Its ground elder and won't go away without weed killer (someone will tell tell you which is best).

Dreadful weed. I dig mine out and remove every piece I kind find, but it it comes back. It comes under the fence from next door where it is rampant. All I can do is control it on my side. As it gets in among shrubs and plants I find I cannot use a weedkiller. Sometimes I have to dig a plant up strip it of this weed and replant.

If you use a glysophate weedkiller, eg roundup, it does no harm to the soil or plants that you subsequently put in. If it does come up from under a plant, whether shrub or small tree or perennial, place a pop or milk bottle (with the base cut off) over the offending elder, and spray into the neck thus only spraying the elder foliage.
DRC726, I had this problem many years ago and I dug a small trench about 1' deep and lined it with strong plastic sheeting, Builders damp course was what I got hold of. I then backfilled the trench sieving the soil as I went. Any loose straggly bits of elder were easily eradicated or sprayed as above.

Thanks 2ndhand that would mean digging about 200 feet of trenches. As someone on 'Gardeners world'once said "you have to learn to live with some ground elder"and I find digging it out when I see it works for me.