Finding a Dogged Dogwood
Thursday, August 28, 2008
Q We live on a three-acre wooded lot that used to have a beautiful lower canopy of dogwood trees that have died off over the years. Are there any new varieties that are resistant to the disease that has been killing them? Many of our red oaks are in decline as well. Is there anything we can do to reverse that?
A Appalachian Spring is a variety of our native dogwood that has proved resistant to dogwood disease. The parent tree was spotted when dogwood anthracnose killed most of the dogwoods on Maryland's Catoctin Mountain, and it was a lone survivor. It will naturalize nicely in our area and is the variety to use.
Red oaks are dying at an accelerated rate in our region. There could be several reasons for this, including the periods of heavy rainfall we have experienced. Red oaks are well adapted to drought but cannot withstand long periods of saturated soil or poor drainage.
Drought can also pose a problem of a different kind. The oak species is prone to a disease called bacterial leaf scorch, which is spread from tree to tree by a pest called the leafhopper. Symptoms are most pronounced in periods of drought. Typically, it starts on one branch with leaves that have scorched margins. The entire branch might die, and the disease could ultimately spread to the entire tree, causing its death.
Red oaks are one of the most sensitive species in terms of changes in their environment. The loss of nearby trees, a change in drainage pattern, compaction of soil, or irrigation can result in the death of red oak roots, and if many roots die, the branches begin to die back also. In some places, a resurgence in gypsy moth populations and the defoliation of red oaks have been factors. Although they grow more quickly than white oaks, red oaks do not live as long. Some are dying simply because they have reached the end of their life span and are more prone to disease and pest injury than they were in their youth.
The vacant lot next to me has poison ivy that has spread into my own garden, covering my perennials. I have a similar problem with honeysuckle vines. How do I apply herbicide to kill them without harming my ornamentals?
The first step is to stop these weeds from crossing into your garden. Maintain a mowed or edged line around the perennial bed to prevent future invasions.
Once you have established the fire line, you can tackle the weeds on your property. You can wipe on brush killer; this is the most careful means of application. Be aware that herbicide can vaporize in hot weather, and on a still day, could injure other plants. Do this job in cool weather when there is a slight breeze to aid the drying of the herbicide on the treated foliage and to dissipate any concentrated fumes from the herbicide.
Persistence will pay off if you treat any growth that is spotted on either of these weedy vines, and you should be able to eradicate them.
Scott Aker is a horticulturist at the U.S. National Arboretum.