Plants that are identified as suitable for Zone 5 may be damaged when temperatures drop below that threshold. I try to plant Zone 3 and Zone 4 plants when I can find them, but many of my favorites are listed as appropriate for Zones 5-8. That means they may be injured or killed when temperatures drop this low.
An important factor in using the Hardiness Zones to determine plant survivability is how much snow is expected along with the cold temperatures. Snow is a great insulator against the extreme cold. It protects the roots of the dormant plants from the frigid conditions. Plants suitable for Zone 5 and Zone 6 can actually handle temperatures in the Zone 4 realm if they're protected by snow cover. The snow is cold, but it tempers the ground, roots, and base of the plant. The deeper the snow, the better the protection. Believe it or not, the temperature of snow on the bottom is warmer than snow on the top.
The temperature of snow is close to the temperature at which water freezes, 32 degrees F (or 0 degrees C). The surface temperature may be colder than that because it is affected by the temperature of the surrounding air. The temperature of the snow on the soil is much closer to the freezing point, 32 degrees F. So a plant that can handle Zone 5 temperatures of minus 20 degrees F is really only exposed to temperatures closer to 32 degrees F if it is covered by snow. The deeper the snow, the more likely the temperature will be warmer on the bottom. A light layer of snow will not have the same insulating effect.
I get most concerned by frigid temperatures when no snow or little snow is forecast along with the cold. Tonight is such a case. Snow is in the forecast, but not much. So to try and protect my Zone 5 plants, I add my own insulation.
My young lavender plants are the most susceptible to possible damage. Zone 5-8 plants, they may suffer from temperatures below minus 20 F. By surrounding them with insulating material, I hope to achieve the same effect as a deep snow where the bottom temperature is close to the freezing point, because the ground is frozen, while the outer layers absorb the severe cold.
|A cold lavender plant.|
A simple method of insulating plants is to cover them with straw or similar material. The air pockets created within the straw insulate in the same manner as the smaller air pockets in layers of snow. A light layer of snowfall on top of the straw will increase the insulating value.
I covered each of my young plants with a clump of hay. The piles are slightly compacted to increase the quantity of hay blades but loose enough to allow air pockets. There is no reason to worry about restricting sunlight because the plant is dormant and there's no sun today anyway.
|Adding a covering. More is better.|
The plants were protected by tomato cages so that dogs and grandchildren wouldn't trample them. Today the cages do double duty by helping the hay stay in place as winds pick up. It's snowing now and should snow for the next day or two. Though it won't be as deep as recent snow in the Northeast, it should mound over the hay and also help hold it in place.
When all of the weather effects are over, the temperature at the base of the plants should be warmer than the outside air. I only need a four or five degree buffer to get the air temperature back into the Zone 5 range, but with physics on my side, I should expect even greater protection than that.
|My lavender, cages, and hay.|
The cover can stay in place as long as the temperatures are cold. Once the sun comes out and starts melting the snow, the hay needs to be removed. Wet hay can quickly rot and begin to decompose. When it does that around a plant, the plant can also be susceptible to rot and damage. With warmer temperatures there is no reason to maintain the insulation and removing it allows air back in to warm the plant and dry up any potential areas that are too wet.
Tonight and tomorrow may be the coldest point of this winter. Tomorrow's high for Colorado Springs is forecast to be minus three degrees F. Yes, that's three degrees below zero for the high temperature. As we move closer to spring, daily temperatures will increase gradually. If my plants get safely past the extreme lows tonight they should have little difficulty surviving to the warm days ahead. My hay-covering efforts should help. I'll let you know how it turns out.