Use a sheet of heavy duty plastic or rubbering, similar to what is used for covering turf piles or silage. It'll prevent the growth of any weeds in the area due to lack of sunlight, and will help to maintain the integrity of the soil by promoting the growth of saprophytes such as moulds and fungi, which will break down the organic matter, freeing the nutrients for plants at a later stage. It'll also prevent the soil from drying out.