Heavy mature brambles have grown up through 6ft Grissalinia hedge. I can cut back the brambles somewhat but the roots are inaccessible. Would painting the cut bramble branches with Roundup work? I can’t cut them lower than about 4ft.
I'm no gardening expert but I believe regular roundup is best applied to the leaves during growing season, they do a 'concentrated' stump killer version that might be more suited.
I doubt that would work, brambles are very vigorous. Roundup might work applied to the roots but not 4ft away from the roots. Can you not get under the hedge? TBH even if you can get to the roots, they're a bugger to get rid of completely, they just keep growing back.
agree with @shweeney on this. Brambles have very, very strong survival instincts as they are constantly being killed off / hacked back. It's more of a case of managing them rather than fully eliminating them, especially as you can't dig them out.
The lower you can get to the ground to cut them, the better, but usually at ground level they would have multiple stems going in every direction. You will need to do this a number of times throughout the seasons.
I don't think that applying roundup at this level is good for the hedge itself or the wildlife that will be using the hedge, insects etc, that reside in there, but if you are then you might be able to get a specific chemical for brambles - something like SBK Brushwood killer - assume it's something like roundup though, so might impact your hedge also.
Roundup never works if applied to the roots, it needs to enter the plants vascular system where it is transported to the roots to kill them from within. Once roundup contacts the soil it breaks down.
I cut them as low to the ground as possible with a loppers, ideally before they branch out, and then drag them out through the hedge to remove the unsightly look.
Weirdly satisfying to pull a spread out briar out through a hedge by a single stem !
There is a version of Roundup that you can use on an individual leaf by leaf level. It's kind of like a large poisonous pritt-stick. It's fine for precision weeding in an otherwise sensitive bed of plants. I think you would have a hard time getting enough Bramble leaves in a hedge to have much effect, as the bramble is multi rooted, and will have multiple root sources beyond your reach. I have taken to regarding them as part and parcel of gardening, like edging the grass.