This looks soft rush, Juncus effusus, or a close relative, the most common rush in the PNW.  However, I can't really see the flowers on the larger plants.  I have it in my backyard, in full sun and unirrigated. It has deep roots, but I was able to dig it out and it hasn't come back.  But I only had a couple clumps.  It's a native and an important food source for animals. http://web.pdx.edu/~maserj/ESR410/juef.html

However, here is PNW's recommendations for management: https://pnwhandbooks.org/weed/problem-weeds/rush-bog-juncus-effusus-var-pacificus https://willamettebotany.org/juncaceae/

Juncus effusus Softrush

Exposure: full sun

Soil moisture: moist to wet

Transplanting success: high

Growth rate: rapid

Form: scattered tufts growing on thick rhizomes, to 39"Soft rush is the most common rush in the Pacific Northwest. It is widespread in wet pastures and disturbed areas, and occurs less commonly in salt marshes and tideflats, where it may grow intermixed with tufted hairgrass and saltgrass. Soft rush has a reputation for invasiveness, but in our experience it rarely forms monocultures, and can provide fairly rapid cover on marginal emergent sites with disturbed, compacted soils. Click here for photographs of this species on the University of Washington Herbarium website.