I have a few raised beds in my fenced-in garden, and I have been following directions for the square foot gardening which includes lining the bed with landscaping fabric. When Spring comes and it is time to prepare each bed for planting, I notice that they seem to be full of roots that need to be pulled out before I can plant anything else. They seem to be coming from below and invading the bed. The fabric also seems to be falling apart which seems to suggest that it is biodegradable. Why is this happening, and what can I do about this?