Answer: Hydrangea macrophylla 'Nikko Blue' probably attracts more attention and prompts more questions than all the other varieties put together. Nikko Blue is considered marginally hardy in your area, but when given the right winter treatment, it can be magnificent. It is prized for its colored blooms. When grown in acid soil (with a pH below 6.0), it will bear brilliant blue flowers. When grown in neutral or alkaline soils, the flowers are a clear pink. It can grow 4-5 feet tall and wide. The blooms appear in July and August and are usually larger than 6 inches across. Plant Nikko Blue in a protected site as early in the growing season as possible. Water it deeply to encourage extensive root growth the first season. Since the buds form on old wood, pruning a Nikko Blue is an art. If it is pruned too hard, it won't bloom. Before pruning, look carefully at the base of the branches and make sure you recognize the nodes where leaves emerge from the stems. On hydrangeas, there will be two leaves emerging on opposite sides of the stem at each node. In the fall, after a killing frost, prune back the branches to just above the second or third node from the base of the plant. The best winter protection is to form a cage of chicken wire around the shrub in late October/early November and fill it with leaves or loose straw. This will help protect the plant from alternative freezing and thawing during the winter and early spring. After danger of a killing frost has passed in spring, remove the leaves or straw. The most common reason for a Nikko Blue hydrangea to fail to flower is winter kill from subzero temperatures, especially while it is young. Planting in a protected site and providing extra winter protection is important. In extremely cold winters or winters without adequate snow cover, Nikko might die back to the ground and produce only leaves the following growing season.
You can fertilize your hydrangea in the spring with a general purpose garden fertilizer (5-10-10) in amounts as recommended on the package. Enjoy your hydrangea!
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