Plants hardy in Niagara, New York, located in USDA Plant Hardiness Zone 6a, will withstand low temperatures from -10 to -5 degrees F. Bigleaf, smooth and oakleaf hydrangea will all do well in Niagara, as long as they are protected from the hot July temperatures. Plant hydrangeas in Niagara in early summer or late fall.
Choose a spot to plant the hydrangea that receives full sun in the morning and afternoon shade and has loose, well-drained soil.
Test the soil if you plan on altering the flower color of your hydrangea. Bigleaf hydrangea will turn blue if there is aluminum in the soil. The absence of aluminum will turn the flowers pink. The Cornell Cooperative Extension of Niagara County offers soil testing to determine which, if any, amendments you should add to your soil.
Mix a 3-inch layer of compost and any amendments required by the results of the soil test into the soil to a depth of 8 inches, using the gardening fork.
Dig a hole the same depth and twice the width of the pot in which the hydrangea is currently growing.
Turn the pot on its side and gently tip the hydrangea out. Lower the hydrangea into the hole and fill half the hole with soil. Fill the hole with water and, when it drains, fill with the remaining soil. Tamp down on the soil around the base of the hydrangea.
Lay down a 3-inch layer of mulch around the hydrangea, extending it out as wide as the branches.
Provide the hydrangea 1 inch of water a week.