2017-02-08 / Gardening Tips

Coping With Cold

Plants do not have the luxury of heading south for the winter as some people and many birds do, although I do know some snowbirds who pack up and move their houseplants south with them each year. Hopefully the coldest weather of this season is over as we enter February. February can feel like the longest month of the year despite having the fewest days.

Plants have evolved some tricky mechanisms for dealing with freezing and even subzero temperatures. Herbaceous perennials use their leaves to make sugars through photosynthesis, which they turn into starch in the fall to better store carbohydrate reserves in their roots for the winter. Allowing the top growth to die back each fall is a clever way to survive. Once the ground has frozen they simply sleep it out, protected by the thermal mass of soil they live in. Soil temperatures remain pretty stable throughout the winter and they may be as much as 20 to 30 degrees warmer then ambient air temperatures.

The main danger to perennials surviving winter are periods of freezing and thawing that occur when sunlight reaches the soil surface to warm it during the day, followed by freezing again at night. Water is one of the few substances that expands when frozen and that water expanding into thawed soil pushes solid objects upward, towards the thawed topsoil and away from the harder, frozen subsoil. This is why your garden seemingly “grows” new crops of rocks almost every year. The rocks in your garden, which appear on or near the soil surface each May were buried the previous fall. Perennial roots are likewise subject to “heaving” and once they are “pushed” to the soil surface they may be killed by cold temperatures later on. If your garden has lost its protective cover of snow, which insulates and keeps the soil frozen, now would be a good time to apply several inches of mulch to keep the soil frozen. This is why it is a good idea to store some mulch each fall in a placed where it does not freeze into a solid brick.

Woody plants also have their winter challenges, especially broad-leaved evergreens such as rhododendrons. Sunlight triggers photosynthesis, a series of chemical reactions that requires plants to inhale carbon dioxide and exhale oxygen gasses. This exchange of gas occurs in tiny pores on the lower surfaces of leaves called stomates. Water is also required for photosynthesis and water vapor is also lost through these pores. When the ground is frozen the uptake of water is difficult at best and plants can easily become desiccated. The next time we have a really cold snap observe the leaves of your rhododendron. Notice how they curl themselves into little rolls that look like green tootsie rolls. The reason they do this is to limit the amount of green leaf surface area exposed to sunlight and to protect the open stomates on the lower leaf surface. If you use an antidessicant spray such as “wilt proof” make sure you spray the lower leaf surfaces to clog the stomates and limit water loss. The best time to apply antidessicants is on a relatively warm and sunny winter day. Fortunately, the rate of photosynthesis, like most chemical reactions, is slowed dramatically by cold temperatures.

Trees also have bark, an amazing tissue that insulates the water filled vessels beneath and prevents these vessels from freezing and exploding. Ice may form in the space between the inner cells but generally not in the cells themselves. Woody plants also use energy from some of their stored food reserves in the roots to prevent freezing. It may appear that woody plants simply sleep through the winter, like woodchucks, but they are hardly resting at all. Like us humans they are just waiting it out and doing the best they can to cope.

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