I am putting in some new raised veggie beds along a fence line and will also be putting in some climbers (probably passion fruit) between the beds and the fence for screening. To keep the vine/tree roots from sucking all the water and nutrients out of the veggie beds I'm planning to put some thick plastic sheeting down under the raised beds to keep the veggie soil isolated from the tree roots.
So, my question is what depth do I need to make sure the veggies have enough soil? I want to be able to grow most common annual veggies and herbs including tomatoes, carrots, parsley, zucchini, etc. I won't be growing perennials such as chillies or asparagus in these beds.