How do I keep the moles and voles out of my sweet potatoes?

I plant sweet potatoes in black plastic about 1 foot apart. The voles and moles eat them before I get them harvested. How can I keep them out or discourage them? I have tried castor oil and the solar vibrators. Also I normally get a huge crop of sweet potatoes, but this year we had heavy rain and I got very few potatoes. Do you know why?

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