Answer to the September's Horticulture Challenge

This extremely aggressive plant is a Creeping Buttercup, a King County Weed of Concern. It produces bright yellow flowers with usually 5 (up to 10) petals. Growth starts in spring, peaks in late summer.
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Source: King County Department of Natural Resources and Parks, Water and Land Resources Division
Website: https://kingcounty.gov/en/dept/dnrp/nature-recreation/environment-ecology-conservation/noxious-weeds
Email: noxious.weeds@kingcounty.gov