boxwood health

Winter and Boxwood: Bronzing

Several species of broad-leaved evergreens and conifers exhibit winter orange-to-purple coloration, referred to as "bronzing." The production of chlorophyll slows, so purple-to-orange anthocyanin pigments present in the leaves are more visible. Drought stress resulting in decreased transpiration can exacerbate the situation. The discoloration can appear on just the tips, one side, or the whole plant. Whether Buxus, Chamaecyparis, or other species, this pigment change is not harmful to the plant. Bronzing is easily distinguishable from other issues, as most diseases either attack lower foliage, cause defoliation, or result in an obvious discolored branch or sector of the plant, not a relatively uniform color change across the surface - dig into a bronze boxwood and you should find plenty of green.
 
The desiccating effects of wind and sun can result in bronzing as a stress response, especially if the plant is already drought-stressed.  Full-sun exposure tends to result in a greater degree of discoloration – and if in partial shade, the “sunny” side may be bronze. Multiple freeze/thaw cycles and south to southwest exposure present the greatest chance of bronzing.  Boxwood planted in a more protected area including the north or east side of a structure are more apt to avoid bronzing.   

Winter bronzing of boxwood

Example of bronzing on Buxus sinica var. insularis ‘Franklin’s Gem.’ This specimen is located on the southwest side of a building – partly shaded by deciduous trees in summer, but quite sunny in winter.

Species and cultivars of boxwood vary as to the degree of bronzing. Note bronzing is not an indicator of a lack of cold hardiness.  Buxus x ‘Green Gem’ tends to bronze with full sun exposure but is one of the hardiest boxwood, rated to USDA Hardiness Zone 4. Our own NewGen Freedom (cold hardy to Zone 5) can also bronze if planted in full sun, especially if south-facing and exposed to wind, but seldom suffers cold damage such as desiccated shoot tips. Variegated boxwood may only show discoloration on the light-colored leaf margins. The good news for any discolored boxwood is the rapid recovery in spring when warmer temperatures and ample moisture boost chlorophyll production. Green is right around the corner!


The lemon-lime leaf margins of some variegated boxwood such as Buxus microphylla Golden Dream (‘Peergold’) take on an orange cast when temperatures drop. The crisp color contrast returns in spring with warmer temperatures.  

Winter and Boxwood: Diagnosing Cold Damage

We all love boxwood for the beautiful evergreen (and deer-resistant) foliage, but winter injury and discoloration can certainly occur. In this newsletter and the next, we’ll discuss some common issues – fitting, considering the winter we’re having (with more yet to come).
 
We divide cold damage into two camps: frost damage that occurs in fall or spring, resulting in light die-back of the newest growth; and more severe freeze damage because of unusually low or prolonged cold temperatures. Buxus species and/or cultivar cold hardiness is important to note, as there is a fairly broad range of cold tolerance among species. The degree of damage can vary by exposure (sun to shade) and the relationship to topography or structures. Boxwood with southwest-facing, full sun exposure seem to be at greater risk of damage.

Maintenance timing matters as well: early spring or late summer pruning or shearing can stimulate new shoots that will get damaged if there’s not enough time to harden off before a frost or freeze. 

What does cold damage look like? Pale- to straw-colored, desiccated foliage and branch tips, usually at the top and outer tips of the plant. Fresh young shoots are especially susceptible (recall boxwood blight symptoms often first appear in the interior lower branches and base of the plant, as spores spread by rain splash). Additionally, an extreme freeze event in fall or spring can cause splitting of bark - resulting in dead limbs. Snow cover can be fantastic for protecting both foliage and branches if it’s a strongly branched plant. 

This lovely cloud-style planting of Buxus microphylla ‘Green Pillow’ (Zones 6-9) is in full sun located on the west side of the residence. Sun and wind conspired with an extended hard freeze to desiccate the upper, outer foliage. Once growth resumes in the spring, the plants may benefit from a bit of shearing. 


NewGen Independence® (Buxus ‘SB 108’ PP28888) (Zones 5b-8).  Last pruned in July, a warm, wet September encouraged a few fresh shoots. Alas, an unusual early-October frost blasted the tender growth. Simply snip these pale shoots off once the danger of frost has passed in the spring.