Posted: 07/05/2013 at 18:31
I wouldn't use any membrane there at all.
That gravel? Is it limestone gravel? If so it is alkaline. I would remove gravel totally and dig over the soil incorporating some compost, manure, etc and plant up with a structure of acid loving plants. Azaleas, pieris, andromeda, Camelias, dwarf rhododendrons, etc. When I say "structure" I mean a framework to then infill with annuals, perennials, etc. using a mixture of spikey grasses, foliage plants like hostas, Heucheras, and annuals for summer colour.
I dont like bark as a mulch. It blows around and it harbours pests