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) and 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 (Fig. 10), 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.
Establishing radishes with a planter has particular appeal because many farmers have wider planters than drills, seed spacing is more controlled than with a drill and lower seeding rates can be used. Specific planter plate recommendations are summarized in the following table (Table 1).
Table 1. Recommended planter plates for cover crop radish.
White 60-cell sugar beet
Deere small sugar beet 4/64 inch
Case-IH sugar beet
Kinze 2000 and 3000 series small 60-cell milo
Kinze Edge Vac w/ e-sets 60-cell small sugar beet 1/16 inch
Monosem 6020 plate; vacuum set to 15
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 (Fig. 11).
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.
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.
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 (Fig. 12). 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).
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 (Fig. 13) 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. Nitrogen deficiencies have been observed when planting after silage- or grain corn on sandy soils or on soils that do not have a history of manure application. N deficiencies are also likely when excessively high populations are established.
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.
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.