Radish

Introduction

Radish (Raphanus sativus) is a member of the Brassicaceae family, along with cabbage and other brassica crops. Radish is a very old crop, possibly important as early as 5,000 years ago. Present-day radishes include European radishes with relatively small roots as well as the large-rooted white daikons that are enjoyed both as food and, recently, as a cover crop, due to long taproots that can penetrate compacted soil.

Radishes grow very quickly, maturing in 21-28 days under ideal conditions. In cold weather, 40-50 days may be required. Preferred planting dates are early spring and August. The roots (technically swollen hypocotyls and roots) are spicy, due to the presence of glucosinolates, which are sulfur-containing compounds produced by members of the brassica family. High temperatures and a lack of moisture can increase both pungency and the speed of bolting. As a result, production in the spring and fall often results in higher quality radishes than those produced in the summer. Some varieties also have a tendency to become pithy, possibly in response to certain environmental conditions. 

Types and Varieties

Table radishes come in an array of shapes (round, cylindrical, turnip-like) and colors (white, green, red, and black). Daikon is a general term for a group of long, white radishes that need cool temperatures and short day lengths to flourish. Some of these can grow up to 18" long and weigh about 3 lb on average, but can reach almost 50 lb. The black or winter radish is intensely hot. Radish tubers are formed from the basal part of the main shoot and a portion of the root.

Soil Fertility

Apply lime according to soil test results to maintain soil pH at 6.0-6.8. Radishes need loose, well-drained soil for easy root expansion. Although daikon can penetrate heavy soils to depths more than 1', the roots will not be smooth, uniform and tender in heavy soils. In stony or very heavy soil, shorter daikon varieties perform better. Because radishes grow so rapidly, a rich, fertile soil is essential. They prefer cool growing conditions (50-65ºF), so maintain a high level of soil moisture to minimize the adverse effects of summer temperatures. Hot weather reduces quality and increases pungency. Late plantings may bolt before edible roots form.

Less nitrogen fertilizer will be needed if legume sod was plowed down or if manure was applied (see Table 1 and Table 7).

Plant Nutrient Recommendations According to Soil Test Results for Radish

Planting 

The seeding rate for table radish is 18-38 lb of seed/A (depending on variety) or 1.4 oz/100 ft row. The seeding rate for daikon radish is 1 lb/A. 

Spacing. Seed radish 0.25-0.5" deep, spaced at 0.75-1" apart within the row (to obtain 12-15 plants per foot of row). Space rows 8-15" apart. It is not recommended to transplant radish.

In the spring, plant as soon as the soil can be worked; thereafter, plant at intervals of about 10 days. Soft, well-tilled ground will result in the most attractively-shaped radishes. Radishes are often planted in 4' wide raised beds, in 6 rows per bed. Use of floating row covers at time of planting will help control flea beetles and cabbage root maggots. Keep soil moist, especially if growing in hot conditions. 

Harvest and Storage

Yield. Some salad radishes and most storage radishes are marketed with tops removed in plastic bags. Bunched radishes are common at farmers' markets and smaller retail operations. Those sold with tops will lose moisture and quality more rapidly than those without leaves. A good yield of bunched table radish is 2,500 dozen bunches (8-12 radishes/bunch) per acre (about 25 bunches per 30 ft of row). A good yield of film-packed radishes (8-oz bags) is about 15-20 bags per 30 ft of row. Daikon may be marketed in cartons or 20-lb plastic bags and good yields range from 15 to 20 tons/A.

Table Radishes. Harvest on time, beginning about 3-4 weeks after seeding when roots are the size of a large marble (depending on variety). Bunch or top, hydrocool, and refrigerate. Topped radishes will keep 3-4 weeks in good, crisp condition if kept at 32º F, 95% RH, and in breathable packaging.

Daikon Radishes. Harvest by hand beginning approximately 60 days after planting. The roots are harvested at about 12-14" in length for the processing market and 2.5" for the oriental vegetable market. Daikon can be sold with or without the tops attached; usually a better price is paid for tops left on the roots. Daikon radishes will last 3-4 months at 32º and 95% RH.