Answer: I'm not sure if the black tarp is causing the problems on your cedars. Black plastic tarp is usually used for 2 reasons: It warms the soil in the spring and it helps keep weeds down. It is possible that since your plant roots are growing over the tarp, they are suffering from both lack of water and overheating. I'd suggest removing as much of the tarp as you can, and mulch with pine bark or some other mulch to control weeds.
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