P&PDL Picture of the Week for
February 20, 2006

Winter Dessication

Mike Mickelbart, Horticulture and Landscape Architecture, Purdue University

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

Winter dessication on Oregon Hollygrape

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service