By John Lichter, M.S., and Richard Evans, PhD.
I want to share my observations concerning a malady that recently has caused widespread dieback and decline of coast redwood in Northern California. Since the pathogens associated with this malady seem to work in concert with environmental stresses affecting redwood trees, this is also a good opportunity to discuss management practices that encourage healthy tree growth. In this article, I will discuss the above issues and provide recommendations which should help to ensure that these beautiful trees will serve their desired function and thrive for as many years as possible.
Coast redwood is one of the most common landscape trees in Northern California. In its native California coastal fog belt environment it can grow to nearly 400 feet tall, with a trunk diameter greater than 20 feet. They are typically fast-growing and often long-lived in our landscapes, relative to other species. However, these trees rarely exceed five feet in diameter and 120 feet in height in most landscapes. In part, their smaller stature is due to the relatively young age of redwoods in landscapes, compared with the age of the giants in redwood forests. It is also due to growth restrictions caused by above- and below-ground environmental conditions.
Since redwoods are such large trees, providing adequate above- and below-ground space for their spread will help to avoid property damage and difficult management choices down the road (Figures 1 and 2). I recommend planting redwoods at least 15 feet from any hardscape (foundation, patio, walkway, etc.) to avoid future damage caused by roots. In addition, I also recommend spacing trees at least 15 feet apart to allow for adequate root growth and support for each tree. Obviously, the spacing between redwoods and other trees will influence aesthetics, as well as shading and the ability to grow plants within their shade.
In their native environment, redwoods receive enough moisture through rainfall and fog drip for their survival. However, outside of their native range, they require regular irrigation to avoid drought stress and withstand attacks by opportunistic pathogens (see below). Root injury from demolition and construction can also lead to drought stress or destabilization. Drought stress results in smaller foliage size, needle browning and drop, twig and branch dieback and, eventually, death. The amount of irrigation required will vary according to tree size, soil characteristics and environmental conditions. In the Central Valley, a weekly irrigation to a depth of two feet, spread uniformly under and beyond the tree canopy, is generally sufficient.
While regular irrigation is required, irrigation that is too frequent, particularly in a slowly drained soil, can stress these trees, leading to foliar yellowing (chlorosis), root dieback, twig and branch dieback, and tree death. I do not recommend irrigating redwoods more frequently than three times per week. Further information regarding irrigating trees can be found in our previously published article, “Practical Guidance for Effective Tree Irrigation.”
In the late 1980’s, almost the only cause of redwood decline involved root injury, inappropriate soil moisture (water deficit or excess), or soil chemical limitations (salinity, toxic ions) (Figure 4). Other issues which were problematic for redwoods included insufficient above- and below-ground space, and poor structure. However, in the 1990’s, I started to see an increased incidence of top and limb dieback caused by the redwood canker fungus (Botryosphaeria dothidea), especially with trees which had undergone drought stress and/or root injury (Figure 5).
Redwood canker is an opportunistic fungus, meaning that it attacks trees that are under stress from drought or root loss. Once the fungus gets established in a tree, its spores are spread by rain. If untreated, the fungus usually spreads through the canopy, causing disfigurement and/or death of the tree. Management of the disease includes providing for the horticultural needs of the tree (appropriate irrigation and soil chemical modification, if necessary) and removing diseased wood well before the onset of winter rains. Other treatments to improve tree health, such as mulching, or loosening compacted soil, may assist in a tree’s recovery.
In the last several years, I have also seen a greater incidence of foliar dieback in coast redwood, either with or without top or primary limb dieback (Figure 6,7). I recently spoke about this issue with Suzanne Latham, Plant Pathologist with the California Department of Food and Agriculture Plant Pest Diagnostics Lab. Suzanne indicated that there are different species of the fungus that cause redwood canker (e.g., Botryosphaeria lutea) which infect coast redwoods, one of which causes a needle blight. This can be quite damaging to the tree, as often a large percentage of the foliage is killed, rendering the tree very unsightly. Unfortunately, removing infected wood is impractical because of the multitude of infection sites on the tree. The prognosis is poor for redwoods that are badly infected with redwood canker, or that exhibit needle blight caused by another species of Botryosphaeria.
In conclusion, we can provide our redwoods the best chance for a long, healthy, disease-free life by providing for their horticultural needs. These needs include adequate soil chemical properties, moisture and volume; avoiding root injury; and, if feasible, removing canker-infected wood.