We see this around Seattle greenbelts, alleys, and other public land. A homeowner clears the blackberry along their fence, and a few months later, canes are coming through again. The visible growth may be on your property, but the main patch could be on land next door.
Start by checking who owns that land. An alley or greenbelt may belong to the City, but don’t assume it does just because it looks public.
If the blackberry is growing on City property or in a public right of way, report it through Seattle’s Find It, Fix It app or web portal. Choose Overgrown Vegetation, pin the location, and include photos showing the blackberry and where it crosses your property line. Describe any blocked access or safety issue. The City lists overgrown vegetation on City property and in public rights of way as a reportable issue. Filing a request gets the issue in front of the City; it does not guarantee that the City will remove the whole patch or work on your property.
While you wait for a response, you can keep the canes on your side cut back so they don’t take hold across the yard. If crowns and roots have established on your property, those may need to be dug out as well. Cutting the canes at the fence will make the area usable again, but it won’t remove the source on the other side.
Be clear about what you want a contractor to do. We can quote removal and cleanup within your property. Work on City land is a separate matter and needs the proper permission.
Field note: Report the source, then make a plan for your side. Those are two different jobs, and a City service request is a way to raise the issue, not a promise that the blackberry will be cleared.