Answer: This plant blooms on both old and new wood, so it develops flower buds ongoing during the summer. The original blooms were probably formed on wood that grew last year, the new flowers are more recently formed. To maintain the blue coloring the soil pH must be acidic; it is possible your plant is rooting out beyond the original potting soil and the soil it is planted in is not acidic enough to maintain the blue coloring. However, it often takes hydrangeas a year or so to settle into a new location and soil, so I would suggest you wait and see what happens by the end of the season. If the color has gone to pink then you know your soil is somewhat neutral to alkaline in pH. If you would prefer blue, then you would need to gradually acidify the soil, you can also use a water soluble fertilizer for acid loving plants. This pH lowering can be done slowly over time using sulfur -- sold at garden centers. (Some people will suggest aluminum sulfate but the aluminum can build up in the soil and become toxic.) So, at this point I would still be a bit patient with it. Prune off the flowers as soon as they begin to fade and you won't have to witness the change from blue to pink to brown. Pruning encourages a dense, bushy habit so pruning to maintain (or achieve) shape will only result in lots of new flowering stems. Go ahead and prune the plants as required. Good luck with your hydrangea.
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