Root. In the computer world, root refers to the top-level directory of a file system. The word is derived from a tree root, since it represents the starting point of a hierarchical tree structure.
Use a fertilizer with high levels of phosphorus. Phosphorus helps to prevent aluminum from creeping into the system of the hydrangea. Choose a fertilizer close to the ratio of 25/10/10 (Phosphorus is the middle number).
Every spring, after the hydrangea has leafed out, work some 10-10-10 fertilizer into the soil around it. Endless Summer hydrangea also requires about a cup of soil sulfur applied around the plant in order to bloom blue. Put an inch or two of a peat moss and compost mix all around the plant. This is top dressing.