I spent an entire evening making an elaborate composting container with tons of aeration holes and found that when growing wheatgrass inside, inevitably there will be some mold growth come and it is often around the base of the sprout so you can safely cut the top green and not have any problem with it. The problem was, composting the mold and realistically wanting to re-use it again was not nearly as appealing and healthy as I thought the small expense of purchasing the vermiculite and organic soil and starting my trays fresh each time.