Still Time to Kill Those Perennials?

With the frost and freeze that took place last Friday, some folks may be wondering whether it is worth their time to spray those perennials or biennials still this fall.   I was driving past some hay fields in Centre County yesterday and could not help but notice the cold temperature damage on some plant species.  Before you waste your time and money on fall herbicide, check the health of those plants and ensure that they are healthy enough to absorb any herbicide you apply.  What I observed yesterday was that it is probably too late for species such as pokeweed, horsenettle, and milkweed (see accompanying image).  However the Canada thistle and the biennial thistles as well as cool season grasses and legumes are still quite healthy.  Hemp dogbane may be a good example of an intermediate species where you may still have some success or it could be too late in some regions.  In some research trials we did back in the 1990’s in upper Dauphin County with hemp dogbane, an early September timing of glyphosate vs. an early October timing produced similar positive results, although the September timing may have had a slight advantage because of cold weather and a light frost just prior to the October application.  With Paul Knight’s prediction for mild temperatures and some Indian summer next week, this remains an excellent time to still achieve some fall weed control if those perennials and biennials remain healthy and you can avoid the rain showers during application.