When to Plant Watermelon in Zone 5

Plant watermelon seeds in May in USDA Zone 5.

To enjoy homegrown watermelons (Citrullus lanatus), gardeners in U.S. Department of Agriculture Plant Hardiness Zone 5 must wait until spring and the danger of frost to pass before planting watermelon seeds. Watermelon plants develop into low-growing, ground-hugging vines that need lots of sunlight and heat to prosper. For raising watermelons, choose a fertile sandy loam soil with excellent drainage. Wet soils and too much shade lead to plant fungal diseases and root rot.

USDA Zone 5

The USDA Zone 5 designation refers to regions where winter low temperatures drop down into the minus 10 to minus 20 degrees Fahrenheit range. Since watermelons are not grown in winter nor do they survive frosts, the zone rating itself isn't applicable to raising the melons. Such a cold winter does suggest that the warmth of spring takes a while to build, therefore delaying outdoor watermelon planting. What's most important in the zone is waiting until the expected last spring frost date. Only after then is it safe to plant and grow melons.

Time Frame

Depending on latitude and elevation, the last spring frost in Zone 5 occurs between April 21 and May 10, according to the website Plant Maps. Once the last frost date passes, allow the garden soil to warm to at least 60 degrees Fahrenheit. If soil is too cool, watermelon seeds won't germinate. Moreover, any young seedlings transplanted into cool soil become stunted and stressed, likely succumbing to root rot. Most varieties of watermelon need 80 to 90 days to produce harvest-size watermelons. Plenty of growing time exists in Zone 5 if watermelons are planted outdoors between May 20 and June 10.

Starting Seeds Indoors

Eager gardeners may start watermelon seeds indoors in deep, single-cell seed trays about three weeks before the last spring frost and the garden soil outdoors becomes at least 60 degrees Fahrenheit. Sow seeds 1 to 1 1/2 inches deep and place in a warm location, between 75 and 90 degrees Fahrenheit, in bright indirect light. Keep the soil evenly moist. Seeds germinate in three to 12 days. Maintain evenly moist soil once seedlings pop up and relocate them to a full-sun window, around 80 to 85 degrees Fahrenheit. When transplanting outdoors, avoid disrupting or breaking any part of the root ball.

Planting Guidelines

Sow watermelon seeds 1 to 1 1/2 inches deep. Watermelons plants needs lots of room to spread their vines and be productive. These melons are not sown or transplanted into neat, tight rows. Rather, they are planted into widely spaced hills. When sowing seeds, create a shallow planting basin or hill that is 6 inches wide. Place four or five seeds in it and cover with soil. Space additional hills in a row 6 feet away. Multiple rows of hills must be seven to 10 feet apart.

References

• "University of Illinois Extension"; Watermelon; Ron Wolford, et al.
• "Colorado State University Extension"; Vegetable Planting Guide; David Whiting, et al.; June 2010
• Plant Maps: Interactive USDA Hardiness Zone Map for the U.S.