Posted: 25/08/2016 at 15:35
Its under a Spiraea which I have just pruned. But I think you may have found the answer - the next door garden has a Rhus/sumach tree right near my fence and I think the roots may have grown underneath - the leaf is certainly similar. At least its not Japanese Knotweed!!
I get all sorts of things coming from under the fence as these neighbours moved in 3 years ago and have not touched this long bed next to my fence - however they keep their patio and bbq area immaculate!!