It’s true that redwoods planted in Redding shed leaves during the summer months. Even in their native environment, redwoods, like all evergreens, shed older leaves a couple times throughout a growing season. This year, however, I noticed the redwoods planted on Turtle Bay property seemed to be shedding more leaves than usual. Given the recent wet and cold winter, I didn’t give it much thought at first. But once July rolled around and small branches were dying, I began to worry. Taking a few samples, I looked at the dry, browned leaves under a microscope. What I saw were small black, tar-like spots, whitish oval spots and white, gunky blobs. Not being a phytopathologist, I sent the samples to a lab. While waiting for the results, I spoke to a colleague of mine and he said he’s noticed the same phenomenon in many redwoods around town. A week later the results were in and they were very surprising. No pathogen was found! None! So, I put down the sprayer and hit the books.
Coastal redwood has an extensive list of fungi that attack it. Perhaps the most common is Botryosphaeria canker. This causes a blight twig dieback. Another disease sometimes seen in well-irrigated landscapes is root rot caused by Phytophthora cinnamom. In this case, the entire tree turns brown. Sometimes this is confused with a massive Dothiorella outbreak, where every twig is blighted. The difference is that Phytophthora-infected trees usually do not recover; by the time the tree turns brown it is usually dead. Also, while root rot is associated with soaking wet soils, twig blight is often found on trees growing under drought conditions.
Another pathogen that is occasionally seen in coastal redwood is Armillaria mellea, the pathogen causing oak root rot and root rot of many other ornamental trees. Although not as common on redwood, it has been seen on trees that were stressed, either from over-watering or drought. It is easily detected by the white mycelium found growing under the bark.
We must remember redwoods are forest trees. We have cultivated them for use in non-forest landscapes such as the backyards of many valley-dwellers. These conditions are not always favorable to their growth or long term establishment. Many redwoods planted in Redding will never grow to their full potential, which is unfortunate, because their size is what makes them so magnificent! So, when choosing a tree to plant, it is important to consider the redwood’s origin as a forest tree with shaded root systems, abundant mulch, and a continual moisture supply. Redding’s weather is colder and dryer and mimicking this natural environment can be costly. Water use and soil conditions must be addressed. The ultimate size of the tree needs to be considered to avoid unnecessary pruning. However, if you have existing redwoods on your property, creating some of these conditions may make redwood culture and disease management less problematic, giving a tree that struggles to survive in an unnatural environment a fighting chance.
Marie Stadther’s life in Coachella Valley was void of trees. In 2001, she packed up and headed north. After a drive through the majestic redwoods, she arrived in Redding, where she immersed herself in horticulture as owner of her own landscaping company and as assistant to an arborist. She is now the lead gardener for Turtle Bay’s McConnell Arboretum and Botanical Gardens. Her love of trees is a way of life, and she shares that passion with the community. Send the Tree Goddess your questions at mstadther@turtlebay.org.
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