Readings on Hydrangea and Their Flower Hardiness

“I don’t know why my Hydrangeas don’t flower” is one of the great conundrums that are handed to me by clients. As with all things, I never know exactly, nor do I have an clear answer, I can only offer that coming to understand these things takes place in the process of experiencing and working with the garden. This question almost always comes while looking upon a Bigleaf Hydrangea (Hydrangea macrophylla). Immediately one asks how it may have been pruned, the gardens history and care, considers the soil, and how cold it was the past winter knowing that Hydrangea macrophylla often experiences tip dieback where you loose the flower buds to the winter’s cold. 

A week or two ago I ordered Michael Dirr’s latest edition of “Hydrangeas for American Gardens” (2020) to begin a little study on Hydrangea. I am finding them becoming so popular, even in my own preferences, that some study is a worthwhile investment. At least at the start of this, it was Hydrangea paniculata and its cultivars that I am most interested in.

However, a little tidbit caught me while reviewing Dirr's work on Hydrangea:
Bailey (1989a) provides climatic data for the native habitats that explain Hydrangea macrophylla sensitivity to certain extremes. Honshu enjoys more than 5 months of frost free temperatures. The mean low and high temperatures are 31 and 47 degrees F in January and 72 and 85 F in August, respectively…The conclusions are obvious: H. Macrophylla prefers moderate temperatures…(73).

Most of Dirr's writing on macrophylla is based on studies done at his University in Athens, Georgia and from research done in North Carolina. Even at those southern locations, some of the named cultivars he writes of as not surviving the cold are names I recognize as macrophylla cultivars in WNY nurseries. From this I leapt to: “All these Hydrangea I see that don’t flower - they probably just aren’t hardy enough to begin with.” So. I thought I would do a little research on the cultivars most readily available to me right now and find what research I could on hardiness data.
One key is understanding the “remontants” or re-bloomers. These bloom multiple times because they will bloom on this seasons new wood as well as older wood. A hydrangea that doesn’t bloom on new wood is dependent on old wood to over winter for its flower buds. When they are killed off by frost, no flowers. But with re-bloomers, the old wood can die back to the ground but still produce flowers that season on its new wood.  I believe most traditionally we think of Hydrangea macrophylla as a plant that "blooms on old wood." 


The Study:
Generally, and this is new to me, it seems that plant breeders characterize Hydrangea macrophylla as “undependable.” Thus my curiosity of "why is this such a commonly used plant in WNY?"
To disclose, this isn't the deepest research. A few hours. I made the best sense of things reading a couple blogs, Dirr's latest edition of "Manual of Woody Plants," Dirr's latest on Hydrangea's, and a few wholesale catalogs and nurseries I have here in the office. Here is what I have found on the cultivars:

1. ‘All Summer Beauty’ - Supposedly a rebloomer but noted as inconsistent in its reblooming by Dirr
2. City Line Series. ‘Berlin Cityline,’ ‘Venice Cityline,’ ‘Vienna Cityline,’ ‘Paris Cityline:’ German bread trademarked series. Dirr observes as being heavily covered in mildew in late summer and fall:  writes “I have immense doubts about their hardiness.”
3. Teller Series. Swiss breed. Dirr notes ‘Teller White’ being sensitive to late spring frosts in his Georgia garden. ‘Teller Blue’ - seen locally as ‘Blaumeise’ - is considered the hardiest of the series. Dirr writes, “Most Tellers are not very cold hardy.”
4. ‘Lady in Red.’ Dirr’s 2009 Manual describes this as having “superior cold hardiness,” which at this point, I think means, you are probably going to lose a lot of flower buds to frost and cold in the WNY area. But. I might like to give this a try. (For some reason I feel I saw it referenced as being ideal for container planting - which is how I will try it)
5. ‘Mystical Opal.’ Of the trademarked Mystical Series. Dirr writes of these as “bred for the florist trade,” and their use as pot plants - which I take to mean, these are the ones you see in the super market in one gallon containers. Does not appear that they rebloom. I don’t get the impression these are intended to be a landscape plant. However. I do see them in nursery catalogs.
6. ‘Pink Beauty’ also referred to as ‘Preziosa.” Each nursery I reference lists ‘Pink Beauty’ as a macrophylla however Dirr (2009) lists as as a synonym for ‘Preziosa’ which is actually a hybrid H. macrophylla x H. serrata. The species serrata seems to be getting a good amount of attention the past two decades because of its potential for cold hardiness as its native habitat is in colder, higher elevations.  This doesn’t necessarily translate to ‘Pink Beauty’ being hardier, but maybe.
7. ‘Tokyo Delight.’ This is listed as a macrophylla out of an Ohio nursery but Dirr lists this (2009) as a straight cultivar of serrata. 
8. ‘Nikko Blue. Dirr writes “May be somewhat of a rebloomer.” 
9. ‘Endless Summer’ and ‘Twist-n-Shout.’ First of the repeat bloomers, however, plantain Tim Wood writes of them as still lacking as flowers on new wood don’t bloom until very late in season. 
10. ‘Bloomstruck Endless’ is one of the most recently released cultivars. Cross of “Endless Summer,” which brought the first reality of a repeat bloomer to the market. My study on this seems to record the recent history of plant breeding. 2004 was the year ‘Endless Summer’ was introduced. This has since been improved with ‘Bloomstruck’ series of cultivars which I believe were released in 2014.

What I generally find in the literature combined with my experience is that Hydrangea macrophylla's flower buds, those on old wood, are most often probably killed off. A Hydrangea can be listed as cold hardy zone 4 (for example) but this just means the plant survives, it doesn't mean the flower buds survive. It seems that even all the new rebloomers and remontants, while exciting in concept, they generally seem to be written of as disappointing - however the most recent 'Bloomstruck' series appears to be the best yet and worth my experimentation. 
The thing with a rebloomer is, it becomes a plant that blooms on this season's wood, and thus later in the season, which means a shorter flowering season. It seems the recent improvements in Hydrangea arborescens are perhaps more worth exploring.

​Just some notes.   

Post Script. April 6th, 2021.

I have been watching the Hydrangea this spring. I can see that possibly the buds on the tips get through the winter as they have the opportunity to acclimate to the deep cold of winter. But now, the beds are starting to swell at the end of March, beginning of April. The inevitability of a hard frost here in early April - well - these swelling buds are probably most susceptible to frost/cold damage now. More so now than the first week of February.