Proper irrigation practices can help ensure long-term tree health.  In fact, over-watering can cause diseases to attack your tree and under-watering can starve limbs and cause them to fail. Watering correctly can prevent these secondary problems from occurring.

The most important rule to remember with trees is to water deeply and infrequently.

Soil should not be constantly moist, especially around the trunk.  Water the root-zone underneath the tree’s canopy by using drip emitters, bubblers, or a hose on a slow trickle.  Young, newly planted trees only need water once a week, about 15 gallons per watering.  Older, mature trees need water even less frequently – usually once a month is plenty during the dry months of summer.  Consider the fact that, roots extend 3 times the width of the canopy, so even if you are not watering, the tree may be accessing water from a neighbooring property.  The rule of thumb for mature trees is to water 10 gallons for every 1 inch of trunk diameter per watering.  Watering infrequently causes the roots to go down and can help the tree access moisture deep in the soil whereas watering every day or every other day ( which is what often happens with trees in lawns) causes roots to stay near the surface where nobody wants them.

Mature native oaks do not want any supplemental irrigation during the months of June-September.  Watering them during this time can cause a root rot disease (Oak Root Fungus) to spread and it can kill the tree over time. The summer is their naturally dormant period which is a characteristic shared by many California native plants and mediterranean climate species.  Non-native trees such as maples and magnolias have incompatible watering needs and should not be planted near native Oaks and visa-versa.

People often ask me about using tree wells and sticking a hose down it to water.  This practice is even used by landscapers and is so prevalent that it has become almost standard.  However, tree wells were never designed for irrigation, they were designed for aeration of compacted soils.  Using them to water just localizes the water to one or two spots and does not adequately hydrate the tree.  Also, 90% of the root mass of most trees is between 3ft and 6″ from the surface. Sticking a hose down there can often put the water out of reach of the roots, especially with fast draining soils. Furthermore, as the tree grows in diameter, it will often crush the wells or worse, the wells, will end up damaging the trunk and girdling the tree which can cause a new set of problems.  A better design is a “tree ring” with drip emitters spaced evenly on the tubing, with several rings spread 2 ft apart under the canopy.

Trees need water under their canopies, or in other words, within the drip-line of the tree.  Think of how rain falls on a tree and trickles down.  The best watering methods simulate nature.  So drip emitters or bubblers evenly spaced under the canopy or moving a hose around several times is the best way to water a tree.  This is, of course, a challenge in areas with limited root space, where the tree is hanging over concrete or asphalt. Trees are very forgiving in general, so it is not like you have to water every square inch under the dripline.  However, by following these principles and methods as best you can, your tree will be happier and healthier.