What is the best spray and when is the best time
Linn County Oregon
I've worked on ecological restoration projects removing introduced blackberry.
You can learn more in "Methods to control blackberry thickets" and dig deeper into this topic in "Managing Himalayan Blackberry in Western Oregon Riparian areas," EM 8894.
Most likely you will have some blackberry come back for a few years even if they appear to be gone. The treatments may need to repeated over time. Good luck!