By Laura Gardner, Master Gardener in Training
This spring as I walk around my neighbourhood, I have noticed quite a few evergreen conifers with brown needles. The species that are commonly affected are mainly the dwarf and ornamental varieties such as Picea glauca ‘Conica’ (Dwarf Alberta Spruce), Thuja occidentalis ‘Smargd’ (Emerald Cedar), and Taxus spp. (Yew). Some of the more robust and resistant conifers are the parent species such as Thuja occidentalis (Eastern White Cedar).
- Inadequate moisture
- Inadequate protection from sun and wind
- Rapid freezing/thawing
- Salt spray damage
- Root damage at transplant
- Late season pruning and fertilization
- Late fall transplant
- Genetic maladaptation
Cultural Practices for Recovery and Future Maintenance
Depending on the extent of the damage, these shrubs may recover and produce new growth. This process can be encouraged by additional watering and the addition of mulch. Shrubs should be well-hydrated up until freezing in the fall to prepare them for moisture loss in the following winter. Mulching helps protect shallow roots from drying out, can help limit frost-heaving, and moderates the temperature of the soil.
Prior to winter, consider adding some protection such as a burlap screen with stakes for plantings on the south and/or west side where it is sunny or windy or near driveways and walkways that are salted. Burlap can be wrapped around shrubs but should be kept loose so that moisture is not trapped. The advantage to a screen is that the area remains open to air and light. Salt, sun, and wind can draw out moisture from the needles and because the ground is frozen, the roots are unable to draw in replacement moisture. Planting these types of shrubs on the north and east sides, in less open areas, and away from driveways can minimize damage.
Shrubs that have been dug from the nursery field and then have been repotted for sale may be subject to some root damage/loss. This can be more problematic when transplanting late in the fall as there is limited time for root re-development. In addition, the ability of the roots to draw in moisture before freeze-up can be compromised. Refrain from pruning and fertilizing late in the summer as this can encourage a flush of late new growth—tender growth that is more susceptible to winter damage.
Conifers that are exhibiting winter browning:
Is Supportive Care Enough?
Perhaps—it is required in the first few years after transplant and probably they will continue to require extra support. However, some dwarf and ornamental conifer cultivars are simply not genetically adapted to thrive in this region. This is because they have originated from areas of more moderate climate and hardiness zones. When considering trees and shrubs, while some species are more adaptable than others, it is preferable that stock be grown locally and be from local cuttings and seed. Forest Gene Conservation Association notes that “bringing material in from dissimilar areas often results in low survival from heat stress or winterkill, frost damage, reduced growth rates, and increased insect and disease problems.”[ii] While climate change is indeed allowing us to push the envelope a little and plant some species from the next hardiness zone, and there are assisted migration[iii] programs for species, there can be a risk in transplanting certain plants from further afield. Plants are genetically adapted to follow a particular timed growth cycle. For example, a study of Quercus rubra (Red Oak) found that a specimen grown in Algonquin Park that was transplanted in the Niagara region stopped growing before the end of the growing season. It was genetically adapted to a growing season of 185 days but the growing season in Niagara is around 230 days. Another specimen grown in Niagara that was transplanted in Algonquin Park was genetically adapted to continue growing past the end of the growing season there and as a result suffered frost damage and browning. It would be weakened and be more prone to damage from disease and insects.[iv]
Another study of Picea glauca (White Spruce) in Alberta found that cold hardiness was determined to be the trait with the strongest genetic variation. Seed from plants originating from Ontario had high growth but a poor survival rate. Because they were accustomed to longer growth periods, there were more vulnerable to early damaging frosts. The plants with the highest survival and growth rates were grown from local stock.[v]
Before purchasing, determine your garden’s site conditions: soil, moisture, drainage, sunlight, wind, climate, and whether the trees or shrubs you are considering can adapt readily to the conditions. Climate change also needs to be considered as we experience increased drought and higher temperatures. Determine their origin. Ask the vendor where they were grown. If they originated from an area with very different conditions, consider giving them a pass. Realize that “if a tree is not genetically adapted to your site conditions, no amount of care will help it grow as vigorously as one from the appropriate source.”[vi]
Keep in mind the gardening adage, “the right plant for the right space.”
[i] Winter Burn. University of Wisconsin Garden Fact Sheets. Online: https://hort.extension.wisc.edu/files/2015/01/Winter-Burn.pdf
[iii] Assisted Migration. Forest Gene Conservation Association. Online: https://fgca.net/climate-change/assisted-migration/
[v] Sebastian-Azcona, Jaime, et al. Adaptations of White Spruce to Climate: Strong Intraspecific Differences in Cold Hardiness Linked to Survival. Ecology and Evolution, vol. 8, no. 3, 2018. Online: http://dx.doi.org/10.1002/ece3.3796.
[vi] When Planting Trees. Forest Gene Conservation Association. Online: https://fgca.net/forest-gene-convservation/when-planting-trees/