Answer: The bigleaf hydrangeas (H. macrophylla) such as Nikko blue and most pinks bloom on stems that grew last year, so you do not want to remove those stems if you can help it. Winterkill or dieback does sometimes kill those stems back to ground level, but you need to be very patient about trimming them off, just to make sure. This dieback is most common if the plant is located in a windy spot or in an exposed spot out in the open; fertilizing in late summer or fall can also contribute to it, as can a dry fall season such as we had last summer.
Sometimes newly planted hydrangeas will die back a bit more the first winter while they are still becoming established and then do better in subsequent years. If most of your neighbors' plants are leafing out only at the base, and those plants are located in a similar microclimate, then yours will probably do the same this year.
Once you are sure the branches are dead, cut them off. Allow the new growth from the base to grow and it should bloom for you next summer, so do not prune it.
If your plants do not leaf out at the base or along the stems, then they are probably dead. This you would need to discuss with your retailer.
Nikko blue hydrangeas will be their bluest in acid soil. You could use a granular slow release fertilizer for acid loving plants such as Hollytone according to the label instructions. You might also want to check the pH of your soil to make sure it is in an acceptable range for you hydrangea (a pH of 5.0 to 5.5 is sufficiently acid to produce blue, while 6.0 to 6.5 will encourage a pink coloring); your county extension should be able to help you with the tests and interpreting the results.
Some of the pink hydrangeas were developed to remain pink in acid soil, however you might use a general purpose fertilizer for them rather than the kind for acid loving plants, just to be on the safe side. Again, a complete slow release granular fertilizer such as 10-10-10 should be adequate, read and follow the label instructions.
Good luck with your hydrangeas. Maybe they will leaf out this coming week.
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