Radishes are a fast-growing vegetable, available for harvest about three weeks after planting the seeds. Radishes need to be harvested from the ground once they reach the mature stage to maintain their healthy condition.
As a cool-weather crop, radishes are usually planted during the fall and approximately six weeks before the last frost. Keep 2 inches between the seeds, as radishes do not grow well when they are too crowded. They need a sufficient amount of sun to sprout quickly. Nearby plants can provide too much shade, which prolongs the length of time from seed to sprout. The Old Farmer's Almanac recommends consecutive planting of radishes every two weeks for continued rotation of the vegetable.