i am about to build some wicking beds and need to find a membrane to lay over the pvc , the membrane is to stop the dirt falling back into the pvc holes and to let the water out ... i thought of shadecloth but i think its to coarse ...anyone seen anything around that might work ?

I use the black shade cloth. Is it better than another product - I don't know. I just set up most of wicking bed this year, so, in another year or so, or whenever, they need to be cleaned, I will find out then. If too much soil seeps down into the gravel I will try to find another product. So far the plants seem to get sufficient water. I just have to water the seeds and smal plants when I first plant them.