I am planning a raised bed for growing more veg next year. It will go on our driveway, which is gravel (I can scrape back) over broken up tar. I really don't want to eat anything growing out of tar, so was thinking of lining it with weedproof membrane. But is that enough? I was also thinking of putting cardboard at the bottom to help retain some water.
Would be grateful for any advice - everything I've found on googling assumes the bed is going on top of existing soil, but I want to make sure no plant roots escape from the bottom of this one.
Thanks.