Here is an interesting post from Richard Jauron, the answer line person in Horticulture here at ISU. If you have been following the blog, you know that Richard has been experimenting with Tenacity (Mesotrione) for the control of nimblewill in his lawn. That experiment has been going on since 2011 and he has had good results and has nearly eliminated his nimblewill.
Here is another observation from his work. He has Virginia bluebells (Mertensia virginica) in the beds at the edge of his lawn. The nimblewill and the bluebells both spread by seed and they have been merging at the edge of the bed in recent years. The bluebells flower in early spring and the foliage then dies back by the end of June. He last spot treated with Tenacity in August after the foliage of the bluebells was gone.
The bluebells are just coming up this spring and are in full bloom. Where he sprayed the tenacity, the bluebells are emerging with damaged, white leaves and stems. This is the first report that I have seen indicating damage from Tenacity to this species.
Here are the healthy Virginia bluebells on May 8, 2013.
Here are the bluebells in areas spot treated with Tenacity.