I have a raised bed, about 2 ft. deep, that is filled with a commercial soil mixture. growing in it are 2 rhododendrons, a brugmansia and a feather nandina. The area was planted about 5 years ago. The soil has become filled with a mass of roots, difficult to penetrate. I recently opened a sack of unused soil mix and discovered the same roots. There is no foliage that I can find related to the roots. What might this be and how can I get rid of the roots??