CLEVELAND, Ohio — Freezing temperatures, ice, and snow are challenges for plants and animals during winter in northeast Ohio. Unlike animals, plants are rooted in place. How do they survive?
Plants need certain resources throughout the year, including water, sunlight, and air (especially carbon dioxide). Getting (or keeping) water is the biggest concern for plants during winter in our area. Cold air can’t hold as much water as warm air, which means there is less water vapor for plants to absorb. In fact, plants must battle to prevent cold winter winds from pulling moisture out of their leaves and stems.
One-way plants have dealt with winter challenges are by evolving hormones which respond to the shorter days and lower temperatures. For example, the hormone abscisic acid causes the tree to close the connection between the leaves and the stems, so that the leaves drop from the tree. Water and nutrients cannot flow to and from the leaves so trees stop producing chlorophyll, which shows as the green color of leaves. The sugars that trees rely on for energy come from the work of chlorophyll. It is crucial to the process of converting carbon dioxide and water in the presence of sunlight into sugars (carbohydrates), also giving off oxygen through photosynthesis.
Like water, sunlight is in short supply during winter. Shorter days give plants less time to absorb sunlight. In addition, sunlight is weaker because the sun is lower in the sky. Less sunlight results in less photosynthesis, crucial for plant growth and development. For outdoor plants that “shut down” during winter, this isn’t a problem.
There are several ways that outdoor plants can prepare for tough times during winter. This process is known as “hardening.”
Use anti-freeze. Some plants create a type of anti-freeze by increasing the concentration of sugar, salt, or other products in their cells. This lowers the freezing temperature of water in their cells, protecting them from all but the coldest temperatures.
Move water deeper. Plants move sap (water containing sugar) into their roots and/or into the spaces between their cells. This increases the distance between the water and cold temperatures, and it less likely to freeze. If it does freeze, it causes less damage.
Enter dormancy. Dormancy is when plants nearly stop growing or reduce their rate of growth. When plants are dormant, they need less water and use less food and energy. Dormancy happens gradually in the fall, as nutrients move from the leaves and stems into the roots. Those nutrients will be stored until spring, when they will be moved back up to power leaf emergence. Sometimes an early spring warmup is followed by a sudden hard freeze (one with temperatures below freezing for many hours). If this happens after a plant has moved out of complete dormancy (remember the freeze we had in May 2021 after a warm spring?), the plant is more likely to be damaged. The same thing can happen in the fall; if a plant is subjected to a hard freeze before it is fully dormant, it can be severely damaged or even die. (Many buds of fruit trees and vines, not completely dormant, were damaged in the sudden December freeze we experienced in 2022.)
Use a different growth strategy. Plants have various strategies for modifying their growth and life cycles. Perennials, trees and shrubs store nourishment in their roots over winter. Annuals such as marigolds and pansies die at the end of one season but produce many seeds that will sprout the next spring and start the cycle again. The seeds of some of these plants require a cold period before they will sprout, a phenomenon known as stratification. Plants that are biennials survive their first winter but die before the second. During their first year, they may look like a circle of leaves hugging the ground. But underground, they are developing a large root for storing nutrients. During their second year, they use the food stored in their root to produce a flower stalk and many seeds, and then they die.
Drop leaves. When a tree or shrub loses its leaves (think of a maple or oak), it eliminates much of the surface area exposed to dry, cold air. This saves a lot of water during the winter. These leaves break down over time and return nutrients to the soil. As new leaves emerge in late spring and summer, the increased surface area helps the tree move water from the roots to the new leaves. Perennials like coneflowers and hostas don’t need to drop their leaves because the entire above-ground part of the plant dies. But this is done for the same reason.
Protect leaves. The waxy, needle-like leaves of pines and spruce have a thick coating that helps resist cold and lock in moisture. This is also true of the broad, tough leaves of holly, rhododendron, and magnolia. Some rhododendrons have another trick: their leaves curl when temperatures get very cold and open again when temperatures approach freezing. This exposes less of the leaf surface when the temperatures are coldest. If you have rhododendrons, you have probably noticed this phenomenon a lot this winter!
Lois Rose has been a Master Gardener for over 25 years.
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