depends on the plant. If its something that sends down long roots, you will need a barrier. For something like beans, you would traditionally dig a deep trench

depends on the plant. If its something that sends down long roots, you will need a barrier. For something like beans, you would traditionally dig a deep trench (as in 4foot) and put a layer of manure, with news paper on top, by the time the roots got that far it would be rotten enough. It might be better to use coffee grounds or fine wood chippings, as they are equally as warm but less toxic in the raw state.