Saving the Trees

Healthy urban trees provide critical benefits in our Mediterranean climate. They clean the air and sequester carbon, provide shade, reduce energy use in buildings, reduce water use in the surrounding landscape, slow stormwater runoff and help recharge groundwater, and often add thousands of dollars’ worth to your home value. Unfortunately, drought has jeopardized the future of trees throughout California. Up to 20%, or 120 million, trees may die in our wild forests due to the historic 4-year drought. This makes the need for proper management of our urban forests even more critical.

But mandated irrigation cutbacks have hit our urban trees hard too. Hundreds of urban trees in San Diego have died as lawns are removed and high water landscapes are replaced with drought tolerant plants. According to the Friends of Balboa Park, 10-15% of the historic trees in the Park are dead or dying. The good news is that even in the most severe level of drought (CA Drought Response Level 4), tree watering is allowed on residential and commercial properties. And if done property, trees can be kept healthy within drought tolerant zones, even if they require more water than the surrounding landscape.

Trees in the Dying Green

Transitioning established trees to a drought tolerant regime is a matter of maximizing irrigation efficiency, and watering to encourage deep rooting. Whether you are removing the lawn surrounding your trees, or just looking to reduce watering generally, the following best practices will keep your trees thriving.

• Install an in-line drip, or micro-spray, irrigation system in an expanding circular pattern under the outer half of the drip zone of your trees—usually drip irrigation is allowed without timing restrictions even during drought
• Water long enough to saturate the soil to a depth of four inches, which will encourage deep rooting (see tree watering frequency table below for specific recommendations)
• If the soil is dry at a four inch depth, it is time to water—you can check soil moisture with a screwdriver or inexpensive water meter
• Apply two-to-four inches of organic mulch starting a few inches from the tree trunk to just outside the drip line, covering the irrigation tubing, to keep the soil cool thus reducing water loss, and to prevent sun damage to the irrigation tubing
• Water in the evenings or early mornings to help the trees stock up on water ahead of hot daytime conditions
• Newly planted trees, as well as trees transitioning out of lawn conditions, should be watered one-to-two times weekly during the first year, weekly the second year, and every other week in the third year (watering frequency depends of overall water requirements of the tree species)
• Some very drought tolerant trees, such as native oaks, resent watering underneath their canopies when they are mature. However, these species will benefit from the recommended watering regime from the edge of their drip zone to several feet outside the canopy.

More information about this irrigation approach can be found at the UC Master Gardener page at the University of California Agriculture and Natural Resources website.

Recommended Trees for San Diego County

Watering needs of trees often varies by location and soil type as well as tree species. This list of low to moderate water trees is appropriate for coastal and inland foothill areas of San Diego County, and can be used in conjunction with the Tree Watering Frequency Table below. The few trees that fall into the Very Low Water Need category can often be successful without irrigation after they are established and three-to-four years mature.

If you are gardening in an area with heavy clay soil, as many in coastal San Diego are, research the clay tolerance of prospective new trees before planting.

For more information about water needs of trees and other plants in San Diego County, please visit the Water Use Classification of Landscape Species page at the University of California, Agriculture and Natural Resources.

Low to Moderate Water Trees