Mini watermelons like “Garden Baby” (Citrullus lanatus “Garden Baby”) grow 3- to 4-foot long vines and produce juicy, sweet 6- to 8-pound melons two to two and a half months after the seeds are planted. If you have a fence that gets six to eight hours of sunlight each day, plant a few mounds of them along the fence and let the vines climb. They are annuals so you will have to replant them each spring, if you want to continue to grow them. Start the seeds indoors one month before the last expected frost to get a jump on the growing season. You will have a cool, tasty treat for the whole family at the hottest time of year.
Plant the watermelon seeds in 4-inch wide peat pots with soil-based potting soil. Plant them 1 inch deep, keep the soil moist and at a temperature of 65 to 70 degrees Fahrenheit. They usually germinate in 10 days.
Dig a 1 1/2-foot deep, 3-foot wide hole where the watermelons will be planted around the same time as the last expected spring frost. Dig the hole so the outer edge is right next to the fence. Put the soil removed from the hole into a wheelbarrow. Mix compost or peat-based potting mix into the soil in the wheelbarrow so the resulting soil mixture is about 50 percent compost or potting mix.
Test the soil pH to make sure it is between 6.0 and 6.8. Mix iron sulfate into the soil to lower the pH, if necessary, or lime to raise the pH. The amount of iron sulfate or lime required will depend on how much the pH needs to be changed and the texture of the soil. Put 1/4 cup of bone meal in the bottom of the hole and fill the hole in with the amended soil mixture. Mound up the soil mixture to form a 3- to 12-inch high mound. Space multiple mounds 4 feet apart.
Plant the watermelon seedlings outdoors two to three weeks after the last expected frost when the soil warms to 65 degrees Fahrenheit. Plant two or three seedlings in each hill. Space them evenly around the outside edge of the top of the hill. Do not remove the seedlings from the peat pots before planting them. Set them just deep enough for the soil in the peat pot to be level with the surrounding soil.
Spread a 2- to 3-inch depth of organic mulch on the soil around the seedlings and around the hill to help retain moisture and keep weeds from growing. Give the watermelons 2 or 3 gallons of water once or twice per week, depending on how quickly the soil dries. Keep the soil consistently moist but not soggy. Pour the water on the soil around the outside edge of the hill.
Set the vines on the fence when they grow long enough to comfortably reach it. They will climb on their own once they take hold. Sprinkle 1 to 2 teaspoons of 33-0-0 vegetable fertilizer around each hill about a week after the watermelons first bloom and again in three weeks. Water the vines right after spreading the fertilizer to wash it down to the plant roots.
Slip the melons into old mesh onion bags or the foot of an old pair of pantyhose when they form. Fasten the onion bags to the fence with zip ties. Cut the foot of the pantyhose long enough to tie the top of the hose to the fence. Pick the watermelons when the climbing tendrils start to turn brown and thumping the melons with a thumb causes a dull sound.