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This summer, Mountain View cut down dozens of large trees around the public library and in nearby Pioneer Park because of their poor condition. Officials have now concluded that an herbicide likely contributed to the trees’ demise, according to a city press release.
“Laboratory testing of soil and tree tissue samples from several affected locations detected varying concentrations of imazapyr, the active ingredient in an herbicide previously used by the city for weed control,” the press release said. “The city has discontinued its use and is taking steps to treat and monitor the affected soil and tree health.”
In total, Mountain View removed 33 trees that were in declining condition, including a row of sycamore trees that lined Franklin Street in front of the public library. The city plans to remove two more trees at the corner of Mercy and Franklin streets, the press release said.
However, more trees may also be on the chopping block. Another 150 trees are showing deterioration similar to the ones that were removed in July, according to the press release. The locations of those trees include Stevenson Park, Gateway Park, Castro Street roadway medians and the corner of Castro Street and Evelyn Avenue.
According to the city’s press release, while testing results confirmed the presence of imazapyr, it is not known the extent to which the herbicide contributed to the decline of the trees or whether other factors also played a role.
Mountain View began using imazapyr in 2021, after it discontinued the use of glyphosate, commonly known as Roundup, according to an update posted to the city’s website. At the time, city staff considered imazapyr a good alternative.
Mountain View has now begun a process called “soil drenching” to try and reduce the level of imazapyr in the impacted areas, according to the tree update. The soil will be tested every 90 days to monitor conditions and replacement trees will be planted once city staff determine that the affected areas are cleared of imazapyr and can support newly planted trees.
The new trees will be native species appropriate to each location based on the city’s recently adopted biodiversity and urban forest plan, the update said.
City staff are also working with local, regional and state partners to reduce pesticide use and “identify products that balance potential impacts on trees, landscaping and other vegetation,” the press release said.
There are more than 27,500 publicly maintained trees in Mountain View, according to the city.
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It has been known since the 1980s that imazapyr is toxic to non-target plants including hardwood trees. And in 2006, the EPA assessed it as dangerous to use near trees. The problem is even listed on the product label! So, my question is, how is it that city staff were unaware of this? That sounds like a major screw-up.