Shelley: Are these the "regular" blackberries?  If yes, how big are they at this point?  The regular blackberry can be repeated removal of the canes every week or so. The herbicide glyphosate (Roundup and other names) works well on blackberries after blooming/fruiting or in the fall. Right now, it's right at that edge. If you use Roundup, do not use the "Roundup" that promises "Season long weed control". It is a mix of two herbicides and can, potentially, damage the root system.  The regular Roundup  is fine as long as the fir tree surface roots haven't had their bark removed. The minute Roundup hits soil, it is tied up and can't affect other plants or seedlings. It  has to be absorbed with green/yellow/red  tissue and moved through the plant to the roots. . A seed can germinate the next day where Roundup had been sprayed and do fine.  

I know this sounds complicated. Feel free to call or email me directly if you still have questions. Chip Bubl ODSU Extension Agent/Columbia County (retired)  <personal data hidden> or<personal data hidden>.