How to Grow Watermelons in Compost

Separate decomposed loam from unfinished compost with a garden sieve. Return your unfinished compost to the compost bin and pile your loamy compost in a separate location. Allow this compost to sit for 6 weeks to finish the composting process. While the composted loam finishes, the bacteria inside of the pile that breaks down organic materials will die off. These bacteria can harm the roots of plants in garden soil if unfinished compost is used to amend soil before it is ready.

Break up your soil with a spade to a depth of 12 inches. Shovel finished compost over the soil in a 4-inch thick layer. Mix the compost into the soil with a rake.

Rake the soil into hills that are 12 to 24 inches high. Each mound should be spaced 4 feet apart.

Check the soil temperature with a thermometer that has a probe. Plant watermelon seeds in the mounds of soil once the soil reaches a temperature of 64 degrees Fahrenheit. Place four watermelon seeds in each hill in holes that are 2 inches deep. Cover the seeds with soil.

Put straw mulch over the hills to help hold in moisture and anchor the soil in place. Water the hills so that the soil is as damp as a wrung-out sponge. Thin the plants after they sprout so that only the strongest watermelon vines remain. You should space each vine 2 feet apart on the hills.

Place more finished compost in piles to the side of each plant to fertilize it once the soil warms to 75 degrees Fahrenheit. This technique is called "side-dressing" the plants.

Harvest watermelons when the vine tendrils shrivel, the side that rests on the ground turns yellow and the skin of the melon turns dull.