Radishes Cover Crop

The daikon or Japanese radish known as oilseed radish provides valuable services as a cover crop in farming operations.
By: Nan Chase

Botanical Name: Raphanus sativus longipinnatus 

Scientists now confirm what some ancient farming communities have long known: the oilseed radish can play a valuable role as a cover crop to protect fields of cash crops between growing seasons.

Cover crops can do several things at once during the cooler off-season:

  • Keep fields from being invaded by weeds after a main crop is harvested
  • Capture or “scavenge” available nitrogen in the soil so it isn’t lost to groundwater
  • Prevent erosion of bare soil
  • Control some pests using natural means
  • Break apart heavy or compacted soil
It turns out that an oilseed radish cover crop is especially good at all those jobs, plus several more: daikon grown as a field crop can provide nutritious and easily digestible forage for livestock, and if permitted to flower it can eventually produce seeds that are marketable as a source of biodiesel oil.

A radish cover crop is sown late in the growing season; the seed needs 60 days to grow to maturity, so mustn’t be planted too late in the fall, ideally in late August.

In some cases farmers will plant oilseed radish seed into the bare soil where a cash crop (say, beans or cucumbers) has already been harvested and the stems plowed under. In other cases farmers may sow radish seed right over ripening “standing” crops – such as cotton or soybeans, even corn – and let the seeds fall into the soil as the main crop is harvested and disked.

Daikon, or Japanese, radish has a root structure that is perfectly designed to work its way deep into any soil. The longest part of the taproot may extend as far as six feet beneath the surface, while the thick upper root grows a foot or more long and several inches in diameter. Those roots can break apart densely packed soil, letting in air and moisture.

As the oilseed radishes eventually disintegrate over the winter they leave holes in the soil where the roots had been. Those holes will aerate soil in spring when fields are plowed. Meanwhile, compounds in the radish can repel some nematodes with biochemical means.

When planting oilseed radish as a cover crop be sure to “terminate” the plants by mechanical means at least three weeks before the main crop is to be planted in spring. Otherwise, if new seeds develop they can germinate and produce uncontrollable growth where a cash crop is intended. In warm climates where sub-freezing temperatures won’t be present to destroy the daikon, the remaining roots can re-sprout and cause trouble.

Also, don’t use oilseed radish in fields where the main cash crops are broccoli, cabbage, or related plants, as some pathogens can linger and damage the crops.

Look for seed suppliers who sell oilseed radish seed in bulk.

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