It's a little hard to make out from this photo alone, but this could be a type of foxtail. A commonly found species is Meadow Foxtail (Alopecurus pratensis) that has seed heads with a slight purple tinge.
The City of Portland has a pictorial guide to common grasses and weedy grasses. Take a look and see if any of these match what you are seeing: ftp://ftp02.portlandoregon.gov/BES/Grass%20ID%20Guide/GSS_GrassesGuide%20REV.pdf
We're also happy to take a closer look at a plant sample if you're able to drop at the Extension office. If you go this route, please dig up an entire plant that is currently flowering and bring it in. You can find us at: https://extension.oregonstate.edu/mg/linn-benton/have-gardening-question
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