Spotted knapweed

Spotted knapweed (Centaurea biebersteinii) is a nasty invasive weed that competes well in sandy areas. Although we have not generally been plagued with it at Pleasant Valley Conservancy, it has turned up at a few locations along our gravel road. Its presence there was not accidental, as it was brought in with the gravel.

When we did a major overhaul of our gravel road in 2007 the contractor brought in lots of loads of gravel, and we were careful to ensure that it was weed-free. However, after the major rainstorms in June 2008 one part of our road suffered a serious wash-out. The contractor was able to repair it, but was unable to get gravel from the same source. It was from this new gravel that spotted knapweed is coming from.

Our general procedure is to walk along the gravel road in early June, when the knapweed is just getting started, and spray it with 2,4-D or Garlon 3A (aqueous). However, the plant shown here was hidden along the edge of the gravel and was missed. Fortunately, knapweed is quite easy to recognize when it is in flower and at this stage it is ideal for my Garlon 4 (in oil) basal stem technique. It took less than a minute to spray the bases of the five or six stems of the plant seen here. Two other plants at different locations along the road were also found and sprayed.

This is not our only fight with spotted knapweed. About five years ago the Town of Vermont rebuilt the whole two miles of Pleasant Valley Road. As part of that project, many loads of gravel were brought in for the shoulders. This gravel had some spotted knapweed mixed in with other weeds. It took us about four years to eradicate this infestation, using the foliar spray technique.

The lesson learned here is to catch an infestation when it is still small.