Mountain View has cut down 33 trees near its public library and Pioneer Park after a mass die-off that city officials now believe may trace back to their own weed-control program. Crews removed 23 non-native sycamore trees along with 10 others that had declined beyond recovery and posed safety risks, and two more are slated for removal at the corner of Mercy and Franklin streets.
A possible contributor, according to lab testing described by the San Francisco Chronicle, is imazapyr, the active ingredient in a weed-control herbicide the city used for routine maintenance. Soil samples turned up varying levels of the chemical around roughly 150 trees across town, and Mountain View has since discontinued use of the herbicide altogether. The affected trees stretch well beyond the library grounds, with symptoms in varying stages of decline turning up in Stevenson Park, Gateway Park, on roadway medians, and near the corner of Castro Street and Evelyn Avenue, all showing signs similar to what killed the library sycamores, the Chronicle reported.
City officials have stopped short of pinning the die-off entirely on the herbicide. Per the City of Mountain View, laboratory testing confirmed imazapyr in soil and tree tissue, but officials caution that chemical exposure alone may not account for the entirety of the decline, leaving open questions about other possible contributing factors. The Chronicle's reporting likewise notes the city has not determined the extent to which imazapyr contributed to the losses and says other factors may have played a role in individual trees' health.
Testing Before Replanting
The city is replanting trees when no more chemical is detected in the soil.
Mountain View used imazapyr as part of its weed-control program and has since discontinued its use.
Replanting After Testing
The city is replanting trees when no more chemical is detected in the soil. Plans for the affected sites in Stevenson Park, Gateway Park, and street medians will undergo formal review by the Mountain View Parks and Recreation Commission before any new trees go into the ground.
Rethinking Weed Control Citywide
Beyond the immediate cleanup, Mountain View has discontinued use of imazapyr. Pioneer Park, tucked behind the library, is part of the city's downtown green space.
Mountain View's public tree inventory includes more than 27,500 trees, a canopy officials are now working to protect from the chemicals once used to maintain it.
