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“Spring has newly sprung the hills are full of grass…” -Ogden Nash
The warmer weather invites you to walk outside, listen to birds sing and inspect your landscape. That’s when you might observe, “What’s wrong with my rhododendron? Why do my boxwood shrubs have dead leaves? Why are the tips of the needles on my pine tree rust-colored?” The answer is- winter injury called “winter burn”. Winter burn is a common problem of needle evergreens, such as hemlock, arborvitae, chamaecyparis, pine, and juniper, and those with “broad leaves”, like boxwood, hollies, and rhododendrons. Leaves do not actually burn but rather dry up. Leaves turn yellow or brown in response to low soil moisture, freezing temperatures and blowing wind. On a windy winter day broadleaf evergreens can become water deficient in a few minutes. Evergreen plants hold on to their foliage in winter, and need tons of moisture to keep their needles or leaves green throughout the season. Since the ground is frozen, plant roots are not able to take up enough water from the soil to replace the lost moisture. Winter injury may occur when water is transpired through plant tissue more quickly than it can be absorbed through the roots. A rapid drop in temperature after a warm and sunny winter day can also cause further injury to the plant.
As described by Dr. Sharon Douglas at The Connecticut Agricultural Experiment Station, “The factors that contribute to winter injury are diverse and results often do not appear on woody ornamentals until the following spring and summer. In addition, the extent and severity of the injuries that develop are often more extreme on plants weakened from drought stress, transplant, or from other environmental and site-related stresses.”
Environmental factors affecting plant growth- Most plant problems are caused by environmental stresses. The main factors that affect plant growth are light (sunlight), temperature, moisture and nutrients. Each of these factors can cause beneficial or detrimental effects. The detrimental effects manifest as symptoms, the visible reaction or alteration in a plant that result from the plant being sick or injured.
Light- Photosynthesis, the process by which plants transform light energy into chemical energy, is dependent on quantity, quality, and duration of sunlight.
Temperature- Air temperature affects the rate of photosynthesis, the production of food molecules for energy. Air temperature also affects respiration, the burning of food molecules for energy. Soil temperature affects the rate of nutrient absorbtion and root growth.
Moisture- Excess or insufficient moisture affects the rate of nutrient absorption, as water is the transporter of nutrients. Water is necessary for photosynthesis. Plants still need water during the winter, and when the soil is frozen, the movement of water into the plant is severely restricted.
Nutrients- Excess or insufficient nutrients can have adverse effect on plant growth.
Other environmental factors can be poor planting or mulching practices, mechanical damage from mowers or trimmers, chemical injury from pesticide application, and pollution.
All environmental stress factors are referred to as abiotic, meaning they are non-infectious, non-living; rather than biotic factors such as bacteria, fungi, and insects.
Are your plants going to survive from winter burn?- The signs of winter injury usually don’t appear until months after the winter season. Identification can be difficult because it can look different in every plant. Plants with winter burn usually survive. Once the ground thaws, damaged plants can recover by watering them well if there has not been adequate rainfall. While there is no remedy for damaged areas, new needles or leaves sprout and replace the dead ones.
“Pruning out the browned parts will have different effects on different species of evergreens,” according to Doris Taylor from The Morton Arboretum in Illinois. “A yew or a boxwood can sprout new growth from farther down a branch if you cut off the end, but a pine can only grow from a bud at the branch tip. If you cut that off, the branch will never grow longer.” Taylor recommends, “Don't rush to prune out branches that may not really be dead. On many plants, the leaves are discolored but the buds are still alive. That means the branch is still alive too."
It is best to wait until late May to determine what is alive and what isn’t. Needles or leaves that are affected by winter burn, will drop off on their own as the season progresses.
Winter burn prevention- All plants, and especially evergreens, need to be fully hydrated in fall, all the way up until the ground freezes. And applying three to four inches of mulch at the base of trees and shrubs can also be beneficial since it helps to conserve moisture during the winter.
Now it’s time to celebrate nature, and spend time outdoors to walk or rest, think or dream, or sit under a rustling tree, smell the flowers, and listen to birds sing.