Have you ever heard that bit about there being lots of Eskimo words for snow? Where Eskimos, because they deal with snow so much, supposedly have many more words to describe snow and its nuances. True or not, it is a fact that we more commonly use words for things we experience commonly. So I offer that as an explanation for why I (at least) haven’t really ever heard of hoar-frost. Frost has always just been frost for me. But I recently came across the term, and on further investigation, I learned that not only is there a special kind of frost called hoar-frost, but there are other types too.
Frost is the white crystals that form when the temperature drops enough to make the humidity in the air condense on to objects in the landscape. The more temperate the climate, the less likely and the less variety of frost you will see. But as a climate has colder temperatures, there is greater opportunity for different types of frost to form. Knowing the types of frost that you get where you live, is actually very important if you want to create a safe and magical winter garden.
Types of Frost
Hoar is a type of radiation frost that forms when the temperature drops quickly and water in the air forms crystals without going through a liquid phase. In order to get hoar-frost, there must be a relatively high level of humidity in the air. We don’t get hoar-frost very often in New England (presumably because despite our humid summers, winter is very dry). But when I lived in actual England, I noticed it relatively often. I also remember it to be a little more common back home in Colorado – and I suspect this due to pockets of humidity that can surround melting snow.
White frost and hoar-frost are very similar except that white frost is thicker and therefore whiter.
Designing For Hoar Frost and White Frost
If you get hoar-frost where you live with any regularity, it is worth considering it when planning your garden. Hoar frost is beautiful and it is best seen in the mornings before the sun melts the crystals. Sculptural plants that persist into the winter are stunning with a coat of hoar-frost. Consider grasses, plants with interesting seed heads and distinctive shapes (like clipped hedges and topiary). Plant them enmasse to heighten the effect.
Advection Frost (also called Wind Frost)
Advection frost happens when the cold air that forms the ice crystals moves across a surface in a horizontal direction. This type of frost can be damaging to crops but it also makes some interesting and pretty effects in the garden. Ice crystals tend to form on the edges of flowers and plant surfaces rather than over the whole leaf or petal and the crystals are often satisfyingly directional – pointing to where the air moved.
Designing with Wind Frost
If you have prevailing winds in your garden (as I do!) you might notice that trees and large shrubs might already be slightly lopsided – leaning away from the wind source. The wind in my garden comes up the hill and anything on that side of the garden is always blown to one side. I have considered planting a hedge or planted wind break and if you have this issue, you may want to consider that as well. A fence (which is a nice wind break and uses a lot less space) is providing that purpose in one area of my garden already. Or, you can embrace the wind, grow to appreciate the asymmetry it provides, and in the winter, watch for some beautiful and interesting shaped ice crystals.
Rime and Glaze Ice
Rime ice is a more substantial version of hoar that requires liquid water droplets in the air freeze onto a surface. This allows the ice to grow larger and heavier crystals that can be beautiful but often damage trees, plants and even structures. Similarly, Glaze ice forms when rain freezes and it creates shiny smooth ice that coats everything and builds up over the course of a storm. Both of these require a unique set of meteorological circumstances, but when it happens the beauty and the damage is quite dramatic.
Designing for Rime and Glaze Ice
If you get regular Rime ice, or ice storms (like in New England) it is wise to cut back (or not plant) pines and trees that overhang structures and power lines. Broken trees tend to bring down overhead power lines and sometimes buildings have been known to collapse under the weight. Preventing damage is hard, but some trees (like pines, for example) have a lot of winter surface area (think of all those needles) to hold the ice and can be especially prone to this type of damage. Think about using plants and trees that have strong natural structure and that do not have a lot of surfaces in the winter on which the ice can gather (ie. leaves are dropped and no needles)
Window Frost and Fern Frost
Window frost and fern frost, also called ice flowers, forms on surfaces like windows. Unfortunately, if you regularly get this type of ice on your windows, it means that they aren’t well insulated. The flowers and pretty winter patterns are the up-side of a bad energy situation. The patterns are caused by a difference in temperature on either side of the glass. Windows aren’t generally in the garden, but you can sometimes see this kind of ice on small ponds, puddles or water features.
Designing for Window Ice and Fern Frost
Fern Frost tends to be feathery or in swooping concentric shapes that indicate that there was a progression of cooling at different levels of the water. Also, other interesting patterns can be appreciated in ponds and streams and even fountains left to run in cold temps. As new layers of water freeze and then contract, water beneath fills in, freezes and caused new fissures and lines. These patterns can be quite spectacular. If you have a water feature in your garden the patterns tend to be most visible at the shallow edges.
Black Frost (or killing Frost)
Not really a frost at all, this term refers to when the temperature falls too low for frost to form and instead, plant tissues freeze, die, and subsequently turn black. This is common with many vegetable garden plants like tomatoes and other nightshades, beans, squashes, and peppers. This is a very normal thing to have happen when the first frost of the winter season occurs, but if the frost is very early, you might want to extend your season by protecting plants before the frost happens. This is done with frost blankets or cold frames.
Hoar frost turns a garden into a magical fairy land. Rime (which is a thicker and heavier) tends to rip gardens apart. The difference is dramatic.
We garden writers and designers have a tendency to go on and on about planning for four seasons of beauty, and I’m not knocking that. But if you don’t ever get hoar-frost, you might need to make different considerations for your winter garden. If you get a lot of rime, you should also make special considerations.
Gardens with regular hoar frosts, should plant and leave your grasses and sculptural plants untrimmed through the winter so that you can enjoy their beautiful frosty shapes. Focus on forms that persist through the winter – like trimmed hedges and other shapely plants. But if you don’t get the hoar, I’d argue that it doesn’t matter so much.
Alternatively, if you are prone getting rime ice, you should cut back on the weak limbed pines that will surely lose massive branches under the weight of the ice. Also, watch for any plants that might be near electrical or service lines as the weight of a tree or plant covered in ice will rip down wires and cables as they fail under the stress.
I am envious of places that get hoar-frost. Looking at my tattered and tired grasses that get knocked down by the first snow – and never stand pretty with frost – I have resolved to make myself a happier gardener by cutting down some of the most visible grasses back in the fall. These always greet me with messiness that annoys – but I do leave the rest for wildlife to use through the cold season.
I gave up my hoar-frost hopes years ago and instead I think about what will look “winter-good” with a pile of snow on it or what will look good with just nothing as we have our share of barren winters too. It is worth pointing out that while you are planning your winter garden, you should think about where your snow removal piles go. This is not a place to plant things that will easily be damaged under the cold dead weight. Plant shrubs that can be cut back hard in the spring or fall and perennials that die back to the ground. Midway through any winter I tend to start feeling defeated by ever-growing pile of snow at the end of my driveway. But I think that being realistic about a New England winter – or whatever kind of winter you get – will help you be more out of your garden in all the seasons that you really do enjoy it.
How about you, do you get hoar-frost where you live? Does it make sense for you to plan for a magical wonder land?
images telegraph and maison and jardin via la pouyette, a year from oak cottage, 黄 赛 on Unsplash, Annie Spratt on Unsplash, Sandra Frey on Unsplash, Aaron Burden on Unsplash, Chloe Ridgway on Unsplash