Melons are lush, vining plants that grow only in summertime sun and temperatures. Sweetheart melons, or the "F1 Sweetheart" cultivar, are cantaloupe hybrid melons from France and require typical melon growing conditions. These melons have a long growing season and fail in fall frosts. If your growing seasons run too short, plant the seeds in polytunnels to conserve warmth into fall and protect the melons from fall frost.
Plant Sweetheart (Chanterais) melon seeds in a polytunnel in mid-spring when temperatures reach 15.6 to 18.3 degrees Celsius. These seeds require warm air and soil temperatures so don't plant them before the last frost, even in a polytunnel.
Use any bright, quick-draining site in the polytunnel. Melons do well with the diffuse light inside the structure and don't require light supplements. Set out planting sites at 2 to 3 feet in the row, with 6 feet between multiple rows. Dig into the top 6 inches of soil in the individual planting sites and add 3 inches of organic compost for moisture retention and nutrition. Mix 5-10-10 fertiliser into the top 3 to 4 inches of soil for optimal rooting.
Plant Sweetheart melon seeds 1/4-inch deep in your amended soil. Use 2 inches of water for each seed to settle the planting then put the new plants on a schedule of 2 inches of water every week. Use 1 inch of organic mulch to maintain soil moisture throughout the season.
Give melons ammonium nitrate fertiliser per the manufacturer's directions when the vines develop. Fertilise each plant again at the first sign of fruit development. Don't over-fertilise the melons as this may lead to vegetative growth rather than fruit growth. Always water the plants immediately after feeding them.
Leave windows and doors open, during the summer, to take advantage of natural summertime warmth and sunshine. Close doors and windows when temperatures drop, during fall, to protect the vines when frost arrives.
Harvest Sweetheart melons when their maturity dates approach. Watch for the rinds to change from grey/green to yellow/buff. Don't leave the melons on the vine, after they reach maturity, to avoid fruit rot.
Sweetheart melons grow to grapefruit size but sport sweet orange cantaloupe melon flesh.