By: Lisa Briggs | 6/4/2020
The Plant Desk fields so many questions in May and June about winter damage on evergreens, both needled and broadleaf. Evergreens adapt to winter cold differently than deciduous plants. Trees and shrubs that lose their foliage are able to go into dormancy without those leaves, conserving moisture and exposing less tissue to bitter temperatures. Evergreens do not drop their leaves, but have developed strategies to cope with winter weather.
For instance, the needles of a Yew or a Spruce are thicker than the leaves of a Honeylocust or a Maple, allowing the foliage to hold more moisture. Broad-leafed evergreens have adapted differently. Have you noticed the leaves of your Rhododendron curling inward on very cold days? This is called quilling and it reduces the exposure of the leaf’s underside to chilly temperatures. Many evergreen leaves also sport a waxy coating. This is another way for the plant to control moisture loss through exposed leaf tissue. When you apply Wilt-Pruf to your Rhodies or screen your Boxwood with burlap, you are providing another layer of protection. This is also why we like to see evergreens planted by mid-September. The extra time gives your plants a chance to store as much moisture in the foliage as possible.
So we will often counsel gardeners to wait until the Fourth of July to make a determination on the viability of newly planted evergreens in their garden. It is not an attempt to delay issuing a credit, but an opportunity for the plant to break bud and finish the establishment process.
This year has an added wrinkle for evergreens. If you been to the Garden Center, you’ve surely noticed the absence of that very large bed of Boxwood. The fluctuating temperatures of the last few winters brought concerned Boxwood owners to the Plant Desk for advice and many wondered if they were seeing signs of Boxwood Blight, a fungal disease that has been quickly spreading across the country, devastating both field crops and gardens. It hasn’t been an issue for our area yet, but the blight was found in the state in July of 2018 and in Dane County last summer.
Gardeners have been justifiably concerned when their Boxwood are discolored. We don’t want to confuse the Blight with ordinary winter damage, which causes leaf discoloration. Loss of leaves is the most obvious sign of Blight infection, where winter-damaged leaves do not drop. Boxwood Blight can begin with one plant, but the fungus can quickly spread to adjacent plants. The leaves develop brown spots, often with a darker edge, and the spots coalesce. Leaves then drop. You can also see signs of black, diamond-shaped lesions on the stems.
There is not a treatment for infected plants. If you suspect Boxwood Blight, the plants should be removed and destroyed by burning, burying or double bagging. Research shows that the spores can viably exist in the soil around infected plants for up to 6 years! Uninfected plants can be prophylactically treated with a fungicide and overhead watering should be avoided.
The disease is spread by moving infected plants. It can be transmitted via pruning tools, shovels, gloves, and vehicles. This is why the Bruce Company will not be offering Boxwood for sale in the Garden Center or on landscape jobs until there are proven, disease-resistant cultivars.