How to Grow Icebox Seedless Watermelons in Oregon

Sprouting

• 1

  Fill 6-inch peat pots, one for each triploid seed, with moist, sterile medium 24 hours before planting. Place the pots on a seedling heat mat and insert a soil thermometer in one of the pots. When the medium temperature reaches between 85 and 90 degrees Fahrenheit, adjust the mat’s heat setting so it stays between 85 and 90 degrees. Tent a piece of food-grade plastic film over each peat pot and secure it with a rubber band to help stabilize the soil temperature if needed.

• 2

  Fill 6-inch peat pots, one for each diploid seed, with moist sterile medium. You need to fill one peat pot with room-temperature medium for every two peat pots filled with heated medium.

• 3

  Plant the triploid watermelon seeds, with the pointed ends of the seeds pointing upwards at a 45- to 90-degree angle, 1 to 2 inches deep in the peat pots filled with heated medium five weeks before the last projected frost date of spring.

• 4

  Plant the diploid watermelon seeds, with the pointed ends of the seeds pointing upwards at a 45- to 90-degree angle, 1 to 2 inches deep in the peat pots filled with room-temperature medium. Washington State University advises planting one diploid plant for every two triploids, so, if you want to grow 10 seedless watermelon plants, you need to plant 10 triploid seeds in pots of heated medium and five diploid seeds in pots of room-temperature medium.

• 5

  Place the diploid seed pots in a room with a temperature of at least 70 degrees Fahrenheit. Leave the triploid pots on the seedling-heat mat, making sure the growing medium stays between 85 and 90 degrees Fahrenheit until the seeds sprout.

• 6

  Water the diploid and triploid seeds just enough to keep the medium moist throughout until the plants develop three to four true leaves; a few mists from a spray bottle several times a day often suffices. Water the seedlings until water drains from the pots just before you transplant them outside.

• 7

  Transplant the seedlings outside when they develop three to four true leaves, after the threat of frost passes in spring and the soil temperature stays at 70 degrees Fahrenheit or above consistently. Insert a soil thermometer in the ground to check the soil temperature before transplanting.

Fruiting

• 8

  Test the soil pH in an area open to full sunlight, preferably on a south-facing wall, building or slope, six months to one year prior to transplanting. Amend the soil pH to 6.0, using 7 to 8 pounds of ground limestone per 100 square feet to raise the pH one full point, and 1 1/2 to 2 pounds of elemental sulfur per 100 square feet to lower the pH one full point.

• 9

  Set up a drip irrigation system and condition the soil with 4 inches of matured compost and 4 cups of soluble, slow-release, 10-10-10 NPK or 16-16-8 NPK fertilizer as soon as the ground warms to 55 degrees Fahrenheit in spring of the planting year.

• 10

  Transplant the peat pots into the prepared soil bed when the seedlings develop three to four true leaves, after the threat of frost passes in spring and the ground stays at 70 degrees Fahrenheit consistently. Space the seeded and seedless watermelon plants 18 to 24 inches apart in rows, alternating between diploid and triploid seedlings. Space the rows 5 to 6 feet apart.

• 11

  Cover the plants with clear, polyethylene greenhouse film until the female flowers form if you live in the cooler parts of Oregon and coastal region. Female flowers have a cluster of node-like structures, called stigma, inside the flowers.

• 12

  Water the plants with 1 inch of water once a week through the drip-irrigation system after you transplant them outside. After the fruit sets, only water the watermelons if you get less than 1 inch of weekly rainfall.

• 13

  Mulch the soil around the plants with 2 inches of straw mulch, maintaining a space of 1 inch from the vines where they emerge from the ground and the soil reaches 75 degrees Fahrenheit consistently.

• 14

  Cover the plants with spun-bonded, polyolefin or polyethylene row covers after planting outside until the vines flower to help protect against pests. Watermelons commonly attract spotted or banded cucumber beetles, squash vine borers and aphids. Spotted and banded cucumber beetles have yellow-green bodies and measure around 1/4-inch long, with the former having 12 black spots on its topside and the latter having three black bands on its topside. Squash vine borers often go unseen, but cause sudden wilting and vine death. Remove row covers when the vines flower.

• 15

  Mix together 5 tablespoons of insect-killing soap containing 2 percent potassium salts of fatty acids with 1 gallon of water in a pump-type sprayer if you have a serious beetle, borer or aphid infestation. Spray the insects until covered, repeating every seven to 10 days as needed.

• 16

  Check the seedless watermelons for ripeness 60 to 70 days after planting, or when they weigh 6 to 12 pounds. Ripe watermelons have a withered, brown tendril opposite where the stem attaches to the vine. Ripe watermelons have a matte finish to them, while unripe melons appear shiny. Ripe watermelons usually, but not always, produce a hollow sound when rapped with the knuckles. Cut the stems using pruning shears to remove the watermelons from the vines.