I've cut a hole out of my membrane and planted directly into the ground and put the gravel back around the plants...only done it in a couple of places and it seems to be working. Although I don't expect the plants I have put there to spread like hardy geraniums would. Somebody with more knowledge than I will no doubt give you a better answer (I hope!!)

I would do the same, just cut into the membrane/sheeting.

I have done this in one bed for a neighbour and planted hardy geraniums in the cuts in the membrane and it worked well.

That's what I've done but I'm worried they won't spread properly and will only grow where I've made the holes. Did the geraniums spread Inverglen?

It depends on the kind of geranium you planted. But if you have planted a type that clumps up well you can slowly cut the ground sheet back and back and let the plant do the covering for you.

i would cut a few holes and plant in groups threw the membrane to give the impression its spread naturaly . the plants can spread to a certain dergree as there still being waterd and fed and plants can grow without soil ie hydraponics .