It was recently brought to my attention that a common weed prevention chemical, Imprelis, used by landscapers is causing damage and death to conifers/evergreen trees.  The product is being promoted by DuPont as "an environmentally friendly herbicide", but it appears that additional testing will need to be completed to fully understand how this chemical is affecting evergreens and the long term environmental affects of the use of Imprelis. 

The product is not available in retail settings, however, it is commonly being used by commercial landscapers and country clubs throughout the U.S.  Many commercial landscapers are starting to receive complaints and claims from customers due to death and damage of their trees.

http://www.freep.com/article/20110728/NEWS06/107280512/DuPont-says-its-product-harms-trees