Wednesday, 01 June 2011 06:47

Knapweeds belong to the sunflower family. They can be distinguished from one another by the patterns on their bracts (the cone-like structure located below the flower head), by their growth, and to some degree by their flowers.  

There are three main knapweeds in Pitkin County:

Spotted Knapweed (Centaurea maculosa) has spotted bracts and lavender/purplish flowers.  It is a simple perennial that reproduces by seed and forms a new shoot each year from a taproot.  Spotted Knapweed is one of the most invasive, aggressive weeds to plague the Western United States.  Very rare in Pitkin County, the largest infestation is found at the base of Independence Pass along State Highway 82 above Tagert Lakes.  This infestation has been monitored and treated yearly.  It is imperative that it not be allowed to spread into the fragile sub-alpine ecosystem.  

Russian Knapweed has papery bracts, stiff, ridged stems, and thistle-like flowers that are lavender to white.  The plant flowers June to August and seed is produced in later summer to early fall.  It is a perennial that reproduces by seed and creeping, horizontal roots, making Russian Knapweed very difficult to control once established.  They key is to stress the weed and cause it to expend nutrient stores in the root system.  In integrated management plan should be developed that places continual stress on the weed.  Currently, the best management plan includes cultural control combined with mechanical and/or chemical control techniques.  A single control strategy, such as mowing or an herbicide, usually is not sufficient.  

This weed is beginning to show up on roads throughout Pitkin County with more frequency.  Hwy 133 has numerous small patches.  Snowmass Creek Road has infestations from Windstar up to the end of the asphalt.  Aspen Valley Ranch also has some considerable patches. 

 

 

Diffuse Knapweed (Centaurea diffusa) has bracts with short, sharp spines and the flower is usually white or pink.  It is a biennial or short-lived perennial that reproduces only by seed.  Diffuse Knapweed can cross with Spotted Knapweed, creating a hybrid with spiked, spotted bracts.  Diffuse Knapweed is increasingly common in Pitkin County.  The largest infestations are found at the base of Smuggler Moutnain Road, and in Snowmass Village across from the Snowmass Conoco.  An early detection and early treatment philosophy could eradicate this weed from Pitkin County.  

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