Mountain Hydrangea is a Controversial Species, But Needn't Be
Hydrangea serrata (mountain hydrangea) is very similar to Hydrangea macrophylla (bigleaf hydrangea). So much so that some botanists think it's a variety of macrophylla, rather than its own species.
Because of a few qualities that strongly distinguish it from macrophylla, we treat them as a distinct species. This also follows how most current writers and nurseries differentiate it. We'll delve into those different qualities here.
I mention that nurseries differentiate mountain hydrangea from bigleaf. In the catalogs I peruse, the hydrangea we're discussing is listed as Hydrangea serrata rather than Hydrangea macrophylla var. serrata, which is the way it's listed in old books.
In our nursery, we see a big difference in how we treat the water requirements of mountain hydrangea cultivars. During the summer, bigleaf and mountain hydrangeas get a similar amount of irrigation due to water loss (evaporation) from the potting soil caused by high heat and strong sunlight. In addition, there is water loss from the plant due to transpiration (a metabolic process).
However, during the winter, we micro-manage how much water the mountain hydrangeas get because they don't like it cold and sopping wet. We watch the weather and cover them so they get less water than the bigleafs.
We don't let them dry out, but if the soil is already wet or wettish, and we see a storm coming, we cover them so they have the chance to dry out a little. In our nursery setting, covering and controlling water works great to improve overwintering and spring leaf out of our mountain hydrangeas.
For plants in our garden - in a rich, moist, well-drained soil - we don't see these struggles with winter wet.
If you're growing your mountain hydrangeas in a pot on a patio and are experiencing winter loss, brainstorm a way to control how much water they get during the winter. Drag them under a shelter or cover them where they sit. Keep them moist, but not sopping for weeks like the winter weather imposes on us here in the southeast.
Leaf Shape & Size: The leaf shape of mountain hydrangea is similar to sister bigleaf, though they are typically smaller or narrower. They never get huge and fat in the middle, like, say, a 'Merritt's Supreme' leaf.
Leaf Margin: The distinguishing leaf difference is that the margins are more serrated. "Margin" is the botanical term for leaf edge; and by "serrated," that's a word to describe the teeth along the edge. The teeth on mountain hydrangea are more pronounced and come to a fine point. There seem to be more serrations too.
See what happened? How the botanical name "Hydrangea serrata" describes the leaf margin?
Leaf Luster: Mountain hydrangea leaves are not as lustrous and those of bigleaf hydrangea. They have a matte finish, rather than glossy or lustrous.
Lustrous leaf and larger size of bigleaf hydrangea, as seen on the top/right side of this leaf collection. Smaller, duller leaves of mountain hydrangea are shown on the left/bottom.
Leaf Color: Mountain hydrangeas have green leaves - same color green as the bigleafs - but oddballs turn red here and there on a bush by midsummer. This is a distinctive ID trait, so if you're looking at the leaf size and serrations, then see some red leaves - you will get an A+ on the identification quiz if you put down "Hydrangea serrata" as the species.
The degree of red tinting varies from a smattering of 5-10% of the bush to 50% as seen on this particular Lady in Red hydrangea.
Lady in Red hydrangea has H. serrata in her diverse parentage - as seen in the red tinting in the leaves due to growing in a sunny clearing on Cape Cod. Extra sunlight coupled with cooler temps seem to induce the red tints on mountain hydrangea leaves.
They Hail from Different Geographies
You know how some people grew up in the mountains and they let you know that? Likewise, if they grew up on the beach, they let you know?
That kind of geographical birthright separates mountain from bigleaf too.
Mountain hydrangeas are from the mountains of Japan and Korea - hence the common name mountain hydrangea. Bigleafs are from down the mountain, they are coastal; seaside, even.
Again, the name mountain hydrangea is a hint. It likes a little cooler summer climate and is decidedly more cold tolerant than sister bigleaf who likes it a little bit warmer and tolerates a maritime climate - like where both naturally grow in Asia.
Here in the Deep South, mountain hydrangea isn't as popular because it certainly struggles in too much heat. Where we're located, in the upper parts of zone 8a, this is as far south as you want to go for the peace and happiness of this species.
In our shady spot, we believe we have a microclimate that may put us in zone 7b. It's always a few degrees cooler out here in the country than it is in downtown Athens, GA. This is especially evident in the summer. Our serratas look excellent and I'm becoming more enamored with their vigor and beauty.
I hadn't seen too much of this species due to where I live, so my eyes were boggled when I saw 'Blue Billows' and 'Bluebird' as ubiquitous types on Cape Cod (zone 7b). They are stunning! Absolutely smothered with flowers!
Lacecap and Mophead Inflorescences
The "smothered in flowers" exclamation leads me to the next difference between the two species. The lacecap and mophead flowers on macs can be flagrantly obnoxious in their huge size. Mountain hydrangeas are refined and dainty by comparison.
However, these refined flowers cover the plant from head to toe especially when grown with plenty of sun (yikes! but don't go planting yours in full sun unless you KNOW your climate will accommodate that - full morning sun with afternoon shade is best). I am under the impression that mountain hydrangeas produce more flowers on the bush that macs do. They make up in number what they lack in size.
Interestingly, the mophead versions of mountain hydrangea are rarely seen. I can name only two off the top of my head: the perky Preziosa and the tiny bite-sized mophead named Maiko (a.k.a. Little Geisha).
A constant between macs and serratas is that the flower colors on both are changed by pH and aluminum availability.
Preziosa flowers bob above the foliage. This pastel type is typically seen in pale pink, pale lavender, or pale blue. Here we have pale lavender, with a few pink ones on the right.
We have four mountain hydrangea cultivars in our nursery; three of them being lacecaps and one lovely double-sepaled weirdo, Fuji-no-Taki, that I think is a "modified lacecap" - something between a mophead and a lacecap. To me, a modified lacecap looks like a misshapen mophead.
The tiniest hydrangea flower that I know of happens to be a lacecap mountain hydrangea. 'Diadem' is her name.
Yet another distinguishing feature is that mountain hydrangeas don't grow as large. I saw old specimens on Cape Cod and none were over 5 feet tall. Well, they weren't even close to 5 feet tall. Three to 4 feet is the typical size.
This shorter stature makes them easier to fit into containers and smaller gardens.
Know you know how mountain hydrangeas are different than bigleafs and have absorbed the varied aesthetic nuances. Please consider these many ways mountain hydrangeas can be a part of your garden or hydrangea collection.
A final reason is a competitive one: your neighbors won't have one . . . at least not until they see how cool yours is!
If you're interested to learn more about mountain hydrangea flowers, we wrote an article covering just the showy reproductive organs (a.k.a. flowers): What is a Lacecap Hydrangea?
Hillary & Mike