It only takes a few inches of added soil to kill a sensitive, mature tree. Some species, like certain oaks, maples and conifers may be very sensitive to grade changes. Others, like elm, poplar and willow are more tolerant of grade changes. As a rule, you should not cover the soil beneath the tree with more than 6 inches of fill, and then only if the texture of the fill is the same or coarser than the native soil. Moss and Algae Invade Lawns in Poorly Drained Soils Moss and algae sometimes invade lawns during the winter months.
Mosses are primitive, nonflowering, rootless green plants with simple stems and leaves.
Under favorable conditions they grow like a fine-textured mat on the soil, preventing water and air from reaching grass roots. Algae are a group of primitive, single-celled threadlike plants which are rootless, leafless, stemless and nonflowering. In wet lawns they form a thick, dense green-brown scum, which also obstructs air and water movement into the soil. Mosses and algae are not plant parasites, and do not attack and harm grass directly. Moss and algae require specific soil and climatic conditions to develop.