The Cold Equation – Why Plants Die Over the Winter
By Jim Hole
By the time spring arrives, our gardens have endured a very long and arduous season. Months of sub-zero temperatures can take their toll on even the most stoic gardeners, but at least people spend most of their time on the warm side of the living-room window. Garden plants have little choice but to endure what winter throws their way. They must persevere or die.
What is it that allows a tulip bulb to survive extreme cold while a croton is damaged when temperatures drop to just a few degrees above freezing? The secret lies in water management. Plants have a love/hate relationship with water. During the growing season, plants are very enamoured with water, absorbing vast quantities of it to maintain vigour and encourage new growth. But during the winter, the love affair cools. Plants naturally retain some water within their cells. However, the water trapped in these cells is a recipe for disaster—when temperatures drop, it freezes and forms ice crystals. The expanding crystals burst cell walls, allowing the vital contents of the cell to leak out. The cell dies, and when this happens to enough cells, the plant perishes.
Plants that are native to areas with cold winters have several coping strategies to avoid the ice crystal problem:
One simple and obvious adaptation is the movement of the water out of the cells. Some plants transfer the moisture into the space between the cells rather than letting it lie within the cells and the cells do not burst.
Certain plants adapt by increasing the sugar or salt content within their cells. Water with higher levels of sugars or salts won’t freeze as readily as clear water —the higher the salt and sugar content, the greater the resistance to freezing. Ironically, plants may also suffer winter injury if their cells do not have enough water. Instead of freezing and bursting, these cells shrivel up and die from dehydration.
Running Hot and Cold
The water content issue isn’t the only factor involved in determining whether or not plants survive the winter. Sometimes, they are simply caught off guard. If temperatures drop rapidly following a warm spell, the plants do not have enough time to prepare for freezing temperatures.
Often, it is not low temperatures that kill plants, but rapid temperature fluctuations. It is much easier for a plant to adjust to gradual rather than brisk temperature changes and the ideal situation is for temperatures to cool slowly in the fall, remain moderately cold all winter, and then gradually warm in the spring. Of course, Mother Nature is rarely this benevolent. We’re all familiar with wildly fluctuating temperatures throughout fall and winter, and these conditions really test a plant’s hardiness.
Combating Winter’s Bite
There are ways to alleviate the winter weather woes:
CHOOSE HARDY PLANTS.
Take steps to protect your more vulnerable garden inhabitants. Mulches of peat moss and compost can stabilize root zone temperatures of perennials, while wind and sunscreen fabrics can be staked up to protect sensitive fruit trees.
Finally, give your plants a good soaking a couple of weeks prior to freeze-up. This will ensure that plants strike the right balance between too much and too little moisture. Ultimately, plant survival over the winter is part skill and part luck. Don’t be immobilized by the fear of losing a plant. Take a chance and plant a few of the more tender perennials and trees. A great deal of satisfaction in the garden comes when a gamble turns into a success.