Winter Dieback of Trees and Shrubs

A recent discussion thread launched on the Shady Lane listserv by Justin Evertson of the Nebraska Statewide Arboretum asked observers how trees in their landscapes weathered the negative double-digit cold of winter. Used to extreme cold conditions, native trees and shrubs weathered the tough winter best. Others (that were thought to be very cold hardy) lost all living canopy growth and are now sending out sprouts at the base. Still others are no surprise at all, such as the Japanese maple, a marginal species for this region at best, which had major dieback and/or outright death.

Overall, the winter injury is divided into 3 main groups, with the first being composed of trees and shrubs with sparsely-leafed branches and stems. Woody plants that fall into this category include privet, burning bush, lilac, tuliptree, and sycamore. For these plants, recovery is a strong possibility though it may take several years.

The second group is made up of trees and shrubs that lost their top branches and stems to winter damage, with new growth arising from the base. Trees and shrubs in this group include black gum (also known as tupelo), Korean evodia, magnolia, and Siberian elm. The good news about this group is the plants survived.  The bad news is tree growth is nothing like normal. A more accurate question would be do we really want to save them? That is a tough one to answer.  It involves years of monitoring the tree,  retraining a new central leader, and thinning out sprouts that give the appearance of a shrub instead of a tree. If you are willing to devote the time and effort to take on this task, then do so. Otherwise remove the old plant and replace it with something with greater hardiness.

Conifers suffered dieback too, with the death of the tops of trees. Spruce trees were hit hardest, but juniper and pine were also affected. The dead central leader can be pruned out and a lower branch can be trained upright by tying the branch to a pole placed along the stem.  Lower limbs can be trimmed back to give the new leader time to grow.  Staking and tying materials are to be removed in one year to forestall stem girdling. 

The third group consists of plants lost altogether, like boxwood and doublefile viburnum. Thankfully, the number of plants in this group is not many but planting something believed to be hardy only to lose it is frustrating. In many cases, trees had been growing successfully for years, only to die from freeze injury just when we thought they were large enough to weather most anything.

Oddly enough, not all species were affected uniformly. The paperbark maple in my yard looks just fine while in other landscapes, the tree had serious dieback. Tree health prior to the winter will impact its survival from an extreme weather event and the drought of 2020 compromised tree health.

This past winter emphasized the importance of provenance. Provenance refers to the source of seed, with trees grown from regional seed sources showing higher survivability than trees grown from seeds that come from outside the region. Differences in soil, precipitation, winter cold, and summer heat will negatively impact trees and shrubs grown from seed sources outside the region. When purchasing a tree or shrub, researching the plant’s hardiness and where it came from are essential to plant success.

Colorado Spruce

The Christmas tree shape of Colorado spruce is beautiful indeed, but several common problems make growing them a challenge.

Deep Cold

February’s hard hit of negative double digit cold resulted in the tops dying out of spruce trees. This caused cracks to develop in trunk tissues that then leak sap. This is apparent with the accumulation of white crust on the trunk and branches. Once cracks develop, canker pathogens like Cytospora gain access to conductive tissues within the tree, widening the wound. Unfortunately, sprays and drenches do not counter the effects of Cytospora canker. Prune out the dead central leader and re-train a lower branch to take over as the new central leader.  Branches below the new central leader can be cut back to restore the pyramid shape. How To Prune Coniferous Evergreen Trees (uidaho.edu)

Spruce Spider Mite

The spruce spider mite is a small sucking pest of spruce and other evergreens. With their needle-like mouthpart, the spruce spider mite removes sugars, sap, and chlorophyll from foliage. When mite populations are high, needles present an off-color appearance of dusty green. Leaves eventually turn brown altogether and fall from the tree. The spruce spider mite is active in the cooler parts of the season, with activity peaking in spring and fall. A hand lens or a microscope is a handy tool to identify spruce spider mites, but the paper test can also be applied. Using a sheet of white paper, tap a branch over the paper and look for small dots moving across the page. Miticides are effectively applied when mites are active in spring and fall. Rainfall has a nice way of keeping mite populations down by drowning them. Ins of ev 3-6-09 (unl.edu)

Bagworm

Tear drop-shaped bags and loss of needles are clear indications of bagworm infestations. A few caterpillars can defoliate trees in a relatively short time frame, leaving branches bare and killing next year’s growth buds. Products containing the active ingredient Bacillus thuringiensis (Bt) can be applied in mid-June to early July to target these caterpillars when they are small. As the caterpillars grow, so do their appetites, causing significant defoliation and being difficult to manage as well.  Come August, caterpillars are too big to manage with insecticides and hand-picking bags from trees is really the only option. Be On the Lookout for Bagworms | Nebraska Extension: Community Environment | Nebraska (unl.edu)

Rhizosphaera Needle Cast

This fungal disease, caused by the pathogen Rhizosphaera kalkhoffii, interferes with intake of carbon dioxide through the needles. The needle’s breathing structures, known as stomates, are clogged with the fungus’s spore producing structures. Without the ability to breathe in carbon dioxide and exhale oxygen, trees are unable to photosynthesize, starting a cycle of decline that, if left untreated, can cause defoliation and death to the tree. Cool wet springs provide the perfect conditions for development of needle cast. A hand lens or microscope is handy for spotting infected stomates, which appear as rows of black dots on needles. Good air circulation, keeping irrigation water from wetting foliage, mulching beneath trees, and application of a fungicide will manage needle cast.  Dis of ev 1-20-09 (unl.edu)

Tree Overuse

Colorado spruce is suited to rocky, well-drained soils, in locations that receive 6 or more hours of direct, uninterrupted sunlight daily. Heavily irrigated clay soils and shady sites are situational conditions ripe with opportunity for decline, necessitating costly interventions that may or may not work.

With the tree popular for use as accents in front yards, as windbreaks, and in allées, the Colorado spruce has become the dominant evergreen in the landscape. As with any over-used plant, a major concern is the potential for an emerging insect or disease problem to infest the species (think: emerald ash borer for ash, Dutch elm disease for elm, and pine wilt for pine.) Diversity is always the best choice, especially when utilizing trees with insect and disease resistance. Substitutions for this much-overused tree include juniper, Concolor fir, Douglas fir, Fraser fir, Norway spruce, Black Hills spruce, and Ponderosa pine.