Forage radishes (Raphanus sativus var. longipinnatus) are members of the Brassica family, which also includes arugula, mustard, and cabbage, to name just a few. Forage radishes are also known as Tillage radishes, Daikon radishes, and Japanese radishes. They are marketed under various cultivar names such as ‘GroundHog’, ‘Nitro’, ‘Sodbuster’, and ‘Bio-till’.

Oilseed radishes (Raphanus sativus var.oleiformis) are another type of radish grown as a cover crop. They are related to the forage radish but have a stubbier taproot, more branches, and tend to be somewhat more winter hardy than the forage radish. Oilseed radishes are marketed under cultivar names such as ‘Adagio’ and ‘Colonel’. As the name implies, oilseed radishes were originally grown for oil. Often the names oilseed and forage (‘Daikon’) are used interchangeably, and that can be confusing because they are different. However, most of the traits and growing recommendations are the same for both types of radishes.

Photo Source: Maryland Cooperative Extension, Fact Sheet 824

Alleviation of Soil Compaction

Forage radishes are excellent at breaking up compacted soils, and have earned the nickname “bio-drills.” Planted in the early fall, 3 to 10 weeks before the first freeze, the roots of forage radishes can penetrate compacted soils more deeply than other cover crops such as cereal rye. Under ideal conditions, the thinner part of the taproot can grow to a depth of 6 feet or more during the fall! The thick fleshy part of the taproot can grow 12 to 20 inches (including 2 to 6 inches protruding above ground), creating vertical holes and zones of weakness that tend to break up surface soil compaction. After the plants die in the winter and the roots decompose, the open root channels can be used by the roots of your vegetable crops to grow through the compacted layers of soil. The channels created by the roots tend to remain open at the surface, improving water infiltration and soil warming in the spring. The channels also provide an access route for subsequent roots to reach subsoil moisture, resulting in greater plant resilience under drought conditions. The decomposed roots of the forage radishes improve the soil’s porosity (air spaces) and the general physical soil condition (tilth).

Radish Holes after Winter Kill. Photo Source: Joel Gruver, Western Illinois University

Weed Suppression

A good stand of radishes — more than 5 plants per square foot — has been shown to eliminate nearly all winter annual weeds. Weed suppression from fall-planted radishes typically lasts into April but does not extend much further into the summer planting season.

Nutrient Scavenger

The deep root system, the rapid root growth, and the heavy feeding of forage radishes combine to make them an excellent scavenger of residual nitrogen after the summer growing season. Radishes take up nitrogen from both the topsoil and from deeper soil layers and then store the nitrogen in their shoots and in their root biomass. Because radishes do an excellent job of cleaning up nitrogen left over in the soil from summer crops, they help prevent nitrogen from leaching into groundwater during fall, winter, and spring.

Unlike cereal rye (annual winter rye), whose residues decompose slowly and continue to hang on to nitrogen for extended periods (thus immobilizing the nitrogen), radish roots decompose and release nitrogen rapidly. This means that early spring crops can get an early boost from the nitrogen captured by the radish crop.

In addition, forage radishes have also been shown to be excellent scavengers of potassium (K) and phosphorus (P) left over from the past growing season.

Effects on Nematodes

Research has provided evidence that the residues from radishes reduce the number of plant parasitic nematodes such as root knot nematodes.

Seeding

It is difficult for the home gardener to determine the ideal seeding rate, but several seed packets I reviewed suggested rates of ¾ lb. to 1 lb. per 1,000 square feet. Follow the seed company’s recommendation listed on the seed packet and adjust the sow rate after a season or two of experience.

The recommended depth for seed planting is ¼ to ½ inch, however, seed can be broadcast (remember my brother in-law’s air plane method?) and left uncovered. The recommendation is to sow uncovered seeds about 50 per cent more thickly. So, if the recommendation is to sow 1 lb., sow about 1½ lbs. if the seeds will not be covered with soil.

Radishes germinate rapidly, emerging within 3-4 days when environmental conditions are favorable. Seed broadcasted on the soil’s surface can establish well if followed by a timely rain. Forage radishes do not tolerate very wet soil, so low spots that collect standing water should be avoided. The radishes are tolerant of frost until temperatures dip below 25ºF. It takes several nights of temperatures in the low 20s to kill forage radishes.

Cautions

Deer will be attracted to your forage radish crop. Also, during warm spells in winter and in early spring, decomposing radishes may release a pungent rotten-egg odor.

Since forage radishes are in the Brassica family, it’s best to avoid planting them in areas where you will be planting other Brassica members such as cabbage, cauliflower, broccoli, kale, kohlrabi, turnip, or mustard.

Summary

Another great tool in the cover-crop tool box, forage radishes can provide multiple benefits including: alleviation of soil compaction, weed suppression, nutrient capture (N, P &K), and erosion control. They can also be mixed with other cover crops such cereal rye to add more organic material to your soil.

Thanks for joining us in The Garden Shed and we hope you stop by again next month.

Resources:

“Radishes- A New Cover Crop for Organic Farming,” http://articles.extension.org/pages/64400/radishes-a-new-cover-crop-for-organic-farming-systems

“Forage Radish: New Multi-Purpose Cover Crop For the Mid-Altantic,” Maryland Cooperative Extension, Fact Sheet #824, https://enst.umd.edu/sites/default/files/docs/FactSheet824 Weil et al 2009.pdf

“Radishes: A New Cover Crop Option,” American Society of Agronomy, Crops and Soils, https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_022940.pdf

“Forage Radishes,” Cornell University, http://covercrops.cals.cornell.edu/forage-radish.php

If you have any other questions please let us know.