Drought and Trees
The drought in Southern California is taking a toll on trees. Thousands of trees have died over the past few years and more will be lost in 2015. Trees that become water stressed often are attacked by pests and diseases. Many do not recover. Refer to the corresponding illustrations on page 3.
Which Trees Do I Water?
Landscape trees and fruit trees take years to mature and provide value and benefits to your home. It may not be possible to replicate those benefits in our lifetime. Woodland trees adapt to the environment but are also vulnerable in a drought. The strong survive and the weak do not. Choose natural selection or provide supplemental water.
How Much is an Inch of Rainfall?
- 1” (inch) of rain/square foot is 0.62 gallons of water.
- 1” of rainfall/acre is approx. 27,000 gallons or 2 swimming pools*.
- Normal Santa Barbara annual rainfall of 16” = approx. 432,000 gallons/acre or 32 swimming pools*. We have received half of normal rainfall this year (2015).
- 1 HCF = 748 gallons or hundred cubic feet = 1” rainfall / 30’ x 40’ roof top
- TIER ONE water use in SB is 4 HCFs or 4” rain / 30’ x 40’ roof top
* pool size based on rectangular shape 15’ x 30’ and 6’ at deep end, 2’ at shallow end
- Target the absorption roots
- Apply water efficiently
- Follow up to maintain soil moisture and tree health
Target the Absorption Roots
- The spread and depth of roots is affected by tree size, species, and soil conditions. Most roots grow in the upper 3”-18” (inches), but target the absorption roots in the upper 3”-6” (inches).
Apply Water Efficiently
- Water can be efficiently applied through drip systems, soaker hoses, or a water jet. With these systems, watering targets the absorption root zone and provides an opportunity for you to calculate water use. These methods of irrigation minimize run off and evaporation.
- Flood irrigation works well with small trees that have basins around their root zone (you can use drip hose with bubblers).
Follow-Up to Maintain Soil Moisture and Tree Health
- Monitor soil moisture with a soil tube, auger, or shovel to check for moisture at 3”-6”. This helps determine the timing and need for the next application. If some soil moisture remains, water will be more readily absorbed during the next application.
- Apply 2”-3” of mulch over the target root zone after watering. Once a layer of mulch is applied, more irrigation time will be necessary to get through that sponge-like layer. It is common for people to water the mulch and not the underlying soil. This is where a water jet works best or install drip or soakers below the mulch to reduce water use during the repeated irrigations.
- Repeat watering each month or more frequently with trees developing fruit.
- Avoid quick release fertilizers as these may increase salt levels in the soil and hold back water from roots.
- Pruning should be limited to removal of deadwood, weakly attached, heavy and dangerous limbs.
- Avoid soaking the trunk of a tree due to its susceptibility to fungi from continuous contact with water. Don’t worry if you get it wet, just avoid the soaking.
Soil, Roots, and Water
Movement of Water Through Soil
- Water moves through soil in layers similar to an onion pattern. As water is slowly applied, it moves laterally (sideways) and downward due to gravity and capillary action. Soil type (texture) and soil structure regulate the movement of water as it is applied.
- Soil type or texture is the amount of sand, silt, and clay particles in the soil.
- Soil structure is the how particles are aggregated or clustered together. .
- Different soil types (sand, silt, clay, loam) and structure (compacted, rocky, non-compacted, etc.) effect drainage and root depth. Root growth in clay soils is shallow and spreading. Root growth in sandy soils is deep but narrow. Root growth in loamy soils (mix of sand, silt, and clay) is both deep and spreading. This also depends on the amount of moisture in the soil.
Learn more about your soil type through a feel test.
- Take a small sample of soil in your hand and apply drops of water until you can form a ball. The dominance of grit in the ball indicates high sand content. The dominance of stickiness indicates high clay content. A combination suggests a more loamy soil or one that has benefits and detriments of each soil type.
- Learn more about the feel test at: www.ndhealth.gov/WQ/SW/Z1_NPS/PDF_Files/Soil_Texture_Feel_Test.pdf
Learn more about drainage of your soil through a simple perc test.
- Scrape away the layer of organic matter in approx. a 12” x 12” area to expose the top soil.
- Slowly pour a quart of water onto the soil. Watch it drain quickly, slowly, or run off.
- If it drains instantly, there is likely a lot of sand content.
- If water runs off or drains super slow, it likely has a heavy clay content or is compacted.
- If it is somewhere in between, it is probably loamy (the best).
- This is not a perfect test but it does provide some basic information about how your soil drains and potential patterns of root growth.
Timing and Frequency
- There is no accurate formula for irrigation as it is unique to every property.
- Our goal is to get water to the target root zone (absorption roots) so it will be readily absorbed. The drip, soaker, or water-jet will need to be moved around to wet the target area.
- If using a drip or soaker system, start with 1-4 hours and monitor the depth and spread of moisture in the target root zone. Move the system to cover the entire target root zone.
- The water jet works well to get water to the target root zone but is more labor intensive. Build and use a water jet at: www.santabarbaraarborist.wordpress.com/instructions-to-build-use-deep-water-jet/ Its a great tool that costs about $65 to build and provides many benefits.
- Monitor soil moisture with a shovel, probe, or auger. Add more water if necessary.
- Repeat monthly with large trees and more frequently with small trees or those in sandy soil.
- Fruit trees will need more frequent irrigation to promote fruit development.
- Hot weather will increase the need for more frequent irrigation.
- Trees in clay soils will need less frequent irrigation than those in sandy soils due to the greater water holding capacity in clay than in sand.
Prepared by: Bill Spiewak firstname.lastname@example.org
Registered Consulting Arborist #381 www.sbarborist.com
American Society of Consulting Arborists (805) 331-4075
Board Certified Master Arborist #310B
International Society of Arboriculture