Northern gardeners hesitate to attempt to grow watermelon because this heat-loving crop typically requires a long, warm growing season to set and ripen fruit. There are some tips and tricks to make a Northern watermelon garden successful, however.
To grow watermelon in the colder climates of the north:
Choose varieties with a smaller number of "days to harvest." Try 'Sweet Dakota Rose', which was developed in North Dakota, 'Blacktail Mountain', 'Sweet Siberian' or 'Mini Love', which is both early ripening and "personal" size.
Start from seed, sowing it indoors about four weeks before your typical last frost date and using four-inch peat pots or similar containers that let you skip potting up and that limit root disturbance at transplanting time. Melons with damaged roots will not grow on well. Use a heating mat under your seed pots to boost germination but shut it off after they have sprouted.
Wait until the soil temperature remains above 65˚F night and day to plant out your seedlings. Cover the soil with black or clear plastic to hasten heating. When you transplant your watermelon seedlings into the garden, keep the plastic there; cut holes in it to plant your vines. This will continue to keep the soil warm night and day. Black plastic will block weeds while also retaining heat; clear plastic will retain more heat but weeds can sprout underneath it, since they are receiving light.
Raised beds warm up faster than ground soil, so consider growing your watermelons in a raised bed. A compact variety like 'Sugar Baby' is amenable to raised beds.
Image credit: Jane Dickson, CC BY-SA 2.0 via Flickr.com