If you are dealing with a mass of roots under a tree, dig out the roots and place an empty pot in the hole. Into that pot one can place a potted plant that will grow happily without the competition of tree roots.This method can also be handy if you are experimenting with various plants ability to cope with dense shade. Pot up various species and experiment by placing one in a pot and waiting a few weeks to see how it copes. If it’s unhappy, one doesn’t have to dig it up and cause root damage. Just replace it with another potted plant. One can also use this idea when playing around with colour combinations.