California’s drought marches onward and our trees need more water, or do they?

We are frequently asked how often and how much to water trees. Our native oak species do not require water during the summer months but will need water monthly during a dry winter. However, all other trees are considered introduced species that have adapted to our climate.  Does this mean watering every day or twice per day? NO.  All plant roots require oxygen to maintain healthy growth and watering every day or twice per day reduces oxygen to the root system.

Some trees such as our California Redwood are naturally coast trees and get moisture from the fog. In the Central Valley we do not get the type of fog found along the coast so redwoods can suffer in our hot, dry temperatures.  

Trees develop small cracks in the bark and leaves that allow fungal and other pathogens into the tree.  Examples of drought related disease include the fungal needle blight on redwood, wetwood on maple and oak, bacterial canker on citrus to name a few.  Redwoods need water at least 3 times per week and try to increase the irrigated area under the redwood canopy when temps are 100F for more than 3 days in a row.

The International Society of Arboriculture recommends about 5-10 gallons per inch of trunk diameter. For young trees less than 1” in diameter, water 3 times per week using 1 gallon directed at the root ball for 3 months. Gradually widen the placement of water outside the root ball keeping within the edge of the canopy.  Reduce watering as temperatures cool off but continue to provide water if winters are dry. 

For mature trees (>25 years), or those with a trunk more than 12″ (30 cm) in diameter, water deep and occasionally but do not direct water at the bas e of the trunk.  About 10 gallons per 1 inch (2.5 cm) of trunk diameter per week (ex., a tree with 12″ DBH would receive 120 gallons) during drought.