compost heap conundrum

Hi
I have a double fronted compost heap made from old pallets in an M shape. I have had ongoing problems with creeping thistles on the plot and am trying to get top side of them this year.
The thistles wind themselves around the edges of the bins - looking a mess and difficult to remove. I am about to move the bins. Will it be ok to sit the bins on black plastic to stop the problem reoccuring?
Any other ideas gratefully received.

Thanks Mum...I knew there was a reason but just couldn't think of it. :blink:
Maybe I'll put some around the edges as that is where the problem seems to be.