White Frost, Green Leaves

Rime on trees_BlkForest_20091011_LAH_3876We’ve been enjoying some glorious autumn foliage these past few weeks, but there are plenty of plants that remain stubbornly green. In fact, their leaves stay green no matter what the season—that’s why we call them evergreens. With winter just around the corner, I began to wonder—how do evergreens survive our cold winters? Why don’t they lose their leaves?

Before we answer those questions, we need to make s short detour to clarify some terms. There are two kinds of plants commonly grown in our gardens. Conifers have needles (such as pines and firs) or scaled leaves (such as junipers, cypress, and arborvitae). Most, but not all, conifers are evergreens. (Tamaracks and larches are two examples of deciduous conifers.)

Broad-leafed plants include most shrubs, non-coniferous trees, grasses, and “herbaceous plants” (like annual and perennial flowers). Some broad-leafed plants are deciduous (such as oaks and roses), and some are evergreen (such as rhododendrons and orange trees).

For the rest of this post, “evergreen” refers to plants that keep their leaves all winter, whether they’re a conifer or a broad-leafed plant.

Mahonia repens - Oregon Grape Holly_DBG_10200118_LAH_6955For the most part, evergreens grow in temperate or tropical regions while deciduous plants tough it out in colder climes. The big exception to this are conifers. Some kinds of conifers grow in very cold regions, such as the boreal forest that circles the arctic tundra. But there are broad-leafed plants that survive freezing—firethorn (Pyracantha) and holly (Ilex) come to mind. Even here in Colorado, I grow a number of evergreen shrubs and groundcovers —Manzanita, Oregon grape holly (Mahonia sp., left), and various Veronicas, for instance. How do they survive our winters?

_BlkForest_20091011_LAH_3864It turns out that hardy evergreens have a variety of ways in which they deal with freezing weather. One approach is to concentrate chemicals, often sugars, in the cells: much of the water is pumped out and replaced by sugars, starches, and other chemicals. This lowers the freezing point inside the cell and allows some species to continue to photosynthesize even when the weather is at, or even a few degrees below, freezing. (The extra water ends up freezing between the cell membrane and the cell wall, where it can’t do any damage.) Triggered by shorter days and diminishing temperatures, this process takes time, which is why sudden cold can kill plants that are otherwise perfectly hardy at the same temperatures.

But what if they do freeze? The main problem with freezing is cell damage from the sharp ice crystals. Some sugar solutions can freeze without forming crystals (a process called vitrification), which means that there are no sharp points poking holes in the cells.

Finally, many hardy plants have extra tough cell walls that prevent the cells from bursting when they freeze.

2009-04-18 Snow in yard 017There’s an even bigger problem than freezing—the lack of liquid water. Plant roots can’t absorb ice, so conditions in winter re almost like living in a desert. It helps to have tough, leathery leaves with thick, waxy coatings that act as a moisture barrier. Shelter helps too, whether it’s a thick layer of mulch or a snowbank. There’s a balancing act here, as being buried for too long with no air circulation can lead to a toxic buildup of carbon dioxide and ethanol. That is why many evergreens go dormant over the winter, “hibernating” until spring’s warmth wakes them up. If the plant is dormant, it won’t produce any unwanted by-products. Moreover, it can keep its stomata closed, further reducing the loss of water vapor.

DSCF0008Those of us who live in cold climates already appreciate hardy evergreen for their beauty in an otherwise stark landscape. Now that it turns out these seasonal stalwarts are finely tuned winter survival machines, I’ll be appreciating them even more!
If you find all this interesting, you’ll really enjoy reading this post on “How Do Trees Survive The Cold?” at the olive tree. I did.

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