Monday, August 5, 2013

Weeds: Noxious or Obnoxious? by Rebecca Anderson

Musk thistle (Carduus nutans), photo from CSU Extension

Weeds were a hot topic at the Evergreen Farmer’s Market last Tuesday.  The extra rain this summer has them popping up in everyone’s yards and gardens.  Questions ranged from identifying and controlling specific weeds to triaging which weeds were the most important to remove.  Of course, the noxious weeds need to be a priority for removal.


Noxious weeds are not native to Colorado.  They have no natural controls such as grazers or parasites.  They out-compete native plants because noxious weeds are able to adapt quickly to our environment.  The Colorado Noxious Weed Act has divided noxious weeds into 3 categories:  List A plants need to be eliminated everywhere, List B plants need to be managed to stop their spread, and List C plants are recommended for control.

Yellow toadflax (Linaria vulgaris), photo from CSU Extension
 The first step to managing weeds, especially noxious ones, is prevention.  This includes maintaining healthy lawns and gardens, purchasing weed-free grass and flower seed, and using weed-free manure and mulches.  Cultural techniques such as establishing a population of desirable vegetation to out-compete the weeds and adequate mulching also contribute to managing weeds.  Mechanical control methods (hand pulling,  hoeing, mowing and tilling) are the next line of defense.  Biological weed control is an area that is being developed.  It utilizes the natural enemies of specific weed species.  Biological controls are slower acting, often requiring 5 to 10 years for success, and are often most effective when combined with other control methods.

Chemicals can be great assets in the battle against weeds.  If choosing to use an herbicide always read the label first!  There are many herbicides on the market, so before selecting a product, make sure the target weed has been properly identified.  Your county extension office and weed department are great resources for assistance with identification.  Once identified, the life-cycle of the weed can then be considered.  Is it an annual, perennial, or biennial?  Timing of herbicide application, frequency of applications, and rate of application should be correlated with this information.

The Colorado State University Extension Noxious Weed Management Pocket Guide has great pictures for several common noxious weeds and tips for their control.  It can be found at the following website: 

The Colorado Weed Management Association has a complete list of Colorado noxious weeds at their website with photos and descriptions of most of the List A and B species.  This can be found at:

Don’t forget to visit our booth at the Evergreen Farmer’s Market.  We have a noxious weed of the week and a noxious weed guide book to get you started on planning a management program for your own yard and garden.

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