In 2001, University of Maryland’s Steve Groff and Dr. Ray Weil started developing daikon radish (Raphanus sativus L. var. niger J. Kern.) as a cover crop to improve soil health, break up soil hardpans and control weeds. Although daikon radish — also known as Tillage Radish® (trademarked by Cover Crop Solutions), forage radish or Japanese radish — has many benefits, it’s their tilling and breaking soil hardpans ability that sparked interest on a recent #groundchat.
The daikon radish’s “super carrot” taproot drills down two to four feet into the soil with a pressure of 290 psi, forming channels in the soil after they desiccate and decay over winter. It’s these channels that reduce compaction and improve soil tilth, which improves water infiltration and surface drainage. The channels also allow the soil to warm up quicker in the spring!
“It is the fine secondary roots that do the most good,” [in breaking soil hardpans], says Kevin Elmy of Elmy’s Friendly Acres Seed Farm. When the taproots hit the hardpan, fine roots are sent out to find a crack in the hardpan. Eventually the roots crack open the hardpan.
But there are some growers/farmers that haven’t had success with tilling radish.
Dr. Matt Ruark, soil scientist at University Wisconsin-Extension, agrees with the tilling aspect of daikon radish, but in his research trials, the daikon radish roots didn’t penetrate and break through the hardpan. It must be mighty hard hardpan in Wisconsin because some farmers have had success breaking through their hardpan.
I emailed Kevin Elmy about rumblings and grumblings on the lack of hardpan breakage with daikon radish. He quickly emailed me back:
“In most cases, they have seeded into dry soil, poor seedbed preparation, lack of nutrients, all resulting in nothing growing. From our research, we have shown the roots will grow from 2-3 cm down per day of growth.”
“I have had an Ontario producer saying they did not work on corn silage ground. After asking questions and looking at pictures it did not make sense until I asked about manure. He forgot to tell me that 25,000 gallons of hog manure was added last fall, plus a “normal” amount of fertilizer. The tubers were growing so fast, they could not grow into the ground faster than their growth rate, resulting in a tuber sticking out of the ground about 18” with poor root growth. Plus the seeding rate was too low, resulting in larger tubers.”
For tillage purposes daikon radishes are best planted in late summer. Timing is important. Grow daikon radishes 40 to 60 days before a killing frost, which is three consecutive nights at -8 C. This enough time for roots to grow which allows enough time for the radishes to grow and do their thing. For Northern gardeners, this usually means a late August seeding!
“If the goal is to break up hardpan with a pure stand of tillage radish, researchers recommend using seven to 10 pounds of seed per acre,” says Elmy in his 2012 Grain News article, Three uses for forage radish.
Territorial Seeds translates this into gardener’s terms. Their recommended seeding rate is 3/4 pound per 1000 square feet planted ½ inch into the soil.
Planting daikon radishes in the spring poses problems. It seems that spring planted daikon radishes bolt quickly and grow much less root and shoot than fall plantings. Defeats the purpose of growing long roots to break up the hardpan.
Daikon radishes don’t grow well in low nitrogen soil. There are heavy nitrogen feeders. Daikon radishes grown in nitrogen-deficient soil won’t grow through compacted soil or compete very well with weeds.
They also won’t grow very well in wet soil. Avoid planting on spots that collect standing water or tend to stay wet for a long time.
Of course, gardeners shouldn’t plant daikon radishes in the same spot every year. It will lead to a build up of pests and diseases that feed on Brassicaceae family. And definitely don’t plant any vegetables from Brassicaceae family the next year.
It appears that daikon radishes do work, but you need to sow the seeds at the right time and give them the proper growing conditions. But if you have a super hardpan, well you could be between a rock and a hard place.
More daikon radish information
Great extension article that summarizes of all the tilling radish research
Short Cornell University fact sheet on forage radish
Easy to read Cover Crops Solutions brochure on Tillage Radish®. Outlines all the benefits of Tillage Radish®.