Controlling Weeds with Lasers

Weed growth can now be impaired using an exact carbon dioxide laser beam in the infrared range to destroy the plants’ sensitive growth centers, called meristems.

Increasingly, environmentally safe methods are being used to rid agricultural fields of unwanted plants. Chemical pesticides can be used selectively and are suitable when conventional thermal methods, such as flaming, are either not exact enough, or are too energy-consuming. However, drift and overdosing often result in harmful herbicide residues in topsoil or surface water.

Now, researchers from the Laser Zentrum Hannover eV (LZH) and the Biosystems and Horticultural Engineering faculty of Leibniz University in Hannover are using a selective CO₂ laser beam to zap pesky weeds at their meristems.

Exact positioning of the laser beam (shown in red) on weed model plants in a laboratory setting. (Image: LZH)

Laser radiation has a direct thermal effect on plants. Current lab results indicate that seedlings can be killed using a minimum laser radiation dose of 35 J. This laser energy can be exactly and effectively adapted to the plant species and growth stage.

During experiments, the LZH Department of Materials and Processes, Safety Technology Group used a galvanometer scanner with a flexible mirror system to move laser beams quickly from plant to plant, and to focus with high precision on the near-surface meristems. Under lab conditions, the group achieved an accuracy of <±1 mm; under greenhouse conditions, a laser on a rail carriage achieved accuracies of ±3.4 mm.

The Leibniz researchers employed a stereo camera system to recognize the plants and to optimize the laser beam position. Using a complex processing method, they compared camera images based on threshold-level filtering and edge detection to active shape models of the plants. The meristem’s position is gauged by the location of the leaves. Once the coordinates are verified, a signal sent to the laser triggers a beam to hit the target.

The new method is especially efficient because scientists can determine exactly how much energy is needed to achieve an optimal effect on the target. They are now testing various irradiation times in different weed concentrations for economic viability. Current knowledge indicates that the best results for large areas can be provided using autonomous field robots working in a stop-and-go mode, they said.

The project, “Investigations on the Effect of the Laser Beam on Young Plants for Weed Control Using Image Processing,” was subsidized by the German Research Foundation.

For more information, visit: www.lzh.de