UNIVERSITY PARK — As days shorten and temperatures drop in the temperate zones around the world, many woody plants begin physiological changes to conserve energy and protect their above ground cells from freeze injury.

A very noticeable change is when deciduous trees drop their leaves. Trees also prepare for winter in more surreptitious manners. These include moving water into extracellular space, and changing the chemical makeup of cellular fluid so that it freezes at a lower temperature. The above ground portion of plants generally exhibits more change and can withstand lower temperatures than the below ground portion. There has been considerable research into how trees and shrubs prepare for, endure, and wake up from winter. Let’s look at a few aspects of the cold weather induced changes and see how understanding them can help you in a practical sense in the forest and your homestead.

One concept that has been researched, although more heavily in orchard fruit trees than in forest trees, is chill hours. Deciduous trees begin a process of acclimatizing to winter in early Autumn. This may be referred to as rest or dormancy. Many break this into three periods, early rest, winter rest and after-rest. During early rest the plants prepare for colder temperatures and the inability to use photosynthesis to produce new energy. During the after-rest period the plants are waiting for warmer weather and longer days to spring forth and grow again. The exact physiological process of the stages of dormancy are still being researched. During the winter rest deciduous trees in the temperate zone are accumulating “chill” hours. There have been different scales to enumerate chill hours, but generally every hour the stem spends above 35 degrees and below 45 degrees equals one chill hour. Ranges for chill hours required for different species and cultivars of those species have been determined by cutting twigs and then placing them in favorable bud break temperature, moisture and light conditions. The number of chill hours are then determined from Oct. 1 to the time the twig was cut by temperature measurements. Twigs cut from most apple trees in Pennsylvania in early November will not break bud, and few will break bud if cut from trees in December.

Currently most of Pennsylvania has accumulated 500-700 chill hours this year. By the end of February the state should have accumulated over 1,200 which is enough for species with noted hour requirements. If a tree does not experience enough chill hours, it may have poor bud break, both vegetative and flower buds and poor fruit production. There are apples which only require around 200 chill hours, such as granny smith, but most are only hardy from zone 6 and warmer. Practically chill hours shouldn’t be much of a concern to your established trees and shrubs. If you are attempting to start new plants by taking cuttings and growing them in a greenhouse you should do this closer to spring.

There has also been research on the lowest temperature roots can sustain without dying. Many roots are killed at temperatures under 20 degrees Fahrenheit. Other species’ roots can survive lower temperatures like white spruce which can survive down to -10 degrees Fahrenheit. Temperatures low enough to kill roots are generally only experienced in the first couple inches of the soil. Root death due to cold is not a major concern with established trees but is a consideration if you have potted plants or newly planted small seedings or smaller landscape plants.

Many seeds dropped in autumn are also dormant through the winter. There are several types of seed dormancy: seed coat, embryo, morphological, combined, double, and secondary dormancy. These are covered in more detail in the US Forest Service publication “The Woody Plant Seed Manual.” Some of the required dormancy processes for many species are carried out over winter so they are ready to grow when the days are longer, and the temperatures are warmer. The woody plant seed manual covers dormancy requirements for many common woody species, so it is a great resource if you are trying to grow trees and shrubs from seed.

There are many research and Extension articles covering these topics. So as the winter weather forces you to stay off the roads and stay warm inside, you can take this time to learn more about your favorite species and how they react to and try to remain competitive despite cold weather and darker days.