Radishes are root vegetables that do well in cooler conditions. They can be found in many sizes, shapes, and colors, but most gardeners are familiar with the round red garden radishes that add a spicy kick to salads. Other varieties of radish, such as the daikon, are larger and have more elongated roots. 
Planning | Site Selection | Planting | Care | Potential Problems | Harvest & Storage
Planning
Radishes prefer the cool temperatures of spring and fall. The classic garden radish has a very short production time going from seed to harvest in as little as three to five weeks. Daikon radish takes longer to mature often ready to harvest after 50 to 60 days.
Garden radishes can be planted in early spring for a late spring, early summer harvest and then again in late summer for a fall harvest. These plants also benefit from succession planting. By sowing seed for these root crops every 7-10 days, you have a few radishes coming into maturity each week, rather than an entire crop all at once. This is particularly important for radishes as they can quickly pass their peak quality if left in the garden too long. Radishes become spongy, woody, excessively hot in flavor, and make seed heads readily in the warm temperatures of summer, so avoid growing during this time of year.
In Iowa, plan to plant garden radishes in early April and sow staggered plantings every 7 to 10 days through mid-May. This will allow for harvest from mid-April through early June. Sow seed again starting in early to mid-August with staggered plantings through mid-September to harvest radishes through mid-October. Daikon radish and other large root varieties can be planted in early April for harvest in mid-June or in mid-August for harvest in late October. Many gardeners prefer the taste of daikon planted in late summer and harvested in fall.
Suggested cultivars for radish can be found in this article: What are some good radish varieties for the home garden?
Site Selection
Radishes grow best in moist, well-drained soils that are not compacted. Loamy, sandy soil is ideal to allow uniform root development. When soils are heavy, rocky, or compacted, roots are often smaller and irregular in shape. If soils are not ideal, grow radishes in raised beds or amend the soil with compost to improve soil conditions. Grow in full sun, providing at least six hours of direct sun a day.
While daikon radishes are too large, garden radishes can be grown on a patio or deck because of their smaller size and short production time. Any large container can be used. The larger the container, the better, and window boxes work particularly well for radishes. Utilize regular potting soil and provide full sun and adequate moisture.
Planting
Sow radish seed directly in the garden ¼ to ½ deep in rows 12 inches apart. Seedlings emerge quickly, typically after 7 to 10 days. When the seedlings emerge, thin the planting so the remaining plants are 2 inches apart. Larger varieties, like daikon, should be thinned so they are spaced 4 to 6 inches apart. Do not sow seeds indoors for transplanting as they do not transplant easily.
Growing and Care
Radishes do best in cool growing conditions. Daytime temperatures of 70 to 75°F and nighttime temperatures of 55 to 60°F are ideal. Provide consistent moisture throughout the growing process. Plants need an inch of water a week provided by the gardener if not by Mother Nature. Drought stress will cause radishes to develop a tough texture and poor flavor.
Do not over-fertilize radishes as it can promote lush green tops and little root development. Fertilizer is not needed each growing season, especially if compost or well-composted manure has been added. Because radishes are consumed raw, it is important not to use fresh manure. In addition to having too much nitrogen that will reduce root development, fresh manure can contain disease-causing microorganisms. If soil fertility is very low, an all-purpose fertilizer, such as 10-10-10, applied to the garden at a rate of ¼ to ½ lbs per 100 square feet prior to planting should be adequate for successful growth.
Cultivate lightly to remove weeds without disturbing the roots. Garden radish typically produces a crop in such a short time that weeds don’t become as problematic. Organic mulch like grass clippings, clean straw, or coco mulch can also help reduce weeds, conserve soil moisture, and moderate soil temperatures.
Potential Problems
Overcrowded plants or those grown in compacted or heavy soils produce small, misshapen roots. Hot, pithy radishes may result from hot weather or harvesting too late. Excessive nitrogen, the rapid onset of hot weather, or overcrowding may produce plants that are all tops (lush foliage, little or no root development). Inconsistent moisture can cause the roots to split or develop an overly hot flavor.
Radishes are relatively disease and insect pest free. Occasionally, flea beetles will feed on the foliage, or root maggots may feed on the developing roots. Because of their short production time in the garden, insects rarely pose a significant threat. When grown in poorly drained soil, root rot may develop. Good cultural practices such as using compost to improve drainage, keeping gardens free of weeds, and fall clean-up can often prevent any significant insect issues.
Harvesting and Storage
Radishes can be harvested 3 to 5 weeks after planting. Periodically check their development by pulling one or two plants as they approach maturity. They can be pulled anytime they reach a usable size (about 1 inch in diameter). Radishes get pithy and hot when harvested too late.
Larger varieties, like daikon, can be harvested 7 to 10 weeks after planting. Carefully dig with a spade or garden fork once they reach their mature size, typically 1 to 2 inches across and 8 or more inches long. Make sure all plants are harvested prior to frost, and don’t allow plants to become overly mature as they develop a fibrous texture and intense flavor.
Radishes are best used fresh and do not store well for long periods of time. Because they are eaten raw, clean and wash radishes after harvest and again before being eaten as a practice in safe produce hygiene. Store roots in the refrigerator for 2 to 3 weeks. Prior to storage, cut off the foliage to within ½ inch of the roots.