Mike Mickelbart, Horticulture and Landscape Architecture,
This year we have certainly
experienced above average temperatures. According to the National
Oceanic and Atmospheric Association, this January was the warmest
on record since 1895. The average nationwide temperature of 39.5°F
was 8.5°F higher than average. In Lafayette, Indiana, our average
high was 48°F in January, when it is normally around 32°F.
This is great for us, but what about plants? Are our plants enjoying
the exceptionally mild winter as much as us? Maybe not.
With warmer winter temperatures, evergreen trees
and shrubs continue to lose water through transpiration. This is
especially true if we have lots of sunshine, which acts to heat
the leaves, raising temperatures above ambient conditions. Whereas
leaf temperature can change quickly with absorbed sunlight, soil
temperatures change much more slowly. This creates a situation
in which the air temperature is high enough for water loss through
transpiration, but soil temperature is still cold. This, in turn,
limits the amount of water that roots can extract. Low relative
humidity (common in winter) results in even greater water loss
because dry air results in greater water loss from the leaves.
In extreme cases, the leaves will lose water while the ground is
still frozen. Under these conditions, leaves become chlorotic,
and sometimes scorched. Conifers and other evergreens with needles
will develop brown tips or entire needles, they may drop needles,
or exhibit shoot dieback.
This Mahonia aquifolium (Oregon Hollygrape) is showing
symptoms of winter damage due to water loss, probably exacerbated
by the warm temperatures we have experienced this year. It is exposed
to full sun and is surrounded by walls and concrete walking paths.
Under these conditions (coupled with low humidity and relatively
high air temperatures), this plant can lose a substantial amount
of water that cannot be replenished from the cold soil it is sitting
in. This year was an exceptionally warm year, so it is not surprising
to see this type of damage. If this is a problem in the landscape
over multiple years, a solution would be to move this species (and
other similar species) to a more protected microclimate.
Click image to enlarge