Panicle Describes the Cone-Shaped Flowers of Two Hydrangea Species
The botanical term "panicle" doesn't have anything English-language-like about it.
When I heard somebody say mophead hydrangea for the first time, I got it because I can conjure up a mophead shape.
Panicle is Latinate from the word "panus" so, unless we've studied a lot, we don't conjure "an ear of millet" - which is what the word panicle is meant to tell us if we knew the language (dead).
Botanical terminology uses the word panicle to describes the complicated structure of these hydrangea flower heads - the cones of flowers we love in our midsummer gardens.
Panicle Flower Structure
Botanically, a panicle is an inflorescence made up of many smaller "branches". Simply defined in botany texts as a much-branched or many-branched inflorescence, but I'm going to go into more detail.
Each branch of the panicle is a raceme. Sometimes racemes stand alone, like in lily of the valley flowers. On these hydrangeas, racemes are grouped together to form panicles.
The inflorescence is the entire cone-like structure, while the individual flowers are called florets. They are called florets instead of flowers when they're grouped together in a compound flower head, like these panicles (and the corymbs described under mopheads and lacecaps).
As we just learned, a panicle inflorescence is comprised of racemes. Along the racemes the florets are borne.
The florets are attached by stalks (called pedicels) and a very interesting phenomenon happens - the florets at the base open first. Gradually, the florets open along the raceme until they are open at the top and the ones at the base are aging.
Just like with lacecap and mophead flowers on bigleaf and mountain hydrangeas, the florets are either sterile or fertile and the petals are actually modified leaves called sepals.
Panicle Flower Colors
Unlike bigleaf and mountain hydrangeas, the flower colors on both panicle and oakleaf hydrangeas are unaffected by the pH of the soil and, likewise, unaffected by the presence or absence of soil aluminum.
However, age causes both panicle and oakleaf hydrangeas to change colors - the white inflorescences antique to beautiful shades of pink, rose, mauve, and even nearing red.
They antique different shades of rose depending on the cultivar type, so if a light, medium, or deep color are important to you, do your research on each cultivar.
Flower Colors on Hydrangea paniculata, specifically
In the case of panicle hydrangea, the combination of sunlight and cool temperatures promote deeper and richer antiquing. That means if you're growing your panicle hydrangeas in too much shade, they won't deepen their rose tones as well as if they're growing in a sunny location.
It's important to note how important cool temperatures are in the antiquing process of panicle hydrangea cultivars. Rule of thumb is that in zones 7b going northwards, you get increasingly fabulous antiquing of panicle hydrangeas (again, this pertains specifically to H. paniculata cultivars).
Couple these climate factors with a cultivar type plus age of a particular cultivar's flower that year, and some very deep, rich rose and mauve colors (some are even red) manifest themselves in late summer through fall.
In our hot zone of 8, we seldom get to experience panicle hydrangeas turning even pale pink. They stay white for an exceptionally long time, but they only pink (rarely deeply and certainly rarely turning red) in cool-ish summers.
I'll give you a cool temp baseline for zone 8: summer of 2018 we surprisingly experienced an August where the daytime temps would hit into the 90s, yet the nighttime temps went down into the upper 60s - and so we enjoyed some beautiful pinking on most cultivars. Nighttime temps in August are typically in the 70s and in those years the flowers turn from white to cream to brown - usually no pinks for us, but the white flowers are never-the-less lovely and the health of the shrubs themselves is just fine.
As you read this article were you able to keep in mind that the common name for Hydrangea paniculata is named after this type of inflorescence and is called "panicle hydrangea"? I've tried to make a clear distinction between referring to the inflorescence type or to the species. Please comment to let me know when I can improve clarity.
May you enjoy all the water and light your garden needs,
Hillary & Mike