Photo Credit: Linda Langelo , CSU Horticulture Program Associate
Yucca in Phillips County Extension Event Plant Select Garden
In these below freezing temperatures, you and I can put another layer of clothing on and be fine. How do our favorite perennial plants survive? Throw on another layer of mulch or evergreen boughs? The answer is not necessarily all that needs to happen for the plant's survival.
Their best defense against the cold is mainly an accumulation of solutes such as sucrose and other organic compounds such as proline in plant cells. These solutes have the amazing ability to depress the freezing point of water from 32 degrees Fahrenheit to 20 degrees Fahrenheit. More importantly, proline is an amino acid. It plays a critical role during a period when a plant starts to undergo stress. Extreme cold or extreme heat both stress plants. Other environmental stresses are salinity, water deficit, toxic metal ion concentration and UV radiation.
But as far as secreting solutes to help mitigate cold- how cool is that? Wouldn't that be great if we could do that especially in winter to ward off needing to wear a coat? I know. That is a bit of a reach.
So now that we know that plants have this amazing ability to secrete within their cell wall proteins that prevent ice crystals from forming on the outside of their cells. There still is more to this story.
Dehydrins in the cell cytoplasm bind water molecules. This changes the structure of water in the cell thereby stabilizing the cell wall membranes. Even more cool! Lastly, lipids also play a role in protecting plant cells.
Lipids include among many substances in plants fatty acids, fats, oils, steroids, waxes, cutin and so on. Lipids are insoluble in water and serve many functions including storage of metabolic energy, protection against dehydration and pathogens, the carrying of electrons, and the absorption of light.
During periods when the winter temperatures begin to go to freezing or below, the plants alter lipid composition to protect the cellular membranes. Plants have it all figured out!
Photo credit: Linda Langelo, Phillips County Extension Event Center Plant Select Garden
Now that we figured out how plants do it, let's take a look at using mulch or a layer of evergreen boughs as plant protection. Mulch does act like a blanket on top of your soil. It mitigates the freezing and thawing of the soil. So yes, mulch does help keep the soil temperatures more even if done at the proper time - before the soil freezes.
I discovered a few years ago, (even though I know intellectually) that mulch of any kind can mitigate the soil temperatures during winter. When putting up Christmas lights, in particular, lighted candy canes, the soil was not frozen below a thin layer of leaves and it was at night with the air temperature in the low 20's. The leaves were about a quarter of an inch thick. It was so cool. Nature in action. Where the ground was bare, the soil was frozen. The leaves slowed the soil's dissipating heat which accumulates all summer long.
The evergreen boughs can help with our winter's desiccating winds. The boughs also shade the plants from early spring sun.
With hardy plants such as perennials, their ability to come through winter is in their genes. However, it does not hurt to have extra protection around the root zone of your perennials. If they are semi-evergreen or evergreen a little protection over the plants with evergreen boughs helps them anyway. It becomes very impractical and cumbersome if the evergreens you are attempting to protect are 4 feet tall. Then you have to ask yourself why you purchased a plant that was not rated for the zone you live in. Remember it's in their genes. If it is not in their genes to survive in your zone, then expect to treat it as an annual.