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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.

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The bulk of the root systems of most annual vegetables is in the top foot of soil, so 12" (30cm) of soil will be sufficient. Root vegetables will want more -- you may have a hard time growing good carrots or beets in a 12" deep container. Since you're going to "seal" the bottom of the raised beds, you're essentially creating a container for growing. With containers, one thing to be very careful of is drainage: make sure that excess water has someplace to go so that your vegetables don't drown.

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Good point about drainage, I'll look for some kind of thick porous matting instead of plastic sheeting. For root vegetables, how much more depth is needed? 18 inches? 2 feet? –  Bogdanovist Sep 5 '12 at 21:23
    
You can use shallow beds if you're careful about variety selection and harvesting: choose "baby" carrots, don't let your beets go too long, give up on parsnips. I don't know an absolute number for depth. In general, deeper is better: I pulled out a couple of 3" dia turnips this spring that had roots that were at least 2' long (not straight down, but there was compaction at about 12"). You have to balance cost & effort, though, and 12" might be a good compromise -- experimentation will help you decide if you want to go deeper. –  bstpierre Sep 6 '12 at 14:25
    
Thanks. I found a good deal on some wooden collars that are 20cm (8 inches) high. They stack easily though so I'll do a double layer which sounds like it should be enough given your answer. Cheers. –  Bogdanovist Sep 7 '12 at 2:24

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