The Open Space Conservancy Trust on July 17 received a report from the City of Mercer Island on herbicide use in 2024 and a plan for 2025 monitoring and treatments. Natural Resources Program Manager Lizzie Stone presented the report and said staff and trustees share a goal to “minimize herbicide use wherever possible.”

Stone told trustees the trust adopted a herbicide use protocol in May and updated it in January; the protocol requires staff to report annually on recent and planned herbicide use. The nut of her presentation: staff surveyed for knotweed, shiny geranium, yellow archangel, yellow flag iris and spotted jewelweed in 2024 and encountered only yellow archangel on trust property.

In 2024, Stone said, contractors completed comprehensive invasive-plant removal in multiple zones in Pioneer Park and Engstrom Open Space. Treatments included dry-stem injection of imazapyr for larger invasive trees and a cut-and-paint application of triclopyr on smaller stems, which Stone said follows the protocol. She described two project sites where Himalayan blackberry was cut and a concentrated triclopyr product was painted onto cut stems “to minimize any off-target damage and to localize that treatment.”

Stone reviewed the history of yellow archangel in Pioneer Park, saying the species was first treated there in 2010 and an initial effort eradicated “approximately 90 to 95 percent of the infestation.” In 2024 staff found patches along the north edge of the northwest quadrant and the south edge of the southeast quadrant; crews treated those areas in spring and again in fall with the hope of shifting to hand-pulling within one to two years.

Stone also described outreach to adjacent residents: she delivered a weed alert, photos and management advice to neighbors bordering the infestations and said conversations were “very positively met.” Trustees encouraged broader public education because plants often spread from adjacent private properties.

Trustees asked about non-chemical pests; Stone said she was not aware of any insect or animal pests causing tree mortality on trust property, though other city parks have had bark beetle issues. She said 2025 work will include repeating surveys for the same set of species, continued targeted treatment where contractors perform comprehensive removals, and possible spot treatments of yellow archangel depending on post-treatment monitoring.

The trust took no formal regulatory action; staff presented the report for review and discussion and were directed to continue monitoring, prioritize outreach to neighbors, and use targeted treatments consistent with the herbicide use protocol.