Posted: 13/03/2017 at 15:08
It's safe to spray weedkiller as long as you keep it off the foliage of the beech. If you can, spray a foot width on teh far side too to give you some space then, when it's worked, fork or the soil to remove all the weed remains and cut off any that may be creeping back.
Water well and then cover with a good thick mulch (2" minimum) of chipped bark, slate chippings or gravel depending on taste and budget. This will exclude light and stop other seeds germinating but also make it easier to hand weed or hoe any future invasions. Unless you can put a physical barrier several inches deep between your hedge and the land on the other side you will always be invaded.
Consider making this mulched area wide enough for you to walk on to trim your hedge as it grows and contain it with a barrier on your side between the hedge and any lawn or future bed you put there. That will help contain creeping buttercup but you need to be vigilant as it puts our runners very quickly in my experience. Constant battle in my last garden.