How, When, and Why to Water Trees in Southern California

By Richard Flowers, ACCNP-Green Thumb Nursery-Ventura

Water trees deeply and infrequently, soaking the soil over the whole root zone out near the dripline rather than splashing the trunk. Newly planted trees need water often, close to daily at first, tapering to weekly through the first year or two, while established trees want an occasional deep soak, roughly every one to two weeks in the growing season and far less in winter. A useful rule of thumb for an established tree is about 10 gallons of water per inch of trunk diameter per soak. Water early in the morning, keep a 2 to 3 inch layer of mulch over the root zone but off the trunk, and remember that overwatering is just as deadly as drought, because constantly soggy soil suffocates and rots tree roots. Amounts vary with your soil, the species, and the weather, and established California native oaks in particular want very little summer water near the trunk.

Tree stage	How often	How much and where
Just planted (first 1 to 2 weeks)	Daily	About 1 to 1.5 gallons per inch of trunk diameter, over the root ball
Establishing (weeks 3 to 12)	Every 2 to 3 days, then weekly	Same amount, soaking the root zone out to the dripline
Young (year 1 to 3)	Weekly, more often in heat	A deep soak over the whole root zone
Established or mature	Every 1 to 2 weeks in the growing season, less in winter	About 10 gallons per inch of trunk diameter per deep soak, out near the dripline

How to Water a Tree the Right Way

The single biggest mistake people make is watering the trunk. A tree’s absorbing roots are surprisingly shallow and wide: most sit in the top foot of soil and spread out to and well beyond the edge of the canopy, so the water should go over that broad root zone, out near the dripline, not against the base of the trunk.^[1] In fact, the majority of a tree’s roots grow in the top 12 inches of soil, which is why a wide, even soak matters more than a deep narrow one at the trunk.^[2]

Water deeply and infrequently rather than with frequent light sprinkling. Shallow, frequent watering, the way a lawn sprinkler runs, trains roots to stay near the surface where they are far more vulnerable to heat and drought, so a garden hose, soaker hose, or drip system that slowly soaks the root zone is better than a quick spray.^[3] A good method is a slow, deep soak: let a soaker hose run over the root zone for a long stretch so the water moves down into the soil instead of running off.^[4] Soak the root zone, then let the top several inches of soil dry out before you water again.^[1]

How Often and When to Water Trees

A newly planted tree needs the most attention. A good starting schedule is to water daily for the first one to two weeks, then every two to three days through about week twelve, then weekly until the tree is established, giving roughly 1 to 1.5 gallons per inch of trunk diameter at each watering.^[5] Keep up at least weekly watering through the first season, more often in hot or windy weather, and taper off into fall as cooler weather slows the tree down.^[2]

Established trees need far less frequent, but deeper, watering. A widely used rule of thumb is about 10 gallons of water per inch of trunk diameter at each deep soak, so a four inch diameter tree wants roughly 40 gallons.^[1][4] In the growing season that usually works out to a deep soak every one to two weeks, adjusted for your soil: sandy soils may need watering every three to five days, while heavier clay soils may need it only every week and a half to two weeks.^[3] A simple test is to water when the top six to nine inches of the root zone have dried out, and to water early in the morning to limit evaporation and wind drift.^[6]

In winter, trees slow down and need much less water, but in a dry Southern California winter you should not let them go bone dry. During warm, dry, rain-free stretches, give trees an occasional deep watering to carry the roots through, especially younger trees and recent plantings.^[7] For help delivering a slow, even soak, see our guide to drip irrigation systems, and our take on how much water your plants really get when it rains.

Why Trees Need Deep Watering, and the Overwatering Trap

Deep, infrequent watering builds a deep, resilient root system that can ride out heat and drought, which is the whole point of watering a tree well. But more water is not always better. Overwatering is just as harmful to a tree as drought, and in clay or compacted soils it can be worse, because waterlogged soil has no air in it and the roots suffocate and decline.^[6] This is also why overwatering is so often misdiagnosed: as the waterlogged roots die, the tree can no longer take up water and its leaves discolor and wilt, so it looks thirsty even though the real problem is too much water.^[8]

Mulch is one of the best things you can do for a tree, with one important caveat. Spread a 2 to 3 inch layer of mulch over the root zone to hold moisture and moderate the soil, but keep a gap of a couple of inches between the mulch and the trunk. Piling mulch up against the bark, the so called mulch volcano, traps moisture against the trunk, rots the bark, and can deprive the roots of oxygen.^[9] Keeping a small mulch free ring at the base of the trunk prevents that moist bark and decay.^[2]

Finally, watering is not one size fits all, and the clearest example in our area is the native oak. Established California native oaks are adapted to dry summers and can be damaged or killed by frequent summer irrigation near the trunk, which encourages fungal diseases like oak root fungus and crown rot. If you must water an established oak during a prolonged drought, keep the water well away from the trunk, out in the outer part of the root zone.^[10] For a closer look at that iconic tree, see our guide to the coast live oak. And for a large, mature, declining, or potentially hazardous tree, have it assessed by a certified arborist rather than guessing, since a professional can judge what a specific tree needs.^[11]

Not sure how much your trees need, or whether that stressed tree is getting too much water or too little? Stop by any Green Thumb Nursery location and our team can help you sort out a watering plan, the right mulch, and the best way to deliver a deep, slow soak.

Tree Watering FAQ

How often should you water a tree?

It depends on the tree’s age. A newly planted tree needs water daily for the first week or two, then every two to three days, then weekly through the first season. An established tree needs a deep soak only about every one to two weeks in the growing season, and less in winter.^[5]

How long should I water a tree with a hose?

Long enough for a slow, deep soak of the root zone rather than a quick spray. A soaker hose or a hose turned to a trickle, run over the root zone out near the dripline for a good while, lets the water move down into the soil instead of running off.^[4]

How much water does a tree need?

A common rule of thumb for an established tree is about 10 gallons of water per inch of trunk diameter at each deep soak, so a four inch diameter tree wants roughly 40 gallons. A newly planted tree needs less at a time, about 1 to 1.5 gallons per inch of trunk diameter, but more often.^[1]

Should you water trees in winter?

Much less than in summer, but not never. In a dry, rain-free Southern California winter, give trees an occasional deep watering during warm, dry spells, especially young or recently planted trees, so the roots do not dry out completely.^[7]

My tree’s leaves are yellowing and wilting, is that too much or too little water?

It can be either, which is why it is easy to get wrong. Overwatered roots suffocate and die, and the tree then cannot take up water, so it wilts and discolors and looks thirsty even though the soil is too wet. Check the soil several inches down before adding more water: if it is soggy, hold off.^[8]

Sources

• [1] Colorado State University Extension, Watering Mature Shade Trees: extension.colostate.edu
• [2] International Society of Arboriculture (Trees Are Good), New Tree Planting: treesaregood.org
• [3] UC IPM, Watering Fruit and Nut Trees: ipm.ucanr.edu
• [4] Arbor Day Foundation, Top Tree Watering Tips for Summer: arborday.org
• [5] University of Minnesota Extension, Watering Newly Planted Trees and Shrubs: extension.umn.edu
• [6] University of Minnesota Extension, Watering Established Trees and Shrubs: extension.umn.edu
• [7] Colorado State University Extension, Fall and Winter Watering of Plants and Trees: extension.colostate.edu
• [8] UC IPM, Water Excess or Deficiency: ipm.ucanr.edu
• [9] University of Illinois Extension, Mulch Volcanoes Slowly Kill Trees: extension.illinois.edu
• [10] UC ANR, Summer Irrigation of Established Oak Trees: ucanr.edu
• [11] International Society of Arboriculture (Trees Are Good), Find a Certified Arborist: treesaregood.org

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