Researchers killed weeds by blasting them with pressurized grit on a test plot of red peppers in Illinois. The goal is to kill weeds with minimum crop impact. (The Minnesota Star Tribune)

University of Minnesota researchers are confronting the age-old problem of controlling weeds with a new approach: blasting them at high pressure with abrasive corncob grit.

Scientists at the U's West Central Research and Outreach Center near Morris will begin a two-year research project next spring to test the technique on raspberry crops at the center and at two commercial raspberry farms.

"When your plants are young and you've just worked up the soil, you get a flush of weeds from pigweed to lambsquarters to foxtail, and whatever else," said Steve Poppe, a senior horticulture scientist at the center. "For good vigor in any type of perennial fruit crop, you want a clean row so those weeds are not sucking all the nutrients and moisture out of the soil."

Those most likely to use the technique are organic farmers, he said, who have few alternatives to controlling weeds other than hand-pulling them or trying to mow, flame or weed-whack them.

"In fruit crops, particularly in the perennial systems, if you want to grow in organic systems, or even in low input systems, weed control is a huge problem," said Emily Hoover, U professor of horticultural science who is involved in the research. Even for conventional berry growers, she said, only a handful of synthetic herbicides are labeled for use with raspberries, and they can be difficult to use without harming the fruit.

"What we're hoping with grit weeding is that we can get rid of the weeds and have minimal impact on the crop, minimal impact on soils because we're not tilling them up, and use an agricultural byproduct in a positive way as grit," Hoover said.

The way the system works is for growers to move slowly down a row of raspberries, spraying weeds that have sprouted between the plants, or canes. Similar work with other crops is underway at the University of Nebraska-Lincoln and at South Dakota State University.

"Instead of having a tank of liquid herbicide, we have a tank of grit," said Frank Forcella, a research agronomist at the Agricultural Research Service of USDA, who is also part of the Minnesota study.

A metering system feeds the grit through one hose that is connected to another hose attached to an air compressor.

"Grit is fed into the high-pressure air stream right at the nozzle," he said. The nozzles can be pre-positioned and aimed, or activated with a hand-held trigger mechanism.

The technique works best if the first treatment happens when weeds are less than 4 inches tall, Forcella said. The blasting typically needs to be repeated at least once more a few weeks later when more weeds sprout.

"As long the nozzle is about 18 inches from the weed, we get very good shredding of those weeds," he said. "We can just obliterate them."

The technique has also been used experimentally to decimate weeds around young tomato, pepper, corn and soybean plants, he said.

In April, Forcella blasted around raspberry canes for the first time in a trial demonstration, and said the initial results were promising: Spraying grit from corn cobs destroyed weeds just as effectively as the more labor-intensive method of hand weeding.

The research beginning next year will measure whether similar results can be achieved on a wider scale on several larger plots.

The project will collect data both on areas that use grit weeding and those that use traditional weeding practices. It will then compare the effectiveness of the weed control, the labor expended for each treatment, and the relative costs of each. It also will investigate whether using grit damages the raspberry canes, and has any effect on plant growth, development and fruit quality or yield.

If the grit were to nick or injure the stem of a crop, Forcella said there might be risk that a disease could enter the plant, but so far no diseases or reduced yields have been observed in initial trials. He said other types of grit are also effective, including crushed walnut shells and even sand. In fact, sandblasting devices can be modified inexpensively to deliver the grit, he said, pulled by a small tractor or even by hand alongside the crops.

The weeds that need suppression are those between the raspberry canes, which are typically planted 1 to 2 feet apart in rows. Weeds that grow in larger spaces between the rows can be controlled by mowing or tilling or other means.

Forcella said that the research will also test whether the grit will destroy not just annual weeds that pop up right away when new canes are planted, but also perennial weeds such as quack grass and thistle that move in after a couple of years and need to be controlled in more mature raspberry plantings.

The goal of the research, he said, is to determine whether grit weeding can be another tool in the toolbox for farmers.

"It's not a panacea for weed control by any stretch of the imagination," he said. "Every farmer whether conventional or organic needs several tools to use."

Hoover hopes the research with raspberries, if successful, will lead to use with apples, blueberries, grapes and other perennial fruits. "We're hoping this has wide applicability to a number of horticultural crops," she said.

Tom Meersman • 612-673-7388