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.   

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!

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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. 

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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.

"What's Up With My Boxwood?" - Late Winter Edition

Author: Holly Scoggins, Program Manager

Many of us experienced a relatively warm late fall and early winter. Late December in my own Southwest Virginia garden (Zone 6b, 2100’ elevation) saw some perennials emerging that had no business even peeping until early spring, plus a bit of new growth on some evergreens. Then blammo! Temperatures dropped dramatically, winds increased, snow and ice became the norm for January, and the first half of February hasn’t been much better. What an excellent time to review a few common winter issues for boxwood! Note that as with most things Buxus, susceptibility to abiotic or biotic issues is usually species- or cultivar-specific.  Start with cultivars that are cold-hardy in your geographic region – check tags and other resources as to USDA cold hardiness zone.  Here are a few of those frequently asked questions.

FAQ #1: What’s up with the cream/tan/straw-colored foliage at the tips of branches?

New growth can be susceptible to cold damage, resulting in bleached or desiccated foliage.

Chances are this was new growth that hadn’t had the chance to harden off. The same damage can occur on fresh growth in spring that gets hit by a late freeze. Gradual acclimatization to cold (and then to warm) is ideal but doesn’t always happen. And as with many woody plants, don’t encourage late-season growth. Fertilizer applications and pruning in late summer and early fall can stimulate fresh foliage that is susceptible to damage. Be sure landscape or nursery plants are well-watered going into an extreme cold spell to limit desiccation. While this bleached foliage is unattractive, rarely is the health of the plant compromised. Simply snip off damaged foliage and twig tips in spring.

FAQ #2 Should I remove the ice and/or snow weighing down the branches?

Buxus sempervirens branches can bend under the weight of significant snowfall. 

Nothing insulates and protects hardy plants better than a nice blanket of snow.  However, rarely is the snow deep enough to completely cover shrubs. The weight of snow and ice over exterior branches can cause them to snap or split.  The chance for damage is exacerbated by whacking at the wintery build-up with a broom or shovel – a too-common practice. Let it melt naturally if possible.  If additional snow and/or ice is expected to the point of concern for breakage under the load, remove by gently shaking or brushing with gloved hands. 

FAQ #3:  Why is my boxwood foliage bronze/orange in winter? 

Bronzing happens!

“Bronzing” is the boxwood term for the appearance of purple-to-orange anthocyanin pigments that can occur in winter. The discoloration can affect one side, the whole plant, or just the tips. Bronzing is much more likely to occur on boxwood planted in full sun, especially with southwestern exposure. The pigment change is not harmful to the plant, as the foliage greens up in the spring – but you may consider a shadier or more appropriate site for that specimen. Again, susceptibility to bronzing is a bit cultivar-specific. If the aesthetics of bronzing are an issue and your boxwood will be sited in full sun, pick a cultivar shown to be less prone to the condition.

For a list of cultivars and much more information on winter care of boxwood – visit the NewGen® Boxwood site https://www.NewGen®boxwood.com/boxwood-care#winter