How Some Plants Survive Winter Freezing
Why do some species thrive in cold temperatures? Multiple freezing and thawing causes the cells to break because of the expansion of water inside the cells. The water crystals are very sharp and once they are frozen, the cell is disordered and the plant dies.
Cold tolerant plants can withstand frequent freeze/thaw cycles because these plants have lower water content during the coldest periods of the year. While water levels are lower, soluble sugars and salts are increased in the vascular system. The plant vascular system delivers resources (water, essential mineral nutrients, sugars and amino acids) to the plant parts, as well as providing structural support.
Higher salt and sugar content inside this plant system acts similarly to the way salt can be used to melt ice. Salt melts ice essentially because adding salt lowers the freezing point of the water. Plant cells that have a higher concentration of salt or sugar will withstand colder temperatures, even below freezing at 32 degrees.
Plants need sunlight and they also need heat. Heat comes from both solar radiation (the sun) and from the heat trapped and store in the earth. In summer, plant growing tasks often involve trying to get relief from too much heat. In winter, sunlight and heat are not as plentiful and we try to keep what have and use them efficiently.
Plants are also affected by winter’s shorter days and “weaker” light levels. We are in a sense ”farming sunlight” especially when we grow fruits and vegetables. Less light means less growth. If you use row covers or are fortunate enough to have a greenhouse, these structures while protecting plants from cold – also limit light.
When temperatures are around freezing even cool season crops show little or no growth, even though they’re still alive. With a little rise in temperature, the response will surprise you. The temperature of the soil is a good indicator of the temperature that will directly affect the plant. If the soil is dry its temperature will likely be similar to the air temperature above it. However, moisture in the soil acts like a buffer that will provide a level of insulation. There is more mass there, so it will take a greater amount of energy to change the temperature. It’s one of the reasons we manage soil moisture in the summer heat as well – as a buffer. Moist soil will drop to around freezing, but it will be resistant to change.
Row covers create an igloo affect beneath them, and can provide up to eight degrees of temperature protection, depending on the fabric product construction. In most situations rove covers must be managed daily, pulled back on sunny days to let sunlight in and repositioned as sunlight decreases.
Which cold-hardy crops are the best? Well, that all depends on what you prefer to eat. When selecting seeds to plant, carefully read the information included on the seed packet. A good seed company will provide planting, performance and taste preference information on the packet. A happy customer is often a return customer. The more you know, the better decisions you can make and the more successful you will be. For more information on growing cold hardy vegetables, take a look at https://content.ces.ncsu.edu/home-vegetable-gardening-a-quick-reference-guide and https://chatham.ces.ncsu.edu/2016/02/protecting-landscape-and-garden-plants-during-cold-snaps/
Minda Daughtry is Horticulture Agent for North Carolina Cooperative Extension in Lee County.