Clear vegetation and rocks from an area in the garden where the soil is very well-drained with at least six hours of direct sunlight each day. Radishes will rot in waterlogged soil. The best soil for growing radishes is sand or sandy loam. Radishes are planted 6 inches apart for maximum growth so plan the space accordingly. Break up the soil to a depth of 8 inches with a shovel or a tiller.
Test the soil in the chosen site to determine what amendments should be added to the soil for the best radish production. For assistance with taking a soil test, contact your local county agricultural extension office. A soil test will tell you exactly what amendments are necessary. Your extension office will also have a list of recommended radish varieties for your area and the suggested radish seed planting dates.
Spread a 2-inch layer of well-rotted compost along with any other amendments recommended on the soil test over the planting space. Work the soil amendments and compost into the to 4 to 6 inches of soil and rake the soil until it is smooth.
Plant radish seeds 1/2 inch deep and 1 inch apart in marked furrows when the soil temperature is between 45 and 85 degrees Fahrenheit. Once the soil temperature is above 95 degrees, radish seed will not sprout. The best daytime temperatures for growing radishes is between 50 and 65 degrees F. Warmer temperatures effect the taste of the radish by increasing the pungency, or hotness. Radish seed germinates in less than one week. Because radishes have a short storage capacity, plant successive crops over several weeks to have a steady supply of fresh radishes.
Thin the radish plants to 6 inches apart when they are 1 to 2 inches tall. The small greens make a tasty addition to fresh salads. Keep the soil moist, but not waterlogged, during the growing process.
Harvest radishes when they are 3/4 to 1 inch in diameter, approximately three to four weeks from planting. The larger the radish, the stronger the flavor. Small radishes are considered the best-tasting. Check the size of the radish by lightly brushing back the soil at the base of the green top. Gently cover the radish with soil if it is not ready to harvest.