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Natural resources manager reports limited herbicide use, targeted treatments planned for 2026
Summary
City natural-resources staff reported 2025 survey results and 2026 plans for Pioneer Park: of five targeted invasive species the crew found only yellow archangel in 2025; staff and contractors performed targeted treatments using approved methods and plan continued monitoring, site-specific treatment and erosion-control follow-up.
At the July 16 meeting, natural resources program manager Lizzie Stone presented the Open Space Conservancy Trust’s annual herbicide-use report and the department’s plan for 2026.
Stone summarized the trust’s herbicide-use protocol (adopted in May 2010 and most recently updated in January 2023), which requires annual reporting. In 2025 staff surveyed for five species — Japanese knotweed, shiny geranium, yellow archangel, yellow flag iris and spotted jewelweed — and reported that only yellow archangel persisted in the park. Staff treated yellow archangel patches in the fall following King County best-management practices and emphasized thorough surveying with the goal of eventual manual removal where possible.
Contractors also removed invasive trees and treated difficult blackberry patches where mechanical removal would have damaged nearby native plants. For tree and large-stem treatments staff described using a localized injection method with an imazapyr product (referred to in the presentation) and cut‑and‑paint treatments on stems using triclopyr; in some blackberry work staff applied herbicide to freshly cut stems. Lizzie said all product uses were approved under the trust’s herbicide protocol.
Trustees asked how yellow archangel spreads; Stone explained it propagates from vegetative shoots and rhizomes and is often introduced from garden plantings or hanging baskets. On private-property encroachment, Stone said city employees cannot perform herbicide applications on private property; in some cases King County noxious-weed teams have authority to treat private parcels.
On steep ivy-covered slopes where full removal could destabilize soil, Stone said staff will spray small patches, leave the dead roots in place through winter, then return to install erosion-control fabric and replant with native species to stabilize slopes. Trustees also offered to support neighbor outreach and volunteer follow-up to help manage regrowth on adjacent private property.
Stone said contractors and staff will continue targeted surveys and treatments in 2026 focusing on the same five species, with contractor-led invasive‑tree removal and select blackberry treatments where hand-digging would harm native plants.
Trustees asked for follow-up info on the status of benches and sign inventory; staff said they would provide additional background and identify items for future work-plan review.
