A Gardening Journal
Fabulous in the Fall: Frosted 'Rock Garden' Holly
- Published: December 09 2016
Plants differ in their ability to display frost, snow, and ice instead of just being battered, broken, or buried by them. One sunny morning after a sharply cold night, just the edges of the leaves of this dwarf holly were encrusted with dense crystals. For a few hours until the sun's warmth melted it, the frost brought spectacular white variegation to the foliage of a shrub that's normally solid green.
This shrub is a dwarf spiny holly, Ilex x 'Rock Garden'; I grow six of them in three large troughs. I made the troughs some years back and, at eighteen inches, they are unusually high. This elevates these slow-growing bushes so that they receive plenty of sun. But it also exposes them—roots, too—to deep freezes they would have escaped growing in-ground, where Rock Garden holly is reliably hardy to climate zone 5 provided it enjoys excellent drainage.
In my not-quite-zone-7 garden, this holly needs winter protection to survive in containers. Before the cold season reaches full force, I spray the bushes with antidessicant as well as cover them with a layer of wind-baffle fabric. Plus, I set bales of hay around their troughs to slow down freezing and thawing of their soil masses. This protection means that the hollies aren't exposed to open air January through March. They're invisible—and, therefore, remain viable—but the cost is that they can't display frost, ice, or snow. A December show like this one, then, is a rarity and, so, all the more welcome.
At closer range, you can see the distinctive nature of the frost. Countless needles of it have crowded the edge of each leaf. There is almost no formation in the centers of the leaf blades, and what little there is, is granular; frost along the edges is dendritic, which is the Latin for branched and tree-like.
Seen closer, the spiky nature of this on-the-edge frost is even more striking.
The look of the entire little shrub is striking, too.
When conditions are just right, this particular kind of precipitation—rime ice—accumulates along the edges of almost any exposed object: leaves, flower petals, branches, power lines, and the spokes of bicycle tires. What's needed first is fog or high humidity as temperatures drop quickly to just a bit below freezing. This chills moisture in the air without its immediately consolidating into frozen forms such as snow, ice, or frost, which would likely happen if the temperature plunged more deeply.
Such cold-but-still-liquid moisture is termed supercooled, and it remains suspended in the air until it encounters almost any solid object. Dust particles are particularly helpful, and when supercooled moisture comes into contact with them, the moisture is at last able to enucleate: to change to its solid phase by collecting onto and around that particle as ice or frost. Each particle, then, is the nucleus around which that liquid-to-solid transition can at last occur.
Cooks will recall a similarity in the behavior of sugar as it melts to form caramel. Why do recipe directions always caution not to stir? The implement could release tiny particles that could foster recrystalization of the sugar—enucleation, as it were—and a general siezing up of the caramel. Instead, errant sugar crystals are eased down into the molten mass by washing the sides of the pot with a clean brush dipped in water. Wetting the brush has inadvertently rinsed off some of the potential enucleating particles adhering to it. Plus, the droplets the brush releases onto those sugar crystals help dissolve them so they can slide down into the developing caramel below. Dissolving is always a wise option if enucleating is a risk.
Back to the rime-iced holly. What if the air is clean, with little dust to help enucleation begin? Then, supercooled moisture stays suspended unless it encounters some other object that is comparatively small, such as the thin edges of the holly foliage or the twigs of shrubs and trees. Mysteriously, larger surfaces—the interior of the holly leaves' blades, say—don't facilitate enucleation well at all, whereas narrow spiny edges seem to act like catnip in encouraging it. Once those first little crystals of rime ice form, they in turn are narrow and spiny enough to spur enucleation of still more crystals. The initial dust particles—or the next-best stand in, the thin edges of just about anything—are catalysts for what soon becomes a rime ice free-for-all. The result is the countless little individual stacks—dendrites—of rime ice.
Accretion continues as long as congenial conditions persist, and can result in dendrites that are inches long, not just the millimeters of those along the edges of my Rock Garden leaves. Take a look here at remarkable pictures, and in-depth comparisons of rime ice with another form of precipitation that seems similar—but is anything but—hoar frost.
Exposed locations seem to favor the start of enucleation. This makes sense, in that air movement there will be more free. This makes it all the more likely that supercooled moisture will bump up against something solid that will facilitate enucleation—and that other moisture will continue to encounter the now-forming crystals of ice.
The troughed Rock Garden hollies are out in the middle of the garden and, at least until I protect them with wind-baffle fabric, they are fully exposed. Below is a shot, taken immediately after those of the hollies, of another spiny broad-leaved evergreen forty feet away.
I sited this Osmanthus heterophyllus 'Nana' close to the house so as to protect it from wind. Comparatively little rime ice had formed on the edges of its leaves, whereas rime ice on the Rock Garden leaves was luxuriant.
Only days after the Rock Garden hollies had displayed rime ice, temperatures have begun to drop into the mid-teens Fahrenheit. It's time to spray the hollies with antidessicant, then cover the bushes entirely with wind-baffle fabric that won't be removed until later in March. The season for rime ice isn't over until Spring—but from now on it won't be forming on Ilex x 'Rock Garden'.
Here's how to grow Ilex x 'Rock Garden'.
Here's how handsome Rock Garden holly is when in berry.
Here's how to grow Osmanthus heterophyllus 'Nana'.