Noxious Weed Management in the Open Space

By Liam Cullinane, Park Ranger, Natural Resource Specialist

Springtime has arrived in the Valley bringing longer days and warmer temperatures at last! But with this change in temperature, noxious weeds are starting to pop up in the open spaces, lawns and parks. Noxious weeds are non-native, invasive plants that displace native vegetation and can have impacts on economies and the environment. In Colorado and in Jefferson County, property owners are either required or encouraged, depending on the plant species, by law to manage their noxious weeds. The level of management for specific species depends on how common the plant is within the county. Noxious weed species that are not very common normally require eradication (total elimination) from the property. Noxious weed species that are very common generally are just encouraged to be controlled (e.g., attempt to stop their spread, if possible).

The natural resource rangers have begun their noxious weed management – including herbicide application, hand removal and mowing – in the open space. We take an integrative approach, meaning we use a variety of methods to combat the spread of these plants in the open space. Herbicide applications normally begin in April (weather dependent) and continue into July, with some follow-up spraying in the fall. Mowing operations are scattered throughout the growing season to prevent certain plant species from flowering and producing seed. Hand removal is used on days when herbicide application is not possible (rain, snow, etc.). In addition to the natural resource rangers, a contract herbicide applicator began work in the open space in early May, which will continue into the summer. Please see the accompanying management area map for more information about where herbicide applications will be taking place in the open space. The contractor has already started spraying bare-ground areas like picnic pads, crusher-fine trails and kiosk pads. These areas are treated with a pre-emergent herbicide to prevent weeds from coming up throughout the growing season.

From April to June, the plants that are heavily targeted are mostly biennials. Biennials are plants that complete their life cycle in two years, with the first year being spent in a non-flowering, vegetative stage called a rosette. At this stage (rosette) these plants are more vulnerable to herbicide application due to the increased uptake in nutrients and water. In the second year, the plants shoot up a stem and begin flowering and seeding in June and July. Target biennial species on the open space include scotch thistle, musk thistle and common mullein to name a few.

As spring turns to summer, the target plants shift from biennials to perennials. Perennials are plants that live for three or more years. Each year, the plant flowers, seeds and then goes dormant for the winter. Perennials are usually very deeply rooted plants, so they are difficult to control. One of the most effective strategies is to mow the plants about once a month so they do not flower and go to seed. This will force them to regrow constantly throughout the growing season, so they will not be able to put resources into flowering and seeding. Then in the fall, when they are putting a lot of their nutrients into their roots to prepare for winter, you can spray them with herbicide. This is the most effective treatment for many perennial noxious weeds found in the open space. Target perennial species are Canada thistle and leafy spurge.

The weeds that are managed on the open space are very aggressive invaders that can take over entire plant communities. Priority was given to areas where infestations pose the worst threat to native plant communities and where they are bordering home lots. If you have questions about noxious weed management or the management areas, please contact KCRMA Natural Resource Specialist Liam Cullinane at 720-599-3247 extension 468 or at liamc@kcranch.org.


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