Winter Damage of Conifers

Plant examples in this grouping are firs, spruces, pines, arborvitae, and yews. There are several possibilities on why so much damage is emerging this growing season, and it all relates to environmental conditions.

One of the more obvious is when branches have cracked or broken off because of heavy snow load. The vascular structure (the plant's plumbing system) is damaged and unable to support growth at the extremities. Snow sheds easily off deciduous trees as there is nothing to catch and hold the frozen precipitation. Conifers, especially the multi-stemmed (i.e., junipers) will hold the snow in place. Anyone that has shoveled wet snow off the sidewalk knows how heavy it can be. To prevent damage, brush off the snow with a broom. Other damages can be caused by a variety of possibilities and are a little more difficult to determine.

Heavy winter snow led to branch breakage. Notice the tear in picture center. Photo: Tom Butzler, Penn State

Typical soil profile consists of pore space and solids (ratios of sand, silt, and clay). The pore space, on average, is half air and half water. The makeup of the space fluctuates over time, depending on weather conditions such as drought and rainfall.

The excess soil moisture last year pushed much of the air out of those spaces for long periods. Roots need to breath and did not have ready access to soil gasses. This could have weakened the plants going into winter and opening the plant to weak pathogens.

The other item of note on water-filled pore spaces was that these could have frozen during some of winter's extended cold spells. This could be problematic as conifer roots, which stay green all year long, could not access water. As sunlight falls upon needles or wind moves over the plant, moisture is lost. This needs to be replaced, and the plant draws up water from the soil. If water is unavailable, the foliage above ground can burn or even die as it is lost to the atmosphere.

There is a high probability that this fir tree planted in clay soils had pore space that was occupied with moisture for long periods. Photo: Tom Butzler, Penn State

The final possibility to consider is the issue of hardening off. All plants going into winter need to prepare, at the cellular level, for the upcoming temperature extremes. This allows plants to survive sub-freezing temperatures whereas that same plant could easily die-off if exposed to similar conditions during the growing season.

A general recommendation is to avoid watering trees and shrubs to allow them time to 'harden-off.' Water can promote growth, and it is very tender. Unfortunately, Mother Nature did not follow this recommendation and provided water that pushed late-season tender plant growth. Once cold weather moved in, sensitive plant tissue was unable to withstand freezing temperatures and died.

In some situations, opportunistic fungi such as Pestalotiopsis and Phyllosticta have been detected on dead foliage. These fungi are sometimes capable of causing dieback but are often considered to be invaders of weakened, stressed, or injured plant parts. The situations described above could certainly have been the stressor that these organisms need.

Hopefully, this year's growing season will be different, and landscape conifers will go into and emerge from winter healthy and intact.