Hostas love moist, but well-drained soil. Add organic matter such as garden compost, leaf mould, well-rotted manure, composted pine bark and peat moss to make the soil slightly acidic. Organic materials provide air space for rapid root growth but should be firm enough to discourage voles and other rodents. If you are battling tree roots, a raised bed is a good idea. Clean out the tree roots and amend the soil (mushroom compost and peat moss are the best in my opinion). Preparing your beds for future healthy, happy hostas is of utmost importance.