Q. Why do my established junipers continue to die? The dieback starts slowly with an individual branch and then the entire plant goes.
A. There are two possible causes for junipers to slowly decline and then die. The primary cause has been root rot from excessive water during the summer months and/or soil erosion. Soil erosion is especially a problem with those plants planted on a slope. These plants end up being buried deeper than was originally intended. They die with no apparent pattern, one here and then one over there. It’s most apparent June through October. It could be a branch or section or the entire plant that collapses over night. An early sign of problems is the change in the color of the plant(s), as the normal color dulls. Established junipers are drought tolerant and survive most summers with little summer water assuming we have a typical rainy season. Once every three to four weeks is sufficient once the rainy season concludes. Correcting root rot problems requires patience. Wet winters and summers are not beneficial. Good water management practices and removing the excess soil, are the primary corrective methods, however it doesn’t occur overnight. It can take months before the plants stop dying and for the turn around to begin.
A second possible cause of the dieback is Flatheaded Borers. Flatheaded Borers are some of the most common woodborers in the United States and Canada. There are more than one hundred and fifty species. Until recently, we were only concern with them on deciduous fruit trees. Today, they’re a problem on junipers, arborvitaes, other conifers and cotoneasters. The name is associated with the larva that does the damage. They have a long segmented body with an over size head or prothorax that is flat. With junipers, the damage starts out on an individual branch or branches before spreading throughout the entire plant.
What separates this problem from root rot is that the trail of dead plants is in a predictable pattern. Another distinguishing characteristic is that the damage continues to show up during the early winter, which is not the case with root rot. Flatheaded Borers tunnel extensively in the shallow, cambial tissue. This feeding so near the bark’s surface makes damage more evident, revealed in mushy wood and clumped sawdust-like brown frass. If you carefully scrape off the outer layers of the wound with a knife, you’ll discover the grub concealed within. Larger larvae can also be found deeper within the stem as they prepare to over winter. In the spring, weeping of sap near damage and emergence holes of the adults can be seen on junipers. Flathead Borers is much easier to control than rot root. Bayer and Bonide both have liquid, systemic tree and shrub insect controls. They are conveniently applied to the roots so there is no need to spray. The biggest plus for these systemic insecticides is that one application last twelve months.
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