It depends on several things:
It depends on soil type and weather conditions; how much moisture is already in the soil; and what kind of tree it is. In order to get this right, one must observe. It's easy enough to look at the leaves and trunk, but it is also necessary to probe into the soil to see what's going on there. The surface may look dry, but a few inches down it could be moist - this is common when it is sunny after a rainy period. But the opposite could be true, especially if you water frequently but don't let the water soak in deeply: the top layer is moist but it is bone dry down below.
Each time you water the tree it should be a slow deep soak at the edges of the tree's dripline (not directly on the trunk), with water penetrating to the deeper tree roots - 12" deep for a young tree. For most new 15 gallon size trees, this will be about 10 to 15 gallons of water. Then wait and let the surface layer (top 6") dry out between waterings. If you do this you will be able to water less often and the tree's roots will go deeper, which is a good thing.
A soil probe is the best tool to quickly check subsurface moisture. It also has the added benefit of letting some air into the soil. Yes, air - it's one of the ingredients plant roots need in order to live. Also, if you think it best to err on the side of too much, keep this in mind: Most residential plant problems are caused by overwatering.
Keeping all that in mind, we suggest you water your tree once a week in summer; every 10 to 14 days in spring and fall. Keep track of the weather. More frequent waterings may be necessary in very hot weather. In cool, moist weather or heavy soils that hold water (like clay) water less frequently.
If your tree is in a lawn area, it may not get enough from lawn watering depending on watering schedule. Short frequent waterings will keep a lawn green but do not water the deeper tree roots. Water your tree deeply – and less frequently than a lawn. Keep a thick layer of mulch over the roots of the tree, but don't cover up the base of the trunk (also called the root crown). With thick mulch you can water less often. And keep lawn and weeds away from the tree trunk! - at least a 2 foot radius circle around the trunk. They compete with the tree's roots.
The drought and the resulting water use restrictions are causing many people to ask “How can I protect my beautiful trees?” We all can see trees in the Ojai Valley showing signs of stress. Although we are not trained arborists, we will try to provide some information that we have been able to glean from several published articles and our experience on this subject. These tips apply to all trees, including our precious oaks.
Why should we be concerned? Trees can be the most valuable asset in our landscapes as well as the most difficult and expensive to replace. Trees help make our valley beautiful while providing habitat for the many birds and animals who live here with us, not to mention cooling shade for us. We cannot imagine Ojai without trees!
So, what to do? First, you must provide the best possible “home” for our trees. Weeds and grasses under the canopy or at least within 4’ of the trunk should be removed, as these compete with the tree for precious water. It is also recommended to remove rocks and other impervious materials which restrict water and oxygen flow to the roots. Finally, add mulch, ideally under the entire canopy. Oak trees love oak leaf mulch, but any plant-based mulch will conserve soil moisture while adding valuable microorganisms. The mulch should be 4-6 inches deep, but should be kept about 4” from the tree trunk.
Once the tree environment has been optimized, the key questions become “How should I water my trees? How often? How much?” The experts agree in their response: “It depends.” Although that may not initially sound helpful, this does make sense. First, if your tree has good foliage over most of the canopy, with new growth this spring, it is probably getting what it needs so no additional water should be required. Valley oaks, which are deciduous and drop their leaves during the winter, will drop some leaves as early as July as a natural reaction to a dry spell. However, you need to continue monitoring all your trees for potential changes during the year.
Second, if you have young trees which were planted recently (within the past 4 years) or trees which are still maturing (up to 15 years old), deep watering will be needed. Create an earth berm at or just beyond the tree drip line or the outermost circumference of the tree canopy. Make sure that the trunk will be the high spot, as you do not want the trunk to sit in water. The root ball of a newly planted tree needs to be watered, but as the tree ages you want the watered area to expand so the roots expand, so extend the berm area as necessary. Creating a shallow trench just inside the berm will cause more water to soak into the ground there, providing incentives for the roots to grow outwards. With a drip irrigation system, a soaker hose, flow from a garden hose, or simply a bucket, slowly add water to the berm area. Since the majority of tree roots are within 12-18” of the soil surface, add enough water such that a metal rod, screwdriver, or similar device can easily penetrate the ground 8-12”. A newly-planted tree needs its root ball watered every two weeks or so while maturing trees will need water every month or so. Keep in mind that our local clay soil does not drain well and native trees need to have the soil dry out between watering, so check the soil (again, with a metal rod or, if available, a moisture meter) before subsequent watering.
Finally, what about mature trees? We’ve all heard that it is bad to water oak trees, but this is an unusually long dry period, and we love our beautiful oaks. The bottom line is that we will need to water our larger trees this year and maybe into the future if the drought continues. The recommended method is slow, deep watering at or just beyond the drip line about every 1-2 months. Again, drip irrigation, soaker hoses, or slow flow from a garden hose will work. As with the younger trees, check watering depth to ensure 8-12” of penetration. A particular problem is trees which have grown within lawn or turf areas. These have been getting frequent water, albeit shallow, from irrigation systems which now are being restricted or even eliminated. These “turf trees” will need to be transitioned from the shallow watering cycle they have been receiving to a more “natural” cycle, with long dry periods by deep watering every 3-6 weeks during the next year or so to encourage their roots to grow deeper in the soil.
Of course, if you determine you need to water your trees, the question becomes “With the drought restrictions, can I?” There is no answer that will fit all situations. But, if you can afford some water for your landscape and if you have to choose between providing water to grass, ornamental plants, small shrubs or trees, prioritizing the trees would be our strong recommendation. So please seriously consider watering your trees this summer!i
More information is available on the Ojai Trees website (ojaitrees.org) as well as various University of California and other sites. If you are not sure what your particular trees need, or are concerned about the health of one or more of your trees, you should consult a trained arborist. Although they will charge a fee for their consultations, consider it a small investment in the value of your tree.