Answer: Firstly, new hydranges should not need much pruning for the first few years, with the exception of removing dead or damaged stems. You can also remove the faded flowers if you wish.
The Nikko Blue blooms only on old wood, meaning stems that grew the year before. If this plant needs pruning to reduce its size, you would remove some of the longer stems right after it blooms. In spring, you may need to cut it back to remove winter-killed stems. Wait and be sure the stems are truly dead before you remove them because spring pruning will reduce flowering.
The Endless Summer is able to bloom on both old and new stems which allows it to bloom over a longer period of time. Early blooms are on last year's stems, the later blooms going into the fall are on the new growth. Again, you may need to remove winter damaged stems in the spring. If the plant is still too large, you could also remove a few of the older longer stems at that time, or trim it back overall to a size you need.
The blue coloring depends partly on the variety you are growing (some are bluer than others no matter what the soil is like), and partly on the soil the plant is growing in. The bluer coloring appears on acid soil. The best way to do this is to run some basic soil tests to check the pH, then work slowly to adjust it if necessary. You can use a soil acidifier such as sulfur per the label directions. Your local county extension can help you with the soil testing and interpreting the results.
I would not recommend using aluminum sulphate as this can build up in the soil and becomes toxic to the plant, overuse can kill your hydrangea. And, you do not want to overacidify the soil. Be patient and work slowly as the materials need time to work into the soil and the roots need to grow throught the amended soil before you will see the effect.
Avoid using lime near the plant. Also, using an acidic mulch such as pine needles or pine bark will help slowly over time. You can also use a complete, slow release fertilizer for acid loving plants such as Hollytone per the label directions. Again, too much fertilizer is not better so just follow the label instructions.
I hope this helps.
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