Watermelon: Sow and Grow Guide

Watermelon is the quintessential summer food, and with more than 1,200 varieties, you are sure to find one that is perfect for your palate and garden! If you were wondering how to grow watermelon from seed, you've come to the right place!

General Information

The watermelon is related to cantaloupes, muskmelons, and other types of melons and ranges in size from less than a pound, to more than 200 pounds. With a smooth, exterior rind of either green, greenish-gray, yellow, or sometimes white, the juicy interior is usually pink or red, but is sometimes orange or yellow, and contains about 92% water by weight.

Did you Know?: Watermelons contain large amounts of beta-carotene and are a good source of vitamin C.Watermelons with red interior are also a significant source of lycopene. Seedless watermelons are self-sterile hybrids that develop normal-looking fruits but no fully-developed seeds. Watermelon rinds are edible, and sometimes used as a vegetable. In China, they are stir-fried, stewed, or more often, pickled. Pickled watermelon rind is commonly consumed in the southern U.S.. Watermelon juice can be made into wine.

General Sowing

When to sow outside: RECOMMENDED: 1 to 2 weeks after average last frost, and when soil temperature is 70°–90°F.

When to start inside: 2 to 4 weeks before transplanting, when weather is warm and settled. Sow into biodegradable pots that can be directly planted in the ground.

The sweetest watermelons grow during long, hot summers. Therefore, cooler areas may need a little extra help growing sweet fruits. Floating row covers (remove covers when flowers form), and weed barrier or black plastic sheeting may help produce a good crop in cooler climates. Floating row cover keeps seedlings a little warmer and also protects from pests. Weed barrier fabric/black plastic sheeting helps to warm the soil, keep down weeds, and retain moisture. Lay the barrier fabric/black plastic sheeting over the soil in your planting area. Cut 4"–6" diameter holes in it, 4'–6' apart, and transplant seedlings or sow the seeds within the cut holes. The best way to irrigate when using this fabric or sheeting is with a soaker hose or drip irrigation underneath. Remove the fabric or sheeting at the end of the growing season.

How Many Plants Should I Grow?: Depending on the variety, expect 2-3 watermelons per vine. Sowing varieties with different days-to-maturity can lengthen the harvest and add a diversity of flavors.

Indoor Sowing

Use a lightweight seed-starting mix/medium (sterile, and lighter than potting mix), and sow seeds ½" deep. Sow 2–3 seeds per pot, thinning to the strongest plant once leaves appear (clip extra plants at the soil level using scissors). The strongest plant may not be the tallest; look for thick, strong stems and deep color. By thinning early, you minimize the negative impact of crowding, like stretching for light.

Containers

Watermelon roots are sensitive to disturbance. By sowing into biodegradable pots that can be planted in the ground you can minimize any root disturbance. Remove the bottom of the biodegradable pot before planting to allow roots to grow freely.

Transplanting

Watermelons are very frost sensitive, so wait to harden off until outdoor temperatures are frost-free and settled. If your spring warm-up is lagging, use plastic mulch or season-extension products like hot caps or walls of water to warm the soil.

Transplant after all danger of frost has past and weather is warm, about 2 weeks after average last frost. Floating row covers can be placed over transplants to keep temperatures warmer and exclude pests.

Outdoor Sowing

How to sow

Sow seeds into raised mounds once the soil has warmed to over 70°F. Mounded soil warms up more quickly, and offers good drainage. Sow 2–3 seeds per mound and space mounds 4'–6' apart.

Thinning

Once seedlings have emerged, thin to one plant per mound.

Optimal Growing Conditions

Weeding

Keep small plants weeded, and as watermelons grow they generally shade out weeds that emerge later. Mulch also helps to prevent weeds while keeping roots moist.

Water

Watermelons need a lot of water. Water deeply, keeping evenly moist, but not soggy. Allow only the top 1"–2" of soil to dry between waterings. If stressed by lack of water, watermelon plants will not set fruit. Water at the base of the plant; avoid wetting leaves, as watermelons are prone to mildew.

Exposure

Choose an area with full sun (6 or more hours per day).

Fertilization

Watermelons are heavy feeders. Even when grown in fertile soil, watermelons benefit from additional fertilization. Apply a slow-release, balanced fertilizer at sowing time. Once flowers form, cut back on nitrogen and use a liquid fertilizer higher in phosphorus and potassium.

Pollination

Watermelon plants have both male and female flowers and need help from bees to transfer pollen to make fruit. Sow bee-attracting flowers nearby to help with pollination. You may also hand-pollinate the female flowers using a small paintbrush or cotton swab. Female flowers have a swollen base that looks like a miniature melon, while male flowers have prominent, pollen-filled stamens.

Harvesting

It can be tricky to know exactly when a watermelon is ripe and ready to pick. First, know the number of "days to harvest" and begin checking fruits as harvest date draws closer. Signs to look for are: (1) the bottom of the melon (where it lies on the soil) turns from light green to a yellowish color; (2) the surface color of the fruit turns dull; (3) the skin becomes resistant to penetration by the thumbnail and is less smooth to the touch; and (4) light green, curly tendril on the stem near the point of attachment of the melon is brown and dry. All of these indicators may not necessarily occur at the same time.