Innovative Air-Propelled Abrasive Grit Application to Manage Weeds in Perennial Horticulture Crops

Frank Forcella1, Steve Poppe2, Emily Tepe3, and Emily Hoover3

1Department of Agronomy and Plant Genetics, 2West Central Research and Outreach Center, and 3Department of Horticulture, University of Minnesota, St Paul, MN, 55108

May 2020

Weed management, particularly within rows of perennial fruit plants, is challenging. There is a need to develop inexpensive but effective weed control strategies. A new technique and alternative weed strategy for controlling weed seedlings postemergence is known as grit-weeding. Unlike herbicidal management, grit weeding may have the potential to be used on new and established perennial fruit plants grown organically. With this technique, abrasive grit derived from soil (e.g., quartz sand) or agricultural residues (e.g., grape pomace, olive pits, nut shells) is discharged under high air pressure to abrade and kill weed seedlings. With proper aiming, timing, and application rates, abrasive grits can suppress weeds and leave crops unscathed. To date, abrasive grits have been tested successfully for selective control only in annual crops. The goal of the current research was to examine the efficacy of grit-weeding in an established apple orchard and first-year planting of raspberries. 

Several non-chemical tactics are available for growers, such as flaming, mowing, mulching, and tilling; and each can achieve a high level of weed control in certain situations. Soil cultivation is incompatible with mulches of all types, and both polyethylene and biodegradable mulches may increase levels of harmful nematodes in raspberry plantings. Polyethylene mulches also present an eventual disposal problem for growers. Unfortunately, each tactic also has limitations in terms of weed control and/or interference with other management issues. For instance, flaming can damage trees and requires considerable use of propane, mowing results in only short-term weed suppression and usually needs to be repeated, mulching creates habitat for rodents and tillage can damage root systems on apples.

Methods and Materials

Grit weeding applicatorTo sustainably control weeds in perennial fruit plantings, the University of Minnesota tested abrasive corn cob grit applied under high air pressure for the control of weeds in established apple trees over two years at one site and raspberries over three years at three locations. Grit-weeding consisted of applying commercially available corn-cob grit using a portable grit applicator (Figure 1). The applicator was mounted on a small, hand-pulled, garden wagon. It consisted of a gasoline-powered air compressor, a wand tipped with a single nozzle, and a grit tank with a battery-powered grit metering system. Flexible nylon tubes connected the air compressor to the nozzle and the grit tank/meter to the nozzle. A vacuum created by air flowing through the nozzle drew grit from the grit tubes into the nozzle. Grit particles were about 0.5 mm diameter and the nozzle of the grit applicator typically was positioned 30 cm from the target weeds. Since the practice utilizes decomposable agricultural waste it may be cost-effective for these crops. Grit weeding previously demonstrated effective and selective post-emergence weed control in corn, soybean, tomato and pepper without reducing yields.

Grit weeding plots in raspberries were established in the spring of 2017 at the West Central Research and Outreach Center (WCROC), Morris, MN.  Apple and raspberry plots were established in the spring of 2018 and continued through 2019 at three locations: 1.) Fairhaven Farm (South Haven, MN): apples and raspberry; 2.) Forbord Farm (Starbuck, MN): organic raspberry; and 3.) WCROC: raspberry. Corn cob grit was applied with a single-nozzle applicator appropriate for small research plots, as was done in larger research plots of organic field corn with an eight-nozzle applicator. This single-nozzle unit recently was revamped by agricultural engineers at South Dakota State University and is somewhat comparable to easily portable high-pressure washers sold in hardware stores. 

In both apple and raspberry plots three treatments were implemented: weedy check, hand-weeded, and grit-weeded. Plots were arranged in a randomized block design with four replications. In the weedy check treatment, weeds were allowed to grow from early spring until late July. Weeds in the hand-weeded treatment were hoed manually or pulled by hand semi-weekly to biweekly beginning in May until July.

Results

In the raspberry plots hand-weeding and grit-weeding occurred six to ten times during spring to mid-summer depending on duration of weed emergence and growth.  As expected, by late summer hand-weeding nearly eliminated all weeds in the raspberry hand-weeded treatment. Grit-weeding also significantly reduced weed biomass, but some species clearly tolerated abrasion more than others. Growth of all broadleaf weed species, which were the predominate weeds in the weedy check treatments, was reduced to negligible levels. These weed species included primarily annual plants such as common lambsquarters, redroot pigweed, hairy galinsoga, common mallow, wild buckwheat, and Venice mallow, as well as the perennial, Canada thistle.

Surviving weeds in the grit-weeding raspberry treatments were grasses, especially perennial grasses such as bluegrass, brome, and quackgrass. Annual grasses, such as foxtails, crabgrass, and barnyardgrass also survived abrasion, but typically were stunted compared to those in the weedy check treatment. However, in some cases (e.g., WCROC and Prairie Horizon Farm in 2019), control of broadleaf weeds by grit-weeding released grasses from competition so that grass biomass in the grit-weeded treatments was greater than in the weedy check treatments.

Growth of raspberry canes was affected by weeding treatments. Total cane growth (original transplants plus newly emerged canes) was significantly lower in the weedy check treatments than in grit-weeded plots in four of six site-years (2018 and 2019). In 2017 at WCROC, biomass of grit-weeded raspberry was significantly greater than that of the weedy check. Thus, grit-weeding usually prevented weeds from reducing raspberry growth. Interestingly, in two site-years (Fairhaven and Prairie Horizon in 2019), raspberry cane growth was greater in grit-weeded plots than in hand-hoed plots, which may have been due to decreased root and rhizome growth from soil disruption caused by the hoeing operations. Grit-weeding did not disrupt soil, roots, or rhizomes.

In the apple plots broadleaf weeds were most common, representing about 80% of total weed biomass. Hand-hoeing and hand-pulling nearly eliminated weeds, as expected. Application of grit decreased weed biomass significantly, by 70 to 80% between years. Broadleaf weeds were controlled with grit much better than grass weeds (e.g., about 90% vs. 15%), as also was observed in red raspberry. The most common broadleaf weeds in these experiments were annuals, such as common lambsquarters, mallow, fleabane, redroot pigweed, Pennsylvania smartweed, wild buckwheat, but perennials also were common, including dandelion, field bindweed, Siberian elm. Weedy grasses comprised annuals, for example barnyardgrass, crabgrass, foxtail and the perennials bluegrass, smooth brome, and quackgrass.

Apple leader shoot growth was not influenced significantly by weed management in 2018, but it was affected in 2019. In the latter year, apple shoot growth was significantly greater in the grit-weeded plots than in either of the other treatments. Shoot growth in hand-hoed and weedy check treatments did not differ. Presumably, weeds suppressed apple shoot growth in the weedy control plots, and disruption of apple roots in hoed surficial soil inhibited shoot growth in the hand-hoed plots. Grit applications suppressed most weeds, did not disrupt the soil and, therefore, possibly allowed better growth of apple leader shoots. Similarly, tree roots are known to be damaged by mechanical fallow (comparable to hand-hoeing) and when under cover crops (similar to weeds), but not by herbicide fallow.

Summary and Conclusions

As expected, hand-weeding nearly eliminated all weeds. In contrast, grit-weeding achieved about 90% control of broadleaf weeds in apples and about 94% in raspberries, but only 15% control of grass weeds in apples and less than 10% in raspberries were achieved. Much of the time and amount of grit used to affect control was devoted to suppressing grass weeds. Harder and more angular grit materials than corn cob grit may be more efficient for control of annual and perennial grasses.

For weed control, grit-weeding clearly has potential to control broadleaf weeds in horticultural crops such as apples and raspberries. It represents a new tactic for weed control in perennial cropping systems that can be added to the arsenal of tactics employed by growers.  However, it was not effective for controlling grass weeds. Lack of effectiveness for grass control possibly reflected the use of low-density corn cob grit in these experiments. Corncob grit is a “soft grit” typically employed for abrading grime from machinery, etc., where hard grits cannot be used safely. Corncob grit was used in these experiments because of its availability in the Upper Midwest and because it represents an under-used agricultural residue. Denser and more sharply angled grit, such as walnut shell grit or quartz sand likely will affect better control of grass weeds. Corn cob grit was used, but in high-value horticultural crops purchase of walnut shell grit may be justified. Lastly, grit-weeding leaves soil undisturbed, which may have beneficial effects on growth of crops coming from rhizomes or root buds.