Answer: In my experience, both of these varieties will typically be killed back to the ground or close to it during the winter in your zone -- your zip code puts you in USDA winter hardiness zone 6A or the coldest part of zone 6. Depending on your microclimate, it might be as cold as zone 5. So they will most likely never reach their full size due to the annual "pruning" by weather. If however they do begin to get too big, the time to prune is after they bloom. Prune out some of the longest stems one by one, then shorten some of the remaining stems. This will control size and also allow better air circulation inside the plant.
Nikko Blue hydrangea coloring is affected by the soil in which it is grown. In acidic soil it is bluer, in a neutral or alkaline soil it is pinker. As your plant grows roots beyond the original soil ball, it may show some color changes over time. It is best to allow it a year or two to settle in and become rooted and established and show its actual coloring in the location where it is planted.
If at that time it is still pinker than you would like, you should run some basic soil tests to check the pH. Then you can begin to gradually adjust the pH as indicated by the test results. To do that you would apply sulfur in small amounts repeatedly until the soil pH is corrected. You may see recommendations for aluminum sulfate, however this can cause toxicity problems and should not be used.
In the meantime, do not apply lime near the plant. (Lime raises the pH and makes the soil less acid.) Use an acidic mulch such as pine needles or pine bark, and use a fertilizer for acid loving plants such as Hollytone in the spring per the label directions. These will contribute to an acid soil over time.
If your hydrangea is planted near a concrete walk or foundation, be aware that the concrete tends to leach and raises the pH as a result. In that situation, you may never be able to get really blue coloring on your hydrangea.
Your local county extension should be able to help you with the soil testing and interpreting the results. Enjoy your hydrangeas!
Q&A Library Searching Tips