Replacements for picky evergreens and what to do about ants damaging a tree
"Fielding Questions" columnist Don Kinzler also answers a question about the hardiness of holly.
Q: The arborvitae in the photo is on the south corner of my house and was planted about four years ago. In the winter it was wrapped in burlap, and weathered the previous years fine, but this year it looks totally dead. I’d like to plant a low-maintenance columnar tree in the corners. I like the look of arborvitae, but they are to finicky and high-maintenance. Any suggestions for replacements? — Jeff G.
A: Arborvitae are beautiful, but growing them is an uphill climb. Rabbits and deer can cause irreversible damage while feeding. They're susceptible to winter injury, as you've found, and snow can break branches.
Although the arborvitae were wrapped, thawing and freezing from the winter sun can still occur if the burlap is wrapped directly on the foliage. For more successful protection, place the burlap on a frame surrounding the arborvitae, so air circulates inside the frame to prevent sunburn and reduce wind desiccation, while keeping the burlap from direct contact with the foliage.
There are never exact look-alike substitutes for problematic plants, but for arborvitae there are close substitutes. At garden centers you'll find what are generically called columnar or pyramidal junipers. There are different named cultivars that vary in shape and color, with many having bluish or silvery tints.
Columnar junipers are rarely bothered by deer or rabbits, and they are much less susceptible to winter injury. Juniper isn't immune, but it's certainly more resistant to winter burn than other evergreens.
Q: Last year I noticed holes along an old scar on the trunk of our ash tree that were being made by many large ants. There was evidence on the ground below of mounds of sawdust and busy ants working. I know that inevitably the emerald ash borer will probably get the tree , but we are not anxious to lose the tree now. What kind of ant is doing the damage? I tried Terro ant dust with no success. — Bill W.
A: For ant advice, I turned to co-worker Pat Beauzay, an NDSU entomologist. Pat says, “These are most likely Eastern carpenter ants. Carpenter ants can be found in living trees (especially mature trees) when there is rotten wood present inside the tree.
“Carpenter ants live in and feed in dead, rotting wood only, and will not cause additional harm to the tree, so in that sense there is no need for control. I would be much more concerned about the structural integrity of the tree and whether the tree could fall and cause property damage.
“Also of concern is the possibility that the carpenter ants may form ‘satellite’ nests in rotting structural wood of the home itself and any outbuildings — these should be inspected for water-damaged wood and the presence of ants. Any damaged wood should be replaced.
“Sevin (carbaryl) dust can be applied directly into the tree cavity, but this is difficult and may not be effective. Consequently, I do not recommend any control measures unless carpenter ants are invading the home itself.”
Q: I would like to plant some winterberry holly in my backyard for birds and something different. Will it survive? — Dale W.
A: Most cultivars of holly are not winter-hardy for zones 3 and 4 of North Dakota and Minnesota. However, there is one species of holly, called winterberry holly (Ilex verticillata) and its cultivars, that is winter-hardy enough to be planted in the microclimate of an established yard. Planting them on an exposed hillside where frigid winter winds sweep away all protective snow could be problematic.
To determine whether trees and shrubs sold at garden centers are winter-hardy or not, we have a great resource in the North Dakota Department of Agriculture. The department maintains a list of non-hardy woody plants, which is a great guide, especially when shopping at national chain stores that might stock trees and shrubs not necessarily suited to our geographic region. Material sold at locally owned garden centers is much less likely to be found on the non-hardy list.
The list of trees and shrubs to avoid can be found by searching "North Dakota Department of Agriculture non-hardy woody plant list."
If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at firstname.lastname@example.org. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.