Freezing Weather and Why It Harms Plants
Since we have been “enjoying” freezing weather this week I thought it the perfect time to discuss some strategies plants employ to survive these arctic temperatures. But first, let’s explore why freezing temperatures are harmful to plants. The damage happens when ice crystals form inside plant cells. Ice causes two major problems in plant cells. The individual ice crystals act as tiny pins, which poke thousands of holes into the delicate cell membrane. This is a problem because plant cells are highly organized. When ice crystals form, poking all those holes, it causes the highly compartmentalized stuff inside the cells to ooze out of the cells. This causes the cells to die, and if the entire plant is affected, the plant dies.
The other problem ice causes for plant cells is a little more challenging to understand. The problem is actually dehydration. As ice forms, it leaves less water available to dilute salts, sugars, and other “stuff” inside the plant cell. This increased concentration of salts can damage the cell structure and also cause plant cells to die.
Some plants have adapted to the challenges of sharp ice crystals and cellular dehydration. These adaptations allow the plant to survive freezing conditions. However, not all plants can survive winter weather. For example, tropical plants do not survive freezing conditions. Summer annual plants survive by producing seeds. Though the plant can not survive freezing conditions, the seeds can start a new generation the following spring. In fact, often these seeds have to experience freezing temperatures before germinating. This strategy keeps the new seed from germinating in the fall and being killed by the freezing temperature. By experiencing freezing conditions, it signals that winter has passed and it is time to begin germinating in the warm spring soil temperature.
Some perennial plants have adaptations that allow them to live straight through freezing winter weather. A recent article published in the journal Science, Michigan State University researchers Christoph Benning, Eric Moellering, and Bagyalakshmi Muthan describe how plants can withstand freezing temperatures. The plants produce a substance that protects the cell contents from freeze damage. Part of what makes this discovery interesting is that same mechanism that helps plants cope with freezing temperatures may also help with drought conditions too. This is because the mechanism they discovered allows the plant to cope with higher salt concentration inside the cells when ice begins to form. The hope is this mechanism can assist plant survival whether the dehydration is caused by actual drought or freezing temperatures.
In practical terms, with plants in our landscape, winter weather that switches from warm to cold can confuse otherwise hardy plants. This quickly changing weather, warm to cold, can trick the plants and make them more tender. Cold injury on landscape plants shows up as a blackening or browning of the leaves on evergreen plants like hollies. Deciduous plants and evergreen plants can have their stems and shoots damaged. With this, the thin bark will be damaged and, in effect, the plant part becomes girdled. This damage is not always immediately evident. It typically doesn’t appear until a week or more after the cold weather has occurred. In some instances, the winter damage will not be evident until late spring or early summer.
Mulching can help reduce winter damage by evening out soil moisture and temperature. For plants in the landscape, I recommend three to four inches of wood mulch. This will also help with weed control, too. Think of the mulch as a blanket for your plants.