To Wildflower2, Please don't buy the cheap membrane, usually brown and looks a bit like brown woven paper, you will be wasting your money as it doesn't last. Get the black woven material which usually has coloured checks across it in big squares. This is the stuff the professionals use (they call it Geotextile fabric but don't let that put you off). You can plant through this by inserting a cross in the material and puting the plant through this. Same applies for perennials. Just ensure you lay the material peeled back from the cross carefully around the plants, then cover with a mulch. Of course you will have to remove every perennial weed before you put down any material, otherwise you will have problems.
If you have a large area to cover it is worthwhile considering buying an entire roll. May be an initial expensive outlay but you won't have to bother with all the irritating joins you would have if you buy it in small sections. There are also pins you can purchase to secure the fabric to the soil. This is worthwhile.
Good luck and happy planting.