I have purchased a home with acreage. The blackberries are growing everywhere and especially troublesome in the hedge of arborvitae. I have used roundup where there were no desirable pIants with great success. Is there a better chemical to use under the hedge. After the treatment, is there a chemical registered in our area to put on the soil surface that will keep the blackberries from growing without hurting the hedge?
Blackberries can be troublesome because they grow off underground nodes so require follow up to assure they stay gone. The two most effective herbicides are glyphosate and Triclopyr. Unfortunately these will also kill your Arborvitae. Also unfortunate is these herbicides work by moving into the growing plant and become inert in the soil, so do not prevent return of the pest. The good news is late summer, early fall is the best time to apply herbicides to blackberry.
Safety is important when working with herbicides. If it is possible to safely brush the herbicide onto growing leaves, that is one way to prevent spray killing your Arborvitae. OSU publications also recommend Crossbow, but it is no longer produced.
It sounds like some mechanical control may be necessary to get the growing plants out of the Arborvitae. Blackberry does not like shade. Hopefully the shade should have reduced the plant vigor in the Arborvitae enough that mechanically pulling the plants up is an option.
Long term control should include adding mulch under the Arborvitae because shade will discourage blackberry re-growth. Eliminating blackberry should be considered a work in process. Be prepared to be diligent about preventing re-growth.
I could not find a soil herbicide that would be safe to use with your Arborvitae.
Because blackberry is such a pest, there is plenty of additional information available: