Brassica Cover Crops - Key Growing Information
Uses: Cover-crop brassicas grow fast in cool weather, make lots of biomass, protect against erosion, break up compaction and smother weeds. They also contain compounds that can help suppress some diseases and soil pests. They are excellent at scavenging nutrients, though nitrogen can be lost before the next crop if they winterkill; including them in an overwintering mix holds nitrogen longer. Some species double as forage for livestock and wildlife.
Culture: Avoid planting them just ahead of other brassica crops, since they share many of the same pests and diseases. Sow ¼–1" deep depending on seed size and variety, either with a grain drill or broadcast and lightly covered. Because they like cool weather, spring and late-summer sowings perform best. Plant radishes and turnips in fall so they don't bolt early and have time to size up their roots.
Seeding Rate: Use the higher end of each range when broadcasting. Mustard: ¼ lb. per 1,000 sq. ft.; 5–15 lb. per acre. Oilseed radish: ½ lb. per 1,000 sq. ft.; 10–20 lb. per acre. Turnips: ¼ lb. per 1,000 sq. ft.; 5–8 lb. per acre.
Soil Requirements: Most brassicas do well across a pH of 5.5–8.5 in fertile, well-drained soil. Seed germinates well with soil temperatures between 45 and 85°F (7–29°C).
Grazing (Forage Turnips and Oilseed Radish): Start grazing once roots are 2–4" across and continue through winter.
Harvest (Oilseed Radish): For oil, cut the tops once the seed has ripened and work the roots into the soil.
Termination: Mowing or tilling at flowering kills brassicas effectively. For the biofumigation effect, mow at flowering and till or tarp right away. Radish and turnip roots are easiest to deal with if left to rot over winter. Brassicas reliably winterkill in Zone 6 and colder, though turnip roots can survive down to about 10°F (-12°C).
