# Protect redwood trees from drought, disease

Leimone Waite
Master gardeners

Q: I have several redwood trees in my yard and the lower branches are turning brown and dying. Can you tell me what's wrong with them?

A: Have you cut back on watering these trees in the last couple of years? Lack of water is the cause of most die back I see in coastal redwoods — Sequoia sempervirens — in our area.

The last couple of years of drought, especially during the winter months, have caused trees to become more water stressed — even with regular irrigation in the summer.

Redwoods have many shallow roots, so they are more easily drought-stressed. They need regular irrigation whenever the top three inches of soil are dry, even during winter months. If possible, increase the weekly amounts of water provided to the trees.

To determine how much to water your trees will need each week, use the formula below to calculate the amount of water they have lost due to evapotranspiration — the trees' water loss due to evaporation and transpiration.

Gallons of water needed = ETo × PF × LA × 0.623

• ETo: Evapotranspiration is found on the California Irrigation Management Information System website at bit.ly/3v7eN27. Once on the website, choose the Shasta College station number 224 or a another CIMIS station near you and click “run a report." You can choose from hourly, daily or monthly reports. I find the daily reports to be the most helpful because they estimate how much water was lost that day in inches for fescue grass.
• PF: The PF in the equation is the plant material adjustment factor. For most trees this is 0.5, which represents the percentage of water the tree uses in relation to fescue grass. If you would like to learn more about plant material adjustment factors, check out the Water Use Classification of Landscape Species website at bit.ly/3Hfc9Kg.
• LA: The LA is the landscape area in square feet. For a tree, use the area under the dripline of the tree for this measurement.
• 0.623: This is a conversion of inches of water to gallons of water.

Depending on the size of the redwoods, they may transpire up to 500 gallons of water per day during the summer. Adding at least three inches of mulch around the trees will help conserve water and keep the roots cool.

Another cause of browning branches on redwoods may be due to a couple of diseases.

• If the tree branches are dying more on one side, or more randomly throughout the tree, the dieback may be due to Botryosphaeria dothidea. This is a plant pathogen that causes the formation of cankers on a wide variety of trees and shrub species, including redwoods. I see this disease more on giant sequoias, but it can also infect coastal redwoods. In our climate, heat and water stress the trees, making them more susceptible to this disease.
• Another disease that can cause branches to die is the Botrytis fungi. This fungus infects needles during the winter when humidity is high, causing grayish-brown spores to form on the needles. The infection often results in the needles turning brown. In bad infestations entire branches will turn brown.

To help redwoods battle disease and help stop them from losing branches, increase the amount of water they get and check their soil pH. Redwoods like more acidic soils.

The Shasta Master Gardeners Program can be reached by phone at 530-242-2219 or email mastergardener@shastacollege.edu. The gardener office is staffed by volunteers trained by the University of California to answer gardeners' questions using information based on scientific research.