The thing you always worry about when you spray a herbicide around trees is “will this affect the tree?” “How long will it stay in the ground?” Well, the staff at the Milwaukee County Parks got the answer to those questions for one herbicide the hard way when they realized 300 Black Walnut, maple and ash trees had died at the Wehr Nature Center after they had sprayed for an invasive noxious weed. See Journal Sentinel article entitled More than 300 hardwood trees at Wehr Nature Center unintentionally killed by herbicide.
Polaris AQ (from the Material Safety Data Sheet (MSDS))
The weedkiller used was Polaris AQ Herbicide produced by Nufarm Americas Inc. It’s main component is Isopropylamine salt of imazapyr. It is a clear blue liquid with faint ammonia-like odor.
“Imazapyr is degraded by microbial metabolism and can be relatively persistent in soils. It has an average half-life in soils that ranges from 2 weeks to 5 months. Half-lives tend to be shorter in forest litter and soils. Imazapyr is water-soluble and variably binds to organic materials in the soils. Although the potential to leach is high, leaching is limited under typical field conditions. In water, imazapyr can be rapidly degraded by photolysis with a half-life averaging 2 days. Due to its rapid photodegradation by sunlight, water contamination by imazapyr is generally not of concern….Acutely harmful to aquatic plants. Practically non-toxic to fish, aquatic invertebrates and terrestrial organisms.”
What is Half-Life?
The National Pesticide Information website explains that “A half–life is the time it takes for a certain amount of a pesticide to be reduced by half. This occurs as it dissipates or breaks down in the environment. In general, a pesticide will break down to 50% of the original amount after a single half–life.”
“The half-life can help estimate whether or not a pesticide tends to build up in the environment. Pesticide half-lives can be lumped into three groups in order to estimate persistence. These are low (less than 16 day half-life), moderate (16 to 59 days), and high (over 60 days). Pesticides with shorter half-lives tend to build up less because they are much less likely to persist in the environment. In contrast, pesticides with longer half-lives are more likely to build up after repeated applications. This may increase the risk of contaminating nearby surface water, ground water, plants, and animals.”