Western states then in the Midwest and East. About 85% of watermelons are purchased at the retail level for home consumption. Other processed products include roasted seeds, pickled rind and watermelon juice.
2006 Research Trial. In 2006 we evaluated 117 varieties of watermelon at the Washington State University Vancouver Research and Extension Unit. The field was certified organic and was managed accordingly. Harvested watermelons were measured for weight, length and width, and number of fruit per plot. After each harvest, one watermelon per plot was measured for soluble solids (measure of sweetness) using a Brix meter.
Variety Descriptions. This table includes information for each variety in our trial in 2004 and 2005. Information includes days to maturity, fruit size and weight, rind color, flesh color, ploidy, brix, and seed sources.
2005 Research Trial. In 2004 we evaluated 44 varieties of icebox watermelon, and in 2005 we evaluated 101 varieties. This study was conducted at Washington State University Vancouver Research and Extension Unit. The field was certified organic and was managed accordingly. Harvested watermelons were measured for weight, length and width, and number of fruit per plot. After each harvest, one watermelon per plot was measured for soluble solids (measure of sweetness) using a Brix meter.
2004 Research Trial. 44 varieties of icebox watermelon were grown and evaluated in a replicated field study at Washington State University Vancouver Research and Extension Unit. The field was certified organic and was managed accordingly. Harvested watermelons were measured for weight, length and width, and number of fruit per plot. After each harvest, one melon per plot was measured for soluble solids using a Brix meter.
Icebox Watermelon Rind Thickness. In 2004, we measured the rind thickness of 47 icebox watermelons. Rind thickness may impact suitability for shipping as well as consumer preference.
- Measure yield and size of icebox watermelon varieties in western Washington.
- Investigate the potential of growing icebox watermelons with drip and overhead sprinkler irrigation systems.
- Evaluate local consumer response to eating qualities of icebox watermelon varieties.
National Watermelon Promotion Board provides history, production statistics, nutrition facts, recipes, picking a watermelon, interesting facts and other U.S. watermelon organizations.
North Carolina University Cucurbit Breeding. Crop information, biogeography and origin, cultivar descriptions, breeding information and seed companies.
Oklahoma Extension Service. Extension publication guide to watermelon production.
Oregon State University Watermelon Commercial Vegetable Production Guide. Seeding and transplanting, fertilizers, mulch & row covers, harvesting, handling, storage and pest control.
University of Illinois Extension Urban Programs Watermelon guide to varieties, growing, planting, care, harvesting, common problems, selection and storage, nutrition and health.
University of Missouri High Tunnel Melon and Watermelon Production. Includes special considerations for this production system, including cultivar selection, transplanting, row covers, fertilizer, pollination, and pest management.