Cold hardiness can be a bit on the tricky side

Gardening in Alaska is all about hardiness, right?

But how do we know if a plant is worthy? Typically we look at the hardiness rating from the USDA, find out it’s in the right zone and plant it.

Or do we?

While this is an excellent, at-a-glance guide, it doesn’t tell the full story. It gives us extreme low and high temperatures to follow, but nothing else.

We tend to look at cold hardiness first around here, so let’s take a glance at what’s actually happening inside a plant to make it withstand freezing temperatures.

Plant sap contains a solution made up of a number of dissolved compounds such as sugars, salts and proteins. The higher the concentration of this solution dissolved in the sap, the hardier the plant is to cold (the lower its freezing point).

How does this happen?

As temperatures drop, the sap migrates outside the cells into spaces between cells, called intercellular spaces. Here it collects in an even higher concentration, forming a type of insulation that protects the cells. If there is sufficient time in the fall with temperatures dropping slowly, this solution will make the transfer and insulate the plant’s cells. This, in turn, insures the plant cold hardiness. If a plant’s genetics do not provide it with a sap solution high in the right dissolved compounds, it will never have insulation to withstand extreme cold.

Even with the right genetics, however, bad timing can toss this protection aside.

If temperatures drop too quickly in the fall, the sap solution does not have time to travel to its protective position in between cells. Ice crystals form inside the cells, bursting and killing them. If enough of this cellular damage occurs, it will kill the plant.

If a plant has made it happily through the fall and winter and into spring, followed by an abrupt re-freeze, it once again may die. The same type of thing can happen if our winter temperatures rise dramatically above freezing, stay that way for a week or two, only to plunge once again into bitter cold. The problem with these last two scenarios is that the sap has begun to move back into the cell (out of its protective place in the intercellular spaces), thus cannot fully insulate the cells upon re-freezing.

To complicate things even further, plants may have the right genetic components in their sap and be able to transport it at just the right time to keep its physical adaptations to cold, freezes, thaws and early spring temperature fluctuations, but may have no tolerance for wind.

Plants usually respond poorly to wind because it tends to dry out the surrounding soil, thus robbing them of their moisture. In the summer, we see this most often in fast growing deciduous shrubs and trees. In the winter, it most often affects conifers.

Conifers keep their leaves (needles) all year long, thus continue to photosynthesize and use water all year. When our world is in the deep freeze, these evergreen wonders are still seeping up tiny amounts of moisture through their roots. They survive on frozen reserves during deep colds, drawing up viscous water when the temperature warms just enough to allow this transfer. It is good to remember here, that plant roots actually continue to grow until the air temperature is around 20 degrees, and while growing, continue to pull in minuscule bits of water. When an evergreen winter burns (goes all brown and crispy, looking like fire starter), it is most likely because its already limited winter moisture supply has been reduced due to wind desecration of the surrounding soil. To exacerbate this problem, wind also pulls moisture directly from the plant’s needles.

Once again, if enough of this damage occurs plants will die.

What about that fabulous shrub that made it through a miserable winter full of thaws, freezes, bitter cold and nasty winds?

You thought you were home free.

Drought hardiness, seed hardiness (yes, seed hardiness), damp tolerance, hardiness to disease, and soil adaptation are all to come.

No, I’m not trying to scare you to death, and if you’re in that lucky group of folks who can grow anything, perhaps this is not for you.

As for the rest of us, we need all the help we can get.

Sally Koppenberg is a garden and food designer and the owner of Stonehill Gardens, a nursery & nature conservatory specializing in Alaska grown trees, shrubs, perennials and native plants.