Winter temperature swings confuse plants

Winter season's temperature swings confuse some trees and plants Frederick County experiences North versus South influences, with larger temperature swings this winter than in most years. Trees and other deciduous plants developed winter buds and lost their leaves last fall, as they went into dormancy for the winter. In our area, dormancy is usually triggered by shorter days and cooler temperatures during the fall. These conditions promote bud formation, leaf fall, and the movement of water and sugars into the root system. The process of entering and exiting a dormant condition is due to the complex interaction of hormones and enzymes in a tree. Some trees, after unusually warm weather in December, were showing increased early bud development.

When a tree goes into dormancy there is a buildup of abscisic acid in the twigs. This chemical causes the leaves to separate and drop off the tree; it also inhibits bud growth and development. During the winter, abscisic acid begins to break down following periods of cold. Once the tree has reached its “chill requirement” the abscisic acid is gone; it can no longer suppress bud growth and the buds begin to produce other substances like auxins, gibberellins, and cytokinins. These promote bud break, flower and leaf expansion, and the tree breaks dormancy. In essence, the tree has to withstand a specific period of cold weather before it will break bud. This chill requirement varies among different species. Trees native to Frederick County like willow, poplar, magnolia, and cherry break bud quickly while oak, walnut and ash stay dormant later. The chill requirement also varies for trees of different latitudes, so that a red maple that was native to Maine might not come out of dormancy if it is transplanted to South Carolina if its chilling requirements are not met. Plants and trees originally cultivated in milder climates similarly respond differently when planted in this area.

At the beginning of January, some trees were showing bud swelling and even blooms, after mild weather during the previous month. Pink blossoms we saw emerging from ornamental cherry trees quickly turned brown as the highs swung from 60's to 20's. These tree species likely originated in warmer locales. Robert Black of Catoctin Mountain Orchard recently observed, “I am concerned about an early White Peach from California called Snow Angel. I did see bud swell on those and feel most, if not all, were damaged last Monday morning when we had 8 degrees. It's not the 8 degrees, it's because this variety is a 'Low Chill' or short dormancy compared to most of the other varieties I have planted here at Catoctin. And it is only 35 trees of my 1000`s of Peaches.” Aside from experimenting with unique varieties, the ups and downs of temperatures in our in our area are one of many reasons to purchase and plant trees that were grown in the same climatic zone that you live.

Evergreens live up to their name, because they do not lose their needles during winter. They normally have small “needles” or a waxy covering on their leaves that prevents drying out if the soil becomes frozen and the plant can not take up water. Evergreens produce enzymes that act like antifreeze to keep their needles and cells from freezing and being damaged by the cold. This maintains the most basic of activities in the plant, but it also means that evergreens can suffer cold damage if temperatures become too cold for lengthy periods. It is not uncommon to see shades of yellow, brown, purple, or red in evergreen needles after their dormancy.

It should be noted that the root system of both deciduous and evergreen plants do not become completely dormant; they can continue to grow during the winter when not frozen in the soil. That is why recently planted seedlings or saplings may show a significant growth spurt during the second growing season after their root systems enlarge over the winter. Spring witnesses an increase in temperature, day length, and available water triggering the hormones in the buds to begin bringing the plants out of their dormant state. The native trees in our area have generally remained in a deep sleep. Hold on out there - spring will eventually come!

Nature Note for 3/13/2016