English Ivy

English ivy is one of the most damaging invasive plants in the Puget Sound lowlands. Washington State classified it as a Class C noxious weed in 2002, and as of August 9, 2025, it is on the state quarantine list (WAC 16-752), making it illegal to transport, buy, sell, or distribute anywhere in Washington (NWCB; King County Noxious Weed Program). Oregon banned sales in 2010 under OAR 603-52-1200. King County lists it as a non-regulated Class C noxious weed, meaning control is encouraged but not required at the county level; the state-level sales ban is the enforceable restriction. The maritime climate here is nearly ideal for ivy: mild winters prevent dieback, year-round moisture supports continuous growth, and the low light under our conifer canopy suits it perfectly. Once established, it spreads vegetatively across the forest floor, creating dense monocultures ("ivy deserts") that smother native understory species, prevent tree seedling regeneration, and reduce habitat for native wildlife (NWCB; King County BMP). When ivy transitions from the juvenile groundcover phase to the arboreal (adult) climbing phase, it produces greenish-white flowers in fall and dark blue-black berries that ripen in spring. Birds (especially European starlings and American robins) disperse the seeds widely, seeding new infestations in intact forest far from the parent plant. In the arboreal phase, ivy adds significant canopy weight that increases wind load on host trees, hides structural defects from arborists conducting risk assessments, and can girdle trunks. The added sail effect during winter storms contributes to branch failure and whole-tree windthrow (Green Seattle Partnership). The region's largest ivy removal effort is the Green Seattle Partnership (City of Seattle and Forterra), which has cut nearly 50,000 "survival rings" around infested trees, cutting ivy at chest height and clearing a five-foot radius around the base. Volunteer events regularly remove ivy from Seattle parks. The recommended control method is hand-pulling in moist soil (effective in our clay), cutting vines on trees at shoulder height and at the base, and removing roots in a wide ring. Chemical control options are listed in the PNW Weed Management Handbook (King County BMP, Rev. 2020). For new plantings, native and non-invasive groundcovers adapted to Western Washington shade include sword fern (Polystichum munitum), inside-out flower (Vancouveria hexandra), wild ginger (Asarum caudatum), piggyback plant (Tolmiea menziesii), kinnikinnick (Arctostaphylos uva-ursi) for sun, and salal (Gaultheria shallon) for dry shade. The NWCB maintains a Western Washington groundcover alternatives list at nwcb.wa.gov/groundcover-alternatives-for-western-wa.

— Chris Welch, ISA Certified Arborist