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Mustard: A weed and disease killer?

Charlie Gray, Newbury, Vt, flail-mows mustard as a soil bio-fumigant in his strawberry and vegetable beds
BIO-FUMIGATING: Charlie Gray knows it’s crucial to follow up flail-mowing mustard with immediate incorporation for his strawberry and vegetable beds.
Timing is everything with weed- and disease-killing mustard.

By Susan J. Harlow

Charlie Gray is a fan of growing mustard — the plant, not the hotdog spread — as a soil bio-fumigant. This Newbury, Vt., farmer knows that to make it work, you need to persevere.

It takes time to learn how to use it effectively, he says. "Some people, in the first year, don't do some of the things they should, then don't see the benefit, so they don't do it anymore."

Gray is not one of them. He farms with his parents, Bob and Kim, his brother Peter and his girlfriend, Marie van der Kar. Their 230-acre 4 Corners Farm grows a wide variety of fruits and vegetables on 50 tillable acres. Most of it is sold at their farm stand. They're especially well-known for their strawberries.

Mustard lessons learned
Gray, a director of the North American Strawberry Growers Association, plants a variety of mustard called Caliente 199 after finishing strawberry harvest, then flail-mows it just before full bloom. The crop is immediately tilled into the soil and rolled for full incorporation. The soil should then be "sealed” with rain or irrigation, and then left undisturbed for several weeks.

When the plant tissue is damaged by mowing, organic compounds called glucosinolates release a gas, isothiocyanate. In small doses, that's the pungent bite in horseradish or hot mustard. In the field, it fumigates the soil, killing weed seeds and diseases.

When Gray first grew mustard six years ago, he followed all the requirements — or so he thought. He fumigated in September and left the field fallow until the next spring. Then he planted strawberries. Right away, Gray saw that weeds took a month to six weeks to get going.

"But I didn't know when to flail mow it," he recalls. "I left it too long, and the stalks dried out."

The second year, Gray irrigated the mustard at planting to improve germination. He was then careful to mow before full bloom. Now, he says, he mows as soon as he sees the first blossom on the bottom of the stalk. "Most people don't have a flail mower, and a bush hog just doesn't work — you need to chop and crush the mustard plant to release the glucosinolates," he says.

He also learned that mustard must be incorporated within 20 minutes of mowing, or 80% of the gas is lost. "We had a lot of bees, so we'd chop it early in the morning before bees got active. Then we’d wait a few hours to till it in. We were waiting too long."

Now, he sets his phone timer for 15 minutes, mows mustard until the timer beeps, jumps onto the tiller for 15 minutes, then hops back on the flail mower. "Then I come back and roll it a week later. Flail mowing and tilling is most important. If you get a hard rain, it's the same as rolling it."

By the third year, the soil was clean come spring. "Now we have it dialed in. We really see the benefits of mustard," he says. "We spend almost no money on herbicides, hand weeding or cultivation. Our plants are amazing, so they're getting some benefit from the biomass."

Cover-cropping the gas
After the first few years, Gray also began drilling a winter rye cover crop two weeks after he "fumigates." He believes the root base of the fast-growing rye locks in the gas below it.

"Now, we realize that what we gained in weed control we lost in nutrients and erosion from being fallowed. So now we plant a cover crop, whether winter rye or oats and peas. The key thing is we don’t rework the soil after fumigation is completed. We grain-drill right through the rolled soil.

"Waiting two or three weeks after bio-fumigating lets the gas dissipate enough so you don’t have to worry about crop damage. Also, it gives you an idea whether fumigation worked. If we see weeds growing in that time, we think back about what we did and if we had missed a step."

Last year, Gray started using mustard before seeding beets and carrots, with great results. Next, he wants to research mustard's effect on disease.

"I’d love to learn more about what’s going on in the soil. We've been planting strawberries on this farm for 35 years. We try to rotate. But when you have five acres of beautiful sandy, south-facing land, you don't want to seed it down for 10 years.

Not pesticide-free
"People think [mustard] is a cure-all — as good as chemicals. But it's a unique method," he says. While chemical fumigation kills 100% of everything, biofumigation sterilizes 70% to 80% of weed seeds, then delays emergence of 20% to 30% by several weeks.

Gray uses no fumigation herbicides, but does use pre- and post-emergence herbicides to help fill in the gaps where they don’t use mustard. "We've noticed that we don’t have to use as strong pre-emergence herbicides after mustard fumigation," he says.

For 4 Corners Farm, mustard is a winner. "We’re looking to incorporate the most natural processes that are all around cost-effective, more sustainable, and better for the environment."

Harlow writes from Westminster Station, Vt.

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