As we read in my post last week, it has been a difficult year for bigleaf hydrangeas (the mophead and lacecap hydrangeas) due to the late frosts in the spring which killed off many of the flowering buds. While weather is often the culprit for the demise of good flowering in hydrangeas, sometimes other factors come into play.
The lessons from this past year are clear: hydrangeas that flower on second-year wood (i.e., where the flower buds form in the previous season) are best sited in protected areas that are not exposed to harsh winter elements, such as strong winds and extreme temperature fluctuations.
Other factors can contribute to a lack of flowers. Too much shade will also hamper flower production, as will fertilizing too often with high nitrogen fertilizers—this will simply produce leafy growth with few flowers. Over- and under-watering your hydrangeas can also weaken the plant and prevent maximum flowering.
Earlier this season, Todd Forrest posted a blog entitled “Waiting for Hydrangeas”. He spoke of the cold spring we had where temperatures plummeted below freezing, destroying many hydrangea flower buds that were waiting to bloom this season.
While panicle hydrangeas (Hydrangea paniculata) and smooth hydrangeas (Hydrangea arborescens) flower on new wood, the bigleaf, mophead, and lacecap hydrangeas (Hydrangea macrophylla) that we all adore, flower primarily on old wood with the flower buds forming the previous season.
The re-blooming or remontant type that flower on both old and new wood have, as predicted, fared very well. We have a large planting of Endless Summer® at the train station entrance of the Garden at the Moshulu Gate. It has been flowering beautifully for the past month with new flowers starting to form as older ones fade. The first year these hydrangeas were planted the flowering was sparse, but now that they have established a solid root system and settled in they are floriferous.
Todd Forrest is the NYBG’s Arthur Ross Vice President for Horticulture and Living Collections. He leads all horticulture programs and activities across the Garden’s 250-acre National Historic Landmark landscape, including 50 gardens and plant collections outside and under glass, the old-growth Thain Family Forest, and living exhibitions in the Enid A. Haupt Conservatory.
Frost-killed hydrangea stems should be pruned back to live wood.
On April 16 at 6 a.m., the Garden’s weather station reported a low temperature of 30.2°F. The freezing temperatures were accompanied by about a half inch of icy slush that coated the greening turf and accumulated in the chalices formed by the opening flowers of saucer magnolias, which had just emerged after a string of warm April days that, I hoped, signaled the end of our seemingly interminable winter. Needless to say, the Garden’s venerable saucer magnolias did not have their best spring.
For many, memories of that hard April frost will be erased by this week’s temperatures—approaching 90°F as I write—and the reappearance of seersucker suits in midtown. Those of us who love plants will be reminded of April 16 every time we see an old-fashioned Hydrangea macrophylla over the next few months. With the exception of remontant (re-blooming) varieties such as Endless Summer® (more on these later), Hydrangea macrophylla flower from buds formed during the previous growing season. Dormant through the long winter, these buds began to swell as temperatures finally rose in early April, only to be zapped by the hard mid-April frost.
