Irrigating Avocado Trees

Irrigation is important to avocado trees in California because avocados are native to the humid sub-tropical and tropical regions of central and northern South America where rainfall is abundant. In comparison, California is an arid Mediterranean climate with low rainfall. For best growth and yields avocado trees need a minimum amount of water each year, approximately 40-50 inches of rain, and moist soils in order to support the number of roots needed for healthy avocado trees. The avocado tree does not search for water therefore water needs to be provided to the trees at the right times. In addition, the majority of avocado roots are in the top six inches of soil under the canopy out to the drip line — this soil is prone to drying out.

Irrigation can be the greatest cost of production so needs to be used as efficiently as possible. The exact amount of water differs for each grove due to differences in soil type, micro-climate tree size and health, and the aspect of the grove.

When to irrigate avocado trees

• Before the avocado trees become water stressed
• At key phenological times like flowering and fruit set followed by the rapid fruit growth phase
• Deciding when to irrigate can be done using weather-based methods, soil-based schedules or a fixed frequency every so many days

Determining how much water to use when irrigating avocado trees

• Often the depth of irrigation matches the location of the avocado roots
• To calculate the amount of water and the best irrigation schedule for your grove, the CIMIS system can be used to calculate the daily water use: (http://wwwcimis.water.ca.gov/cimis/welcome.jsp)
• Properly maintained 12” and 24” tensiometers can be a used for direct soil measurement to refine when to start irrigating and to see if you are over irrigating.
• Depending on the soil type irrigation can generally start when 12” tensiometers read 25-30 centibars, earlier if the soil is very sandy and later if the soil has high clay content.
• Different soils need different amounts of water as sandy soils hold less water and have less lateral movement of water than clay based soils
• Large and small trees require different amounts of water, enough water for a large tree with a good layer of mulch can be less than for a small tree
• Use less water on sick or defoliated trees as they have smaller root systems than healthy trees
• Take care not to irrigate for too long on slopes or there is a risk of washing soil away
• Avocado roots are sensitive to waterlogging, do not over irrigate avocado trees  

The design of an irrigation system is a job for a professional, seek advice when deciding what irrigation system to install. To control costs, efficiency of the irrigation system is important — aim for about 80% efficiency with a high distribution uniformity. Most California avocado growers use under tree individual low-volume sprinklers for high distribution uniformity and to minimize over-wetting the soil potentially spreading root rot.