Q: I live in northern Vermont and I like to grow things that are not indigenous. We had many large, rich blue flowers on our Nikko hydrangeas for their first three years, but then in their fourth year we stopped getting flowers from them. All our other plants and flowers continue to do fine. The Nikko’s look better and better every year, but no flowers.
I know the plants need to grow on old wood, but they haven’t in prior years and our harsh winters never seemed to bother them before, so do you have any ideas as to what could be the problem?

A: I didn’t know any hydrangea could survive winter in Zones 3 & 4 without some serious winter protection! ‘Nikko Blue’ is one of the most cold hardy but I’d think Zone 5 would be as far north as it would survive. My bet is that the Nikko just got enough warmth the first couple of years to bloom from a few surviving old wood stems. Why not try ‘Endless Summer’ or ‘Dooley’ hydrangeas (which bloom on new wood) and let me know how they do.

See this Vermont Hardiness Map and this Plant Hardiness article.