With another week of observations, the winter damage to hemlocks appears to be worse in the City of Pittsburgh that the surrounding areas.. Winter injury, winter desiccation, or winter burn usually occurs during midwinter or early spring when water is lost from the needles and cannot be replaced in sufficient amounts. As a result, needles turn brown and often fall from the tree as temperatures start to warm in the spring. Those needles that do stay on the tree are likely not to recover their green color.

Rapid fluctuations in temperature, extremely low temperatures, and very windy conditions may increase winter injury on conifers. Injury is most severe on the south side of the tree where exposure to the warm sun is greatest and on the side that receives most of the prevailing, drying wind. Browning of the foliage is usually restricted to areas above the snowline. Having said this, I have observed situations that did not follow these guidelines, but is still the result of winter infury.

Some needles and branches will be lost, but most buds on the affected branches will develop normally. In many cases, needle discoloration and loss due to winter injury cannot be prevented. However, there are few methods to help reduce the extent of injury in conifers. Mulching around the base of the tree will help hold in heat and prevent deep freezing around the tree. Well-drained, aerated, and fertile soils will help to improve tree vigor and thus a greater likelihood of keeping winter injury to a minimum.

source: Michigan State University

Pictures to the left: (top to bottom)

Tsuga canadensis - Canadian Hemlock
Tsuga canadensis - Canadian Hemlock
Hedera helic - English Ivy
Hypericum - St. Johnswort
Euonymus kiautschovicus - Spreading Euonymus