Drone Technology

Invasive plants often establish and flourish on steep, rough terrain that is difficult to access. This makes early detection and management difficult and hazardous work. Nigel Davis, a commercial applicator in Helena, Montana, understands the challenges involved with treating invasive plants in natural areas with ground-based equipment. “Some of our application sites are really remote, and between the snakes and steep terrain it’s labor intensive and dangerous” says Davis. “We needed to find a better, more efficient way to treat these infestations.”

Davis became interested in the use of drones, or unmanned aerial vehicles (UAV), to treat invasive plants about a year ago. He looked at various companies and models available and selected a model produced by a company located in the United States.

“I wanted to work with an American company and needed a drone that could handle at least a four-gallon payload capacity. There are lots of different options for tank capacities, but I opted for a 4 gallon unit for longer battery life that would still let us meet objectives” Davis explains.

The drone is outfitted with a 4-gallon tank, pump, a six-foot boom, and six nozzles. Davis did his own testing to determine which nozzles provided the best coverage for areas he was typically spraying. The nozzles he selected produce a 12-foot pattern with a 24-inch average overlap when spraying. He calibrated the drone sprayer output and checked its application pattern with spray cards, to ensure accuracy and coverage. He also had to make a few adjustments to the boom angle, and is considering taller landing gear to protect the booms.


The drone can treat from one to four acres with the four-gallon tank, depending on site conditions and customer specifications for application volume. “I have complete control of the volume output and speed, so I can manipulate application rate dependent on each unique situation” says Davis.

In a typical application, the ground speed is five meters per second at a height of five meters. It takes about four minutes to spray one acre. Davis explains that the speed and altitude can be adjusted depending on the environment where you are working.

Along with studying for and passing the Federal Aviation Administration drone certification exam, it still takes time and practice to become proficient flying the drone. “There is always a learning curve with any new technology, and flying a drone is no different,” says Davis. “We felt comfortable after about 10 to 12 hours of flying time, but that will vary among different operators.” He currently uses the drone to treat invasive plants on steep, rocky terrain—areas that are unsafe to traverse on foot or with all-terrain vehicles.

“We limit flying time to about five hours a day, from early morning to about noon, depending on wind and temperature, he says. “I still find that my motor skills get tired working the controller, and my brain gets tired operating flight controls”.

About half of the target areas can be treated by uploading the infested area to the drone GPS and using autopilot to fly the established grid. After treating the area, the drone returns to the launch spot and lands. It is completely GPS controlled if that option is selected.

Infestations that can’t be uploaded to the GPS are flown by hand, which requires the operator to see the entire infestation. In these situations, Davis positions himself near the target infestation where he can also see his launch-based ground crew. When the drone needs refilling or a new battery, Davis flies it back to the launch site to be resupplied then returns it to complete the application. Trees are also an issue, so he hand-flies the drone for spot-treating infestations near trees.

“It’s nice not to have to worry about rattle snakes anymore! Well, not as much anyway!” says Davis.

Davis plans to add a camera to the drone after he gains more experience. “Flying a drone via a camera and drone goggles is more difficult, so we wanted to visually fly it for a while” he explains. “A camera will allow us to locate weeds, set a GPS point on the infestation, and then program the drone to go back and spray those locations. The capability is there, we just haven’t done it yet.”


Davis treated a total of 35 acres of invasive plants with the drone this season, primarily targeting spotted knapweed and leafy spurge. The application rate for spotted knapweed is 3.3 ounces of Opensight® specialty herbicide per acre. Leafy spurge on grazed land is treated with 1 quart of Tordon® 22K herbicide per acre. Liberator was added as a surfactant and drift agent at 0.25 percent volume-to-volume. Visual assessments of spotted knapweed and leafy spurge control the season of application indicate that drone-applied herbicide treatments provided good to excellent control of both species.


Davis believes that using a drone for herbicide application may have distinct advantages, especially on rugged terrain. These include easy access to remote infestations, zero foot print (wheel tracks) on the ground, reduced applicator exposure, reduced herbicide drift, and cost-effective control.

“A really important advantage of the drone is that it keeps our employees from attempting hazardous terrain with all-terrain vehicles or backpack sprayers. We can also reduce the amount of herbicide solution we apply because we can put the herbicide exactly where we want it without having to treat the entire hillside or field” he explains.

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