1 - Fire prevention pruning may increase chances of bark beetle attack, but pruning paint won't help.
2 - Siberian elm may be used to create a "shrub" hedge.
Q. We are about to clean up our lot in Silver City, based on Wildland/Urban Interface fire reduction requirements. The NM Dept. of Natural Resources has discovered that their previous recommendation to limb up all trees has led to increased morbidity, especially amongst pinon (pinyon) pine. The fresh wound allows easy entry for bark beetles.
In this case, I would like to apply pruning sealer, even though many advise against its use these days. What would be your recommendation? We have a lot of beetle kill on the lot and surrounding areas.
A. I talked to Dr. Stephani Sandoval, Forest Health Specialist with NMSU Extension Service and New Mexico State Forestry. She said that pruning can result in release of chemicals (through the pruning wound) that attract beetles to the pruned tree. However, the beetles do not enter through the fresh cut, so pruning paints are not helpful in keeping them out. The naturally produced pitch and drying of the wound will be the best protection for the trees.
There are other factors that will also create bark beetle problems. When trees are stressed (by drought, overcrowding, severe pruning, or other injury) the tree produces airborne chemicals indicating that they are weakened. These chemicals attract the beetles whose job in nature is to remove the weak plants from the environment so that healthy trees can grow in their place. These beetles usually enter by boring through the intact bark because they will feed on the layer of tissue (phloem and cambium) just below the bark. Once again, the pruning paint will do little to prevent the problem.
Q. I have tried growing many different hedge plants. Nothing works in my very poor soil. However, the elm trees grow well with no care around here. Can I make an elm tree into a hedge no more than 10 feet tall?
A. Yes. Many trees will not tolerate it, but the Siberian elm that is so common in New Mexico will grow as a shrub if managed to maintain shrub form. I first saw it grown as a shrub (about 6 feet high) alongside a driveway in Bozeman, Montana. I have also seen it as a shrub in Siberia (its homeland).
To manage it best as a shrub, train it from its earliest years to shrub form. That is, while it is still young, trim it to a foot or so above the ground its first or second growing season to encourage many branches to form near the ground, then also trim them to encourage more low branches and spreading before you allow it to grow upward to form the hedge. This may take three to four years. Then trim it at hedge height after that. You may want to periodically remove some of the larger, interior stems to encourage new basal growth and new foliage at the base.
The good thing about a Siberian elm tree is that it will grow under almost any conditions. However, that is the thing that makes is a nuisance tree as well; it just grows anywhere. Your idea is a good way to take advantage of this plants persistence and toughness.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: firstname.lastname@example.org, office: 505-865-7340, ext. 113.
For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!