Testing "Electric Mulch" for Weed Control
“…in this world nothing can be said to be certain, except death and taxes.”
- Benjamin Franklin
Ben forgot about weeds. Weeds are certain.
For conventional and organic farmers, home gardeners, park and school managers, road-managing public works departments and many others, weeds are a constant, concerning and certain challenge.
While there is not – nor will there be – a silver bullet that effectively controls all weeds in all situations, researchers are actively developing “silver birdshot” approaches – weed-control tactics and technologies that work in specific arenas and can be used as one element in a broader, varied, integrated management strategy.
Electric mulch is one of those new technologies.
Under development by Erik Lehnhoff, an associate professor of weed ecology and management at New Mexico State University, electric mulch uses small solar panels to charge stainless steel screens with a low-power electric current to prevent weed growth in vineyards, orchards or other high-value crops like blueberries.
“We’re using two different types of screens and testing them against pre-emergent herbicides and plastic weed barriers for weed control,” Lehnhoff said. “We installed them in April. It’s now August, about a month from harvest, and so far the screens seem to be working great. We’re seeing very good weed control compared to the control plots, and even to the herbicide plots which now have an abundance of weeds in them.”
The experiment is being conducted in a cabernet vineyard in the university’s research farm in Las Cruces, with a second trial in a blueberry field in Oregon. The two types of screen have different-sized mesh – one a tighter weave and one more open. Each type was cut into 4-foot by 5-foot sections then a hole was cut in the center (so it doesn’t touch the grapevine or blueberry bush) and each screen was slit from the hole to the edge to slip it around the plant for installation. They are staked down with plastic stakes driven through PVC spacers to raise them slightly off the ground, then connected to solar panels with thin wire.
“The general concept of how these screen work is that it takes a weed to complete the electrical circuit,” Lehnhoff explained. “The solar panel generates the electricity which flows through a wire to the screens, but it needs a connection to go from the screen to the ground. The weed is that connection. There is no actual electricity flowing until a weed comes up and touches the screen and that completes the circuit.”
In the vineyard, it’s easy to see the concept in action. The narrow-mesh screens are almost entirely weed-free while a few thin, spindly weeds poke through the larger holes in the more open-mesh versions. Those skinny weeds have been lucky enough not to make contact with the screen, but eventually get large enough to touch the metal and die.
“The finer mesh costs a little more because it takes more material to make those screens,” Lehnhoff said. “The larger mesh is cheaper and while there are more weeds in those plots they die before they get too big. They’re doing a job I think would be acceptable to most growers.”
At harvest, the weed growth in each plot will be measured and grapes from each vine in the experiment will be compared to look for any volume or quality differences that could be attributes to the weed control methods used. The screens will also be removed and soil samples taken to make sure the electric mulch system isn’t causing unwanted changes to the soil microbial community.
Limitations and Concerns
One limitation of the system is irrigation. If the ground is too wet through flood irrigation or excessive rainfall, the system essentially shuts off and only starts working again when things dry out. The Oregon trial of the system didn’t appear as effective as the New Mexico trial and excessive moisture may have been the culprit. The New Mexico experiment has also had occasional small animals chew through the wires linking the solar panels and the screens which had to be fixed.
Safety and fire are the two common concerns, but Lehnhoff believes neither should be a barrier to adoption.
“The way this is set up, the system is totally safe,” he said. “It’s running at a low power level that is safe for humans so even if you touch these screens – and I have – you get a little bit of a shock but nothing that would be dangerous.”
As for the fire concern, the electric mulch screens are installed on bare ground and essentially act as a pre-emergent herbicide, killing weeds while they are still small and have very little biomass. It could theoretically be a fire risk if installed over a dense stand of large weeds, but it isn’t designed to be used that way.
Next Steps
Lehnhoff’s next experiments are designed to determine how large a screen can be effectively powered by a single small five-watt solar panel, and he plans an economic analysis of the system compared specifically to plastic weed cloth, which lasts for a few seasons then has to be removed and disposed of.
“Our initial estimates are the systems are similar in terms of purchase and installation costs,” he said. “We’re hoping for longer durability, a longer lifespan and greater overall effectiveness of the electric system.”
One thing that would bring installation costs down would be installing the system during the initial planting of a vineyard or blueberry field. Instead of cutting small sections of screen and holes and slits to slip them around existing plants, a row-long section of screen could be rolled out first, then holes punched through it at the required planting distance. The screen could be narrower than the experimental 4-foot width, perhaps just a foot or two wide just to control weeds within the row.
“It’s going to be most economically feasible in high-value crops, and one of the best applications would be in organic systems where herbicides are not used,” Lehnhoff said. “This could eliminate the need for a lot of hand-weeding within rows, which has a huge labor cost.”