Watermelons

Citrullus lanatus (lanatus variety) (Cucurbitaceae or cucumber family)

Production

Soil preparation

Watermelons should not follow after other cucurbits or nightshades, but they follow well after cole crops, leguminous green manures or sweet corn.
Where Phytophthora (Phytophthora capsici) is an issue, rotate away from this land. Biofumigation has shown some effectiveness to reduce incidences.
Apply compost and other amendments based on the results of a soil test.
Spread lime as needed to increase both the pH and Ca levels. Dolomite lime will also increase Mg levels. Spread gypsum when calcium levels are low but pH is correct.
In northern regions, plant watermelons in high raised beds covered with plastic mulch for the best results.

Common recommended fertilizer rates¹

Nitrogen	Phosphorus	Potassium	pH
100	0-160	0-160	6.0-6.4
¹Rates are for New York and are from Cornell University’s Nutrient Guidelines for Commercial Vegetable Production (2019). Check the Cornell website for updated guidelines, or consult with local experts for recommended rates outside New York.

Varieties

Washington State variety trial

Greenhouse guidelines

EZ Seeder seeding plate #5
Use a 1020 tray with 24 pots for more light, which results in stockier plants. When using a 38-pot tray make sure you transplant before they get leggy.
Start seeds in the germination chamber or when you can keep your greenhouse warm both day and night. Germination is poor at lower temperatures.

Cell pack tray	Germination temp	Growth temp	Hardening off	Notes
24 or 38	80°–90°	85°	Less water and a lower temperature for 1 week	Place plug trays on a wire mesh bench to encourage air pruning.

Number of successions

Start seedlings 3 weeks before the last frost date.
For continuous harvest, plant the next successions on a 10–14 day schedule until 2 ½ months before the first frost.

Signs to watch for and what to do

Watermelon plants like to grow under warm conditions, which isn’t always compatible with other vegetables grown simultaneously. Find the warmest spot in your greenhouse for them.

Transplant readiness indicators

Harden plants off outside; plants that are lush don’t perform well in the field.
Transplant in the field when plants have enough of a root ball to hold the soil in the pot or cell pack together. When they can’t, their roots will be damaged and the plants have a higher fatality rate.

Transplant tips

Lay plastic mulch at least 1 week before planting to allow weed seeds to germinate and the soil to warm. Planting on the same day as laying plastic will cause weed pressure in the planting hole.
Avoid transplanting in plastic mulch during the heat of a hot summer day. The heat of the plastic can cause damage to the young seedlings. To avoid burning the young plants, transplant in the evening, use stocky transplants and water deeply. If planting by hand, water plants with a watering can. Avoid applying row covers in hot conditions. When hot weather is expected after transplanting, only use insect cover for protection.
To help resist striped cucumber beetle damage, dissolve Kaolin clay (as in Surround) in plenty of water and drench the complete tray in the solution before transplanting.

Water wheel transplanter

Rows	In-row spacing	Planting depth (inches)	Notes
1	12 inches	Normal	Add kelp to water as a 0.5% solution in the water wheel planter.

Cultivation procedures

As watermelons are a tropical fruit, use infrared transmitting (IRT) plastic for your first planting because it helps warm up the soil.
After transplanting, cultivate once or twice in between plastic with a rolling cultivator or Spyder. Cover the wheel tracks with rye straw at a rate of 3 lbs. per foot. (One 600 lb. round bale covers 200 feet of wheel track.) Increase this amount when using an early cutting of orchard grass, because hay breaks down much faster than straw. Over time, hay and straw mulch will release a significant amount of K and other beneficial properties like silicon, contributing to long-term soil health. Mulching also protects crops from soil splashing. This reduces soilborne diseases and time spent cleaning produce after harvest.

Frost, disease and insect protection

Plant early plantings downwind from later varieties, as both powdery and downy mildew are windborne.
For your earliest plantings, use floating row covers to protect against frost and striped cucumber beetles. For varieties that require pollination, remove the covers at flowering. Use 10-gauge wire to support the row cover, since abrasion against the cover will damage the plants. If you don’t need frost protection or a warm environment, you can use the longer lasting insect covers to provide protection against beetles.
Powdery mildew: While powdery mildew is not common in watermelons, it’s an emerging issue and therefore discussed here. Powdery mildew can be somewhat controlled with a mixture of Bacillus subtilis, as in the product CEASE, and potassium bicarbonate, as in MilStop SP. Sulfur causes phytotoxicity in melons, so be careful if you’re applying sulfur in any form to control powdery mildew. Also, consider using tolerant or resistant varieties.
Downy mildew can be somewhat controlled with the biological Pseudomonas aeruginosa, such as the product Zonix.

A full list of OMRI-listed control products on cucurbits, and their efficacy, is available from CALS.

Harvest

Yield	Average of 0.8 watermelon per row foot
Standards	Harvesting¹ 50–75 watermelons per person, per hour
Standards	Washing Use a conveyor washer
Tools and supplies needed	20-bushel bins or large green 1 ¾-bushel boxes
¹Harvest rates don't include the time required to transport crops from the field to a wash and pack shed or storage facility.

Ready-to-harvest and quality indicators

Watermelons are ready to harvest when at least 2 out 3 ripeness indicators occur:
- The tendril closest to the melon is shriveled up (depending on variety).
- The melon has a yellow coloration on the side not exposed to the sun.
- The melon sounds hollow (the “poink”) sound when tapped. This indicator can be hard to get right when your harvest crew is inexperienced with watermelons.

Good quality watermelons should:
- Be symmetrical and uniform in appearance
- Have a surface that appears bright and waxy
- Have no dirt, scars, sunburn, transit abrasions, bruising or other surface defects
- Feel heavy for their size

Harvest procedures

One or 2 people stand on a harvest wagon and the others each take a row of watermelons.
For small watermelons like Sugar Baby: After a picker harvests a melon, they toss it underhanded to one of the people on the truck. Make sure you have eye contact with the catcher before you toss it.
When dealing with large melons, the crew should bring them to a large melon bin (20–24 bushels) and carefully place them inside.
Move the harvest wagon down the harvest lane as you pick.

Washing procedures

Using adequate mulch like straw can keep the fruit clean and make washing unnecessary. Sort and box watermelons in a well-lit packing shed. Many blemishes are discovered during this process.
If washing is necessary, use a rinse conveyor washer or brusher washer. Add a sanitizer like hydrogen peroxide and peracetic acid to the water in the circulation tank of the rinse conveyor vegetable washer. Use the guide How to Wash Produce Using a Peracetic Acid Solution to establish how much sanitizer solution to add to water. Make sure you use the appropriate pressure on the nozzles to avoid crop damage.
Under no circumstance dunk melons in cold water, as this can create cross-contamination of pathogens that can be absorbed through the skin of the fruit.

Additional resources

Packing and storage summary for watermelons

Cleaning in the field	Brush off any soil or straw mulch
Packing in the field	If possible, sort by size in either large boxes or squash bins. Do a quick field inspection for any defects before placing them in the bin.
Packing for delivery	Sort by size and quality. Commercial melons are graded into 30-, 36-, 45- and 60-count sizes. Specialty types can be smaller. The majority of the pre-July 4 market is 36- and 45-count fruit. See the International Federation for Produce Standards for the correct PLU code. Add the prefix 9 for organic crops
Storage	At 45°–59° and 85–90% humidity. Chilling injury will occur when stored below 45° for several days. Watermelons are sensitive to ethylene exposure, which will cause them to soften. Allow for good air exchange, and don’t store watermelons with crops that release ethylene.

This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture through the Sustainable Agriculture Research and Education (SARE) program. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and should not be construed to represent any official USDA or U.S. Government determination or policy.