Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Radishes (Raphanus sativus L.) are a diverse crop grown for their edible storage taproots and sometimes for their edible leaves which are used as cooked greens, the shoots, when grown as a microgreen, and in the case of the rat-tail radish, the edible fruit. Some types are also are grown as forage crops for livestock and as cover crops for soil improvement. Other types are grown for the oil contained in the seed which is used for industrial purposes (radish oil is not an edible oil). They are true root crops and the harvested portion is the swollen upper part of the tap root. They are grown as annual plants but most are biennial in nature and will flower and produce a seed stalk after exposure to low temperature conditions. Radishes are a quick crop that fit well in rotations for market growers.
Radishes can be divided into groups based on their origin; size, shape, and color of the root; utilization; as well as season of adaptation. Three main groups are the small salad radishes of European origin, larger Asian radishes (often referred to as Daikon radishes), and black or Spanish hot radishes.
European salad radishes can be further divided into groups according to their size, shape, and skin color. White root interiors predominate but there are also types with pink, purple, green, and yellow flesh. Skin color is most commonly red, white, or a mixture (there are yellow and green skin forms). There are three main root shapes: globe or flattened globe, cylindrical elongated, and tapered.
Salad radishes mature very quickly (21-28 days commonly) and are easy to grow. Salad radishes are commonly grown in the spring (they are often called spring radishes) but can be grown throughout the year in frost free periods. Best quality roots are produced when temperatures are between 50°F and 70°F under shorter daylengths. Higher temperatures, dry soil conditions, and longer daylengths (summer months) can cause radishes to bolt and can cause roots to be tough, pithy, and of greater pungency. Salad radishes are harvested, washed, and then either bunched with the tops on or topped and bagged. Shelf life for bunched radishes is 10-14 days when stored at a temperature of 0-2° C and at a relative humidity of 95-100%. Radish greens are edible and can be used as cooked greens.
Asian, Daikon, Japanese or Chinese radishes are most commonly found as long white cylindrical radishes with an extremely strong tap root. They are adapted to a wide range of soil conditions and can penetrate compacted soils and are often used as winter-killed cover crops in this region. Some types can grow to over 2 feet in length, 4 inches in diameter, and weight over 40 lbs. While the most common form is long and cylindrical leading to a tapered root tip, there are also stubby, barrel shaped forms, globe (turnip) shapes, and tapered (carrot) shapes. Skin color is most commonly white but there are also green, pink, and purple forms. Flesh is commonly white but with cream, green, pink, red, and purple colors also found. Most forms are mildly pungent, but there are also more strongly pungent types.
Asian radishes mature in 45-60 days and are becoming more popular in the West as cooks become familiar with their use. They are also very easy to grow. Of course, they are an important ingredient in Asian cuisine and found in Asian markets throughout the country. Asian radishes are best adapted to cooler temperatures and shorter daylengths and are commonly grown in the late summer or fall. However, they can be grown in the spring and summer using adapted varieties that are slow bolting. Asian radishes have a longer storage potential than salad radishes and larger roots can be kept for several months stored near freezing under high humidity if the roots are intact. Asian radishes are also used for producing sprouts and microgreens where the shoot is eaten.
Black, Spanish or winter radishes are group of high pungency, hot radishes. They commonly have black skins and white flesh and may be shaped as globes, flattened globes, elongated cylinders, or tapered roots. They vary in size but some types can be larger than one foot in length. Other than black, skin color can be white, pink, red, brown, and purple. Growing conditions are similar to Asian radishes and they have long storage potential.
All root radishes are grown in seasons where they will not bolt (produce a flower stalk). The storage reserves in the root will be used up in the flowering process.
Another radish variant is the rat-tail or Madras radish (Raphanus sativus var. mougri or Raphanus sativus var. caudatus) which is grown for its edible fruit, which botanically is a silique but is commonly called a “pod”. While all radish pods can be eaten, these types have been selected specifically for their edible pods. They are allowed to flower and pods are harvested in the immature stage and eaten raw or cooked. These radish pod crops are most commonly found Indian and Southeast Asian cuisine. This is an interesting crop for growers looking for specialty products to offer consumers.
Seed salad radishes as early in the spring as soil can be worked, then at 8 to 10 day intervals through September (or through November in high tunnels). Fall adapted Asian radishes are planted from mid-July through September. Space rows 8 to 15 inches apart and 1 inch between plants in the row for salad radishes or 2-3 inches apart in the row for larger Asian radishes. Radishes are often planted in 4′ wide raised beds, 6 rows per bed. Nitrogen requirements are modest with 50 lbs/a N recommended for smaller radish types and 60-75 lbs/a N for more robust Asian types.
Recommended varieties for our region are as follows:
Red globe; white interior radishes: Saxa, Rover*, Cherriette*, Perfecto, Rudolf (Crack tolerant), Cherry Belle, Pink Beauty (organic), Champion, Crimson Giant (large globe)
Specialty Radishes, Spanish Radishes, and Asian Radishes: Watermelon (white flesh, red interior, globe), Shumkyo Semi Long (Red flesh, white interior, elongated), White icicle (white flesh, white interior, elongated). Minowase (daikon), Mihashige (daikon), China Rose (red flesh, white interior, elongated), Chinese Winter (daikon), Discovery* (daikon), Round Black Spanish (heirloom, dark flesh, white interior, large globe), April Cross* (daikon), Sakurajima Mammoth (white flesh, white interior, large globe)
*F1 hybrid variety