Organic matter holds nutrients and water in the soil and helps prevent stress from fluctuations in soil moisture levels. As with any other shrub, plant the root ball level with the soil surface, and water thoroughly immediately after transplanting. Don't fertilize Hydrangeas at all until they are established. This will be 4 to 8 weeks after transplanting. Once established, feed with an all-purpose fertilizer.
Hydrangeas are water-demanding.
Water whenever the plant begins to wilt in the absence of rainfall. Avoiding this wilt is particularly important during the spring months when the flower heads are forming, so be sure to monitor the soil moisture around your hydrangeas during dry weather. Lots of aluminum makes blue, while no aluminum makes pink. The reason for this is that Aluminum is barely present at all in alkaline soils, or at least the plant can not uptake the nutrient from soil with that pH. You don't need to remember all this chemistry. To make flowers pink, add 1 cup of dolomite lime per 10 square feet and water into the soil.