Radishes – A New Cover Crop for Organic Farming Systems

Over the past decade, radishes have been redefined; once known almost exclusively as a pungent vegetable, radishes have recently gained recognition for their cover cropping potential.

Radishes have made rapid inroads as a cover crop for several reasons. First, the radish phenotype is well suited to perform many valuable cover crop functions—provide soil cover, scavenge nutrients, suppress weeds, and alleviate compaction—while creating few of the residue management challenges associated with many other cover crops. Second, recent research including many on-farm trials has documented beneficial effects of radish cover crops on soil properties and subsequent crops. Third, the seed industry has ramped up production of radish seed, brought new branded products to market, and promoted radish as a cover crop. Fourth—but perhaps most important in terms of the exponential growth in interest by farmers—radish cover crops have become a hot topic of discussion in rural coffee shops and on-line agricultural forums.

Radish Seedstock
Most of the radish varieties currently marketed for cover cropping (e.g., GroundHog radish™, Nitro radish, Sodbuster, and Bio-till radish) are large rooted selections of daikon-type oilseed or forage radishes, but are not the product of formal breeding programs. All are morphologically similar to the large white daikon radishes traditionally used in Asian cooking. Hybrid daikon-type culinary radish seed is prohibitively expensive (more than $100/lb for bulk seed) for use in cover cropping, but open pollinated culinary daikon varieties may have some potential with bulk seed available for about $5/lb. Standard oilseed radish cultivars (e.g., Adagio, Colonel, and Defender) tend to have a stubbier, more branched taproot, greater winter hardiness, and lower seed cost than larger-rooted daikon types. (Ngouajio and Mutch, 2004)
All radishes are insect pollinated and cross-pollinate easily, increasing the likelihood of genetic variability if not grown in strict isolation. In recent years, some farmers who purchased inexpensive radish seed have reported high levels of variability including early bolting.

In response to the growing interest in radishes for cover cropping, some public and private breeding programs are starting to select for radishes with superior cover crop attributes. More research is needed comparing radish varieties with respect to traits such as winter-hardiness, hard-seededness, seedling vigor, nutrient scavenging, root penetration strength, and biofumigation potential.
The information that follows should be generally applicable to all radish cultivars used for cover cropping unless otherwise noted.

Benefits of Radish Cover Crops
Effects on Soil Structure
The radish attribute that has captured the most farmer interest is their robust rooting ability. Under favorable growing conditions, radish roots can extend more than 3 feet deep in 60 days, with the thickened storage portion of the root (commonly referred to as the tuber, though not botanically correct) extending more than 12 inches. Plants with roots more than 1 inch in diameter normally have a significant portion of the root exposed above ground (often more than 4 inches, even in uncompacted soils) (Fig.
After radishes winter-kill and their large fleshy roots desiccate, the channels created by the roots tend to remain open at the soil surface, improving infiltration, surface drainage, and soil warming. Radish rooting effects on soil porosity also extend into the subsoil. This general process, called bio-drilling, can improve root growth by subsequent crops and access to subsoil.
Research at the University of Maryland has shown that radish roots have greater ability to penetrate compacted soil than cereal rye and rapeseed. (Chen and Weil, 2010) Subsequent research found twice as many corn roots penetrated compacted subsoil after radish cover cropping as compared to cereal rye, with both cover crops promoting more rooting than bare-fallow. These results suggest that radishes may be useful as a biological alternative to deep ripping and other mechanical methods of alleviating soil compaction.

Effects on Weeds
A good stand of radishes can eliminate nearly all weed growth both during and for some time after active radish growth. To obtain near-complete weed suppression, radishes should be planted early (6 or more weeks before frost), at a relatively high population (more than 5 plants per square foot) into a clean seed bed. Weed suppression from fall planted radishes typically lasts into April, but does not extend much into the summer cropping season. Researchers at the University of Maryland have concluded that rapid and competitive fall growth, rather than allelopathy, is the primary mechanism of weed suppression by radishes. (Lawley et al., 2011)
Effects on Seed Bed Preparation
After winter-kill (or other causes of mortality), radish residues deteriorate rapidly. As a result, fall biomass production is unlikely to interfere with spring field work. Typically a good stand of winter-killed radishes leaves the soil surface weed free and perforated with open root holes in early spring. As a result, the soil warms up and dries out faster than soils covered by either winter weeds or a growing cover crop and is conducive to earlier spring planting.
Effects on Soil Nutrients
Because of their deep root system, rapid root extension, and heavy N feeding, radishes are excellent scavengers of residual N following summer crops. Radishes take up N from both the topsoil and from deeper soil layers, storing the N in their shoot and root biomass. With favorable fall growing conditions, radishes typically take up more than 100 lb/ac of N. Early planting promotes high biomass production and associated nutrient accumulation but research at the University of Maryland has shown that late planted radishes can still take up substantial quantities of N despite low biomass production due to shifts in plant C:N ratio. (Dean and Weil, 2009)

Unlike cereal rye and other small grains whose residues decompose slowly and continue to immobilize N for an extended period, radish residues decompose and release N rapidly. Timely crop establishment following radishes can result in an early boost in growth and N uptake similar to following a legume cover crop or N fertilizer application. In contrast, if planting is delayed (e.g., northern locations) and weather/soil conditions are conducive to leaching or denitrification, the availability of N scavenged by radishes to subsequent crops may be limited.
Radishes are excellent accumulators of P and K (root dry matter commonly contains more than 0.5% P and 4% K), and elevated levels of soil test P have been measured following radish cover cropping, particularly within 1–1.5 inches of radish root holes (White and Weil, 2011). Despite radish being a non-host of mycorrhizal fungi, mycorrhizal colonization of corn following radish does not appear to be suppressed (White and Weil, 2010).
Effects on Soil Erosion, Runoff and Organic Matter
Radishes grow rapidly when planted in late summer or early fall and 10 lb/ac drilled on 7.5-inch rows can provide full canopy closure in about three weeks. This canopy intercepts rain drops minimizing surface impact and detachment of soil particles. Even after radishes are killed by a hard freeze, a layer of decomposing residue remains on the soil surface throughout the winter and into the early spring providing erosion control. In addition, runoff and sediment transport are reduced because of the rapid infiltration facilitated by open root holes. For more complete protection against erosion, radish can be mixed with other cover crops that are winter hardy (e.g., cereal rye) or winter kill but leave more persistent residue cover (e.g., oats).

Total dry matter production by radish cover crops can exceed 3.5 tons/ac (5000 lb/ac aboveground and 2,000 lb/ac below ground) after 2 months with favorable growing conditions (1.1 lb fresh weight per square foot at 90% moisture = 5000 lb/ac dry matter). It is important to keep in mind however that radish biomass is highly decomposable and increases in total soil organic matter (SOM) levels following radish cover crops are unlikely.

Effects on Crop Yields
On-farm comparisons and limited replicated trials in Maryland, Ohio, Pennsylvania, and Illinois have reported significant increases in corn and soybean yields following radishes as compared to fallow or other cover crops. These yield increases are likely the combined result of multiple effects described above.

Cover Crop Radish Seeding
Good stands of radishes can be established by drilling 6–10 lb/ac or broadcasting at 8-12 lb/ac. When using a drill, seed should be placed ½–1 inch deep. When broadcasting, establishment is enhanced by culti-packing or light tillage. Aerial seeding has been successful using 10–16 lb/ac broadcast into standing corn and soybean canopies when soil surface moisture was favorable for germination for several days. It is important that the seedlings quickly have access to light so aerial seeding should not occur until the crop begins to senesce (~50% yellowing of lower leaves), early harvest also improves growth. Mixing radish seed with other cover crop species (e.g., oats, annual ryegrass and/or crimson clover) can improve seed distribution and stand establishment and reduce total seed cost.
There is growing interest in planting radishes on wider row spacings, often in combination with other cover crop species. This can be accomplished by blocking off rows in a drill or using a planter with appropriate plates or another seed metering system appropriate for radish seed.
Radishes germinate rapidly, emerging within 3–4 days when environmental conditions are favorable. Seed broadcast on the surface can establish well if seeding is followed by a timely rain or irrigation. Radishes have a very flexible and aggressive growth habit and will spread out in a rosette to fill available space. Radish plants (roots and shoots) grow much larger at lower plant densities but it is not clear that giant specimens (e.g., greater than 3-inch diameter roots) have any advantage over good stands of radishes with 1-inch diameter roots.
Radishes grow best when planted early enough to allow 6 weeks of growth before regular frosts. Later-planted radishes tend to be more cold-hardy and less likely to winter-kill. When planted in the spring, most radishes bolt quickly producing much less root and shoot biomass than fall plantings.

Winter Hardiness
Radishes are tolerant of light frosts but generally show injury when temperatures drop below the mid-20s. In regions where winter temperatures regularly drop below 20 F, radishes normally winterkill but it should be noted that overwintering was reported at some northern locations in 2010 and 2012, likely due to early and persistent snowcover and unusually mild winter conditions, respectively. Young radishes in the rosette growth stage are more winter hardy than radishes that have developed a sizable storage root.
Crop Rotations
Radishes fit well following small grains, corn silage, and early harvested vegetable crops (e.g., sweet corn) that allow cover crop planting before September 1. Later plantings can scavenge significant amounts of N but may accomplish little biodrilling or weed suppression. Nutrients scavenged by radishes are released rapidly making radishes a good fit ahead of early planted crops with high nutrient requirements. Caution should be taken when adding radishes to rotations that already include brassicas.

Cover Crop Mixtures
Many farmers and researchers are experimenting with cover crop mixtures that combine radish with other cover crops that fix N, provide more persistent residues or simply have cheaper seed. As a general rule, radish rates should be cut by at least 50% when included in cover crop mixtures because of their capacity to out compete other species.

An alternative method of managing radish competition in mixtures is to plant separate rows of radishes and companion species. This can be accomplished by blocking off or compartmentalizing the rows in the seed boxes of a grain drill or by attaching an additional seed metering/distribution system (e.g., Valmar airflo or Gandy Orbit-air). In addition, some farmers are using split-row planters to plant alternating rows of radish and companion species on 15-inch spacing or planting twice on 30” rows with a 15-inch off-set using GPS guidance.
Spring oats and sorghum-Sudangrass (Sudex) compete well with radish and provide longer lasting residues to immobilize some of the N released from radish residues in the spring. These additional residues may also help maintain soil moisture, reduce weed growth, and reduce erosion during the next growing season. When cereal rye is mixed with radish, the rye overwinters and scavenges N released by the decomposing radish. Hairy vetch is a winter-hardy legume that has also performed well interseeded with radish (both mixed and in separate rows).

Potential Problems
Radishes have little tolerance of wet soils, so planting in fields that collect standing water or are prone to prolonged wetness should be avoided. Enhanced growth directly over tile lines is common and plugging of tile lines has been reported but appears to be a rare occurrence.
Radishes are very responsive to N, and N deficiency limits their ability to compete with weeds, grow through compacted soil, and perform other potential functions.
Radishes are only moderately cold hardy and need about 6 weeks of favorable growing conditions to produce sufficient biomass to achieve most potential benefits.
Lastly, be forewarned that rotting radish residues produce a powerful rotten egg-like odor, particularly during winter thaws.
Summary
Radishes have much potential to perform valuable functions within organic cropping systems. Realization of this potential depends upon timely establishment, favorable environmental conditions, and adequate fertility. As described in this article, a solid research foundation supports the value of radishes as a cover crop but farmer innovation is needed to fine-tune strategies for integrating radishes in specific organic cropping systems.

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