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Fall Weed Management

By Gary Norton, Park Ranger

As the weather begins to turn cooler, the rangers are planning their fall weed management activities. The two most important seasons are in the spring, when plants begin their growth cycles and, in the fall, when they are shutting down and storing reserves for the winter. Weeds, like knapweed and musk thistle, are very susceptible to spring applications due to the uptake of chemical during their early growth stages. Fall weed spraying can be very effective since plants can transport herbicides to their roots along with water. Species targeted in the fall are deeply rooted perennials like Canada thistle, which require several management strategies to achieve control. The rangers have been busy this summer preparing spray areas for fall management.

Mowing is part of an integrated management approach to weed management. Noxious weeds have been known to develop a resistance to herbicides, so using different methods like prescribed burning, mowing and seasonal variations can give us a better chance of controlling noxious weeds. During the summer months, the rangers mow infested areas of noxious weeds to limit seed production and to stimulate the weeds like Canada thistle to “flush out.” This method encourages the plants to use more energy in creating more green foliage and provides a greater surface area for herbicide application. The mowed areas are less likely to survive the winter due to less stored energy and the herbicide applied to the plant surface is quickly transported to the roots where it can do more damage to the plant. Hardy perennial weeds easily survive “top kill” application due to their significant stores of energy below ground.

The ranger staff has been prioritizing fall spraying objectives throughout the summer based on the following: areas that were mowed, resident input and critical habitat areas. A target weed species list is generated by the state, which lists the most noxious weeds and under state mandate and requires property owners to control these weeds. The amount of control is dependent on each species with the goal of a reduction in numbers over time.

The ranger staff encourages residents near open space to monitor and manage weeds on their own properties and in the open space by their homes. Residents are allowed to mow a ten-foot buffer in open space, which can help in weed control. A resident can also use a broadleaf specific herbicide like Weed Be Gone, but residents should never use herbicides like Roundup because it kills all plants in the management area. Before treating weeds in open space, the ranger staff asks that you contact our office and schedule a field visit so that we can evaluate the weed problem and make recommendations.

If you have any questions on fall weed management, please contact the rangers at (303) 979-1876 ext. 170

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