It depends a bit which weeds, and what their method of spread is. In almost any case, the black plastic edging will probably be a waste of time.

Weeds that spread by runners, like daylilies or morning glory, will easily bypass even quite deep edging, and most weeds spread by seed so will come through the fence. As such, if you add some boards to the fence from the ground up you might make the most impact, but even that will be bypassed by seeds to some degree.

But for seed-sown weeds, to some extent you can depend on the shade your plants are going to supply to limit the degree to which weed seeds find hospitable ground on your side of the fence. The better your shrubs cover, the less weed growth you will have... "the trick is to cover the ground," as a nursery near me once wrote in their catalogue. As your plants grow in, their root competition will also help to reduce weeds.

The real key to ongoing success against weeds is to design into your border the capacity for you to get into that area and pull weeds early, and probably also to put down an annual covering of mulch. Either open areas or stepping stones, in other words, which will not show much among your plants in summer, and can maybe add a nice structural/pattern design element in winter.

What you can also do is plant a groundcover that you like that will grow over toward your neighbour and maybe suppress his/her weeds a bit, or a lot. In my climate something like Asarum canadense would do the trick. But you have to want it in your border too!

Ongoing vigilance and access, and discreet use of herbicide on weeds that grow by runners, is the nutshell answer that I would offer. That, at least, is how I approach my neighbour's weeds, which include morning glory and buttercups, and so attack via underground runners, surface runners, and seed through a spaced-board privacy fence.

Karin L

What she said. (Nice answer, Karin.) The only thing I would add is the possible use of chemical "weed preventer"--like 'Preen'--to the arsenal. It might require some experimentation to use it effectively.

Thank you so much for the sage advice. I lurk most of the time, but could not quite find the info that I needed. I will not waste my efforts installing a barrier next to the split rail. Until my shubs take over, I will have to be diligent with weeding and definitely will use preen. I love that stuff.

Regards,
Lynn