The following is a good article on using forage radish as a cover crop in nurseries from the University of Maryland.
Forage Radish: For improving growth of trees in nurseries
Forage radish, also called tillage radishes, is from Daikon radish, which is a traditional Japanese radish. Daikon radishes have been grown for human consumption for a very long time but are being used more and more for natural weed control. The tillage radish is extremely competitive and will outgrow most other weeds. The seed rate is 1 lb per 4000 ft2 and is seeded in September or October. The seed germinates very quickly and light green foliage will expand to cover about 12 – 18” of soil surface. The radish that is produced is huge. It is several inches in diameter and will grow several feet into the soil profile. I (Stanton) have been playing with forage radish at my farm for the last two years. I plant it at the base of fruit trees and blueberry plants where I usually have problems controlling weeds. So far it is working fairly well. It competes with most weeds except Canadian thistle which still manages to thrive even in thick plant of forage radish. In December the plants die down and the root will die over the winter. As the large root dies it leaves an excellent column of organic material in the soil and adds nitrogen back to the soil as it decomposes. If you attend the October 2 Sustainable Nursery Field Day at D.R. Snell Nursery, Inc. you can learn more about the trials the University of Maryland is conducting on the tillage radish in nursery plantings. Do a web search for Daikon radish or tillage radish and you will find seed sources. I bought my seed from Groff seed in Holtwood, PA
Information from Stanton Gill, Extension Specialist, IPM for Nursery, Greenhouse and Managed Landscapes, University of Maryland