Lasers used to zap weeds into submission

Weeds are pesky things. They grow everywhere and, by definition, where they’re not wanted. Whether a large-scale farmer or a weekend gardener, everyone who has tried to raise crops has wished that there was a ray gun that could just blast the wretched things out of existence. Now, thanks in part to researchers from the Laser Zentrum Hannover (LZH) at the Leibniz University of Hannover, Germany, that frustrated daydream is closer to reality. Through the use of low-powered infrared lasers, the team has found a way to inhibit weed growth without harming neighboring plants, providing an alternative to expensive, hazardous and environmentally-damaging chemicals.
laser weed killer
Where there are crops, there are weeds. They are a problem for farmers around the world and billions of dollars worth of herbicide are used every day to keep unwanted plants from competing with crops. Properly used, these chemicals are effective and safe, but they are expensive and since they are usually petroleum based, rising oil prices make them, and the crops, more expensive. It also requires great care to make sure the herbicides aren’t dangerous for the farmers using them and that they don't cause damage to the local environment. The alternative is to pull out the weeds by hand, plow them under or bury them with dirt. Unfortunately, this is labor intensive, time consuming and often counterproductive because these methods involve disturbing the soil, which tends to promote new weeds as fast as the old ones are removed. Also, it’s only effective for removing weeds between crop rows, leaving those around the crops free to compete.
The LZH method is to stunt or kill the weeds in place using a laser. This isn’t a completely new approach. Scientists have been experimenting with weed-killing lasers for years, but early attempts revolved around using lasers to cut weed stems or to boil the weeds in their own juices. This wasn’t always effective and the laser needed a lot of power to get the job done. There was also the constant problem of how to tell the weeds from the crops so the right ones were zapped.
LZH took a different approach. The team, headed by Thomas Rath of the Institute of Biological Production Systems, used a low-powered CO2 laser to strategically heat the water in the weeds’ cells. Instead of slicing through the weeds or burning them, the LZH laser would only heat the weed cells enough to damage them and thus inhibit their growth. This is trickier than it sounds, because if too little power is used, it can turn the laser into a high-tech sunlamp that actually promotes weed growth. As Christian Marx, Research Fellow in the Department of Biosystems and Horticultural Engineering explains, "it has been shown that lasers operating with too little energy are more favorable to weed growth, causing the exact opposite of what we want."
According to LZH, the team succeeded in locating the weeds’ growth centers and inhibiting them as well as adapting the method to different plants and plant heights. But the real hurdle was in finding a way to make the weed-killing laser practical by making sure it killed only the weeds and not the crops.
Making it practical
They achieved this by using new sensors operating on an innovative computer algorithm that determines the plant species by measuring its contours as well as accurately aiming the laser at the weeds’ vulnerable growth centers. "The recognition is the be all, so we just kill the weeds," says Marx . The next step is to develop the system into something that works outside the laboratory. Currently, the weed-killing laser is a “top down” system where the laser and sensors are mounted on a moving track over the plants. While it is possible to install the system on overhead tracks, booms or robot arms, this is only suitable for greenhouses and affords only limited protection. Out in the fields, this isn’t at all feasible and in water protection areas where weed control is also needed, it’s nearly impossible.
Putting the laser on a tractor isn’t practical either because the vibrations interfere too much with the equipment. Instead, Dr. Rath says, "our research is currently on drones - small robots, flying in swarms over the field." He envisions small robots carrying solid-state lasers hunting down and destroying weeds before they get a chance to establish themselves.
If the weed-killing laser ever comes to market, future farms may be a very different sight as swarms of crows swooping over corn fields are joined by robotic counterparts industriously exterminating chickweeds and rape seeds.

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