WateringTimesWatering plan, $19
Quercus douglasii, WUCOLS IV

How often to water blue oak in California

In July in the Central Valley, established blue oak in loam needs a deep soak only about once a month or less: about 55 gallons each time under a 10 foot canopy. Blue oak is a deciduous California native oak of the foothills, found from about Los Angeles to Sacramento and beyond. It usually grows slowly to about 30 feet, sometimes 50, with blue-green to blue-gray leaves and light bark. Leaves turn red and yellow in fall, and on dry sites some drop in summer. Once established it thrives with little care and no water.

TreeCalifornia nativeRated for 4 of 6 zones

How often to water blue oak, month by month

Your city
Canopy width
10 ft
Your soil

Type your city above and the days between waterings and the gallons appear here, worked out from your city's climate table. Or read the figures by climate zone further down.

Every month, on your water days. The personal plan prints this for blue oak and the rest of your yard, on the days your water supplier allows.Get the plan for $19

How this is worked out: established roots fill the top 1.5 feet of soil, and you water again when that soil has dried halfway. Loam soil holds about 1.5 inches of water per foot. Water the whole area under the branches, and skip a watering after a good soaking rain.

Which soil do you have?

Dig down a few inches, take a handful of moist soil and squeeze it.

  • Sandy: gritty, falls apart when you open your hand, will not form a ball.
  • Loam: forms a soft ball that crumbles when you poke it.
  • Clay: forms a firm, sticky ball and squeezes out into a ribbon between thumb and finger.

Still not sure? Pick loam. Most garden beds sit between sand and clay.

How to measure canopy width

A tree with its canopy width marked across the widest branches and the circle under it shaded blue

Measure across the branches, from the tips of the outer leaves on one side to the tips on the other. A tape works, or pace it out.

Water the whole circle under the branches, shown in blue. The gallons here cover that circle.

A wider canopy means more gallons each watering, not more waterings. While the tree is young, measure again each spring.

How to tell it needs water

  • A first-summer blue oak whose root zone is dusty deep down during a heat spell can use one slow, deep drink
  • Some leaves dropping in late summer on a dry site is normal for blue oak
  • Yellowing leaves and dieback near a lawn or drip line point to too much summer water
  • Tender spring shoots on a sapling collapsing in sudden heat may mean it is too dry

How to water it

  • Give a sapling slow, deep drinks spaced far apart
  • Wean it off summer water after the first few years
  • Site it in sun or part shade, in clay or other soil that drains
  • Keep the ground under the canopy free of weeds so leaf litter builds up
  • Pair it with other dry-adapted natives rather than lawn

Through the seasons

For the first few years after planting, give it occasional deep summer drinks. Once established, it grows with little care and no summer water at all, a true dry garden tree. Leaves color in fall and drop for winter. Blue oak gets a very low water rating from WUCOLS along the north-central coast, through the Central Valley and on the south coast. For South Inland the rating is low water. WUCOLS marks it as not suited to the high desert or the low desert.

In pots

Blue oak grows so slowly that a seedling can live in a tall pot for a few years. Fill a deep container with a gritty, fast-draining mix. Water when the upper few inches are dry, then let every drop drain away. During hot months, look at the pot twice a week or so. Do not let water pool beneath it. In fall and winter, rain usually covers it. Plant it in the ground while it is still small.

1 Your climate zone six WUCOLS zones, each from its own reference city
0.27gal per sq ft in JulyVery low water use in the Central Valley zone
A year
1.71 gal per sq ft
Reference
Sacramento climate table
Counts
Plant need, before sprinkler losses
0.250.500.751.00.04JAN0.07FEB0.11MAR0.15APR0.21MAY0.25JUN0.27JUL0.24AUG0.18SEP0.12OCT0.06NOV0.03DEC0.250.500.751.00.04JFM0.15AMJ0.27JAS0.12OND
Gallons per square foot per month, Central Valley. Peak 0.27 in Jul, 1.71 a year.

Monthly water need by climate zone

Blue Oak: gallons per square foot per month, by WUCOLS climate zone

MonthN. CoastC. ValleyS. CoastS. InlandHi DesertLo Desert
Jan0.040.040.080.31––
Feb0.050.070.080.35––
Mar0.090.110.110.52––
Apr0.120.150.140.64––
May0.150.210.160.78––
Jun0.160.250.170.92––
Jul0.160.270.21––
Aug0.150.240.190.93––
Sep0.120.180.150.74––
Oct0.090.120.110.53––
Nov0.050.060.080.36––
Dec0.030.030.070.27––
Year1.21.711.557.37––

How this is worked out: the WUCOLS water rating for each zone times that zone’s climate for the month. A dash means WUCOLS does not rate the plant there. The figures are what the plant uses, before sprinkler losses.

Plants with the same water needs 16

Rated very low water in the Central Valley, like blue oak. Plants that drink alike can share one valve and one schedule.

Oak TreesQuercus agrifoliavery low water, Central Valley0.27 gal/sq ft in JulySalt CedarTamarix aphyllavery low water, Central Valley0.27 gal/sq ft in JulyPearl AcaciaAcacia podalyriifoliavery low water, Central Valley0.27 gal/sq ft in JulyMexican Palo VerdeParkinsonia aculeatavery low water, Central Valley0.27 gal/sq ft in JulyFoothill PinePinus sabinianavery low water, Central Valley0.27 gal/sq ft in JulyInterior Live OakQuercus wislizenivery low water, Central Valley0.27 gal/sq ft in JulyNuttall's Scrub OakQuercus dumosavery low water, Central Valley0.27 gal/sq ft in JulyCalifornia BuckeyeAesculus californicavery low water, Central Valley0.27 gal/sq ft in JulyTree of HeavenAilanthus altissimavery low water, Central Valley0.27 gal/sq ft in JulyOleanderNerium oleander ssp. & cvs.very low water, Central Valley0.27 gal/sq ft in JulyAgaveAgave americana (and thick-leaved relatives)very low water, Central Valley0.27 gal/sq ft in JulyCactusOpuntia spp. & cvs. (CA natives and non-natives)very low water, Central Valley0.27 gal/sq ft in JulySpanish BroomSpartium junceumvery low water, Central Valley0.27 gal/sq ft in JulyBig SagebrushArtemisia tridentatavery low water, Central Valley0.27 gal/sq ft in JulyCalifornia PoppyEschscholzia californicavery low water, Central Valley0.27 gal/sq ft in JulyCalifornia Scrub OakQuercus berberidifoliavery low water, Central Valley0.27 gal/sq ft in July
Minutes for your sprinklers

Turn this into minutes on your water days

Every city page has a calculator that takes blue oak, your sprinkler type and the month, and gives the minutes per water day for that supplier's schedule. Or put it in a zone of the personal watering plan and get all twelve months printed.

Questions people ask

How often should I water blue oak in summer?

In July, established blue oak in loam needs a deep watering about once a month or less in the Central Valley, about once a month or less on the south coast and about every 22 days in the South Inland zone. Sandy soil holds less water, so waterings come closer together. Clay stretches them. Check the soil before each watering and skip one after a soaking rain.

How much water does blue oak need per day?

In July in the Central Valley, established blue oak needs about 0.7 gallons a day under a 10 foot canopy and about 2.7 gallons a day under a 20 foot canopy. Give it in deep waterings spread days apart rather than a little every day, so the whole root zone gets wet.

Does watering for blue oak change across California?

Yes. WUCOLS rates blue oak separately for each of the six California climate zones: very low water in the North-Central Coastal zone, very low water in the Central Valley zone, very low water in the South Coastal zone, low water in the South Inland zone, not suited to this region in the High and Intermediate Desert zone, not suited to this region in the Low Desert zone. The monthly table above shows how the same rating translates into different gallon amounts as reference evapotranspiration changes.

How fast does blue oak grow?

Very slowly, often 1 to 3 inches a year with occasional faster spurts. It is a long-term tree. Patience is part of growing it.

Why does my blue oak drop leaves in summer?

On dry sites it sheds part of its leaves in summer. That is expected behavior. Resist the urge to water heavily.

What is the gray moss hanging on my blue oak?

That is a lichen often called Spanish moss. It lives on moist air, not on the tree. It rarely needs removal.

Where does blue oak grow naturally?

It grows on dry, rocky foothill slopes around the Central Valley. Its range runs north toward the Klamath Mountains and south to the Tehachapi Mountains. It is a classic oak woodland tree.

Does blue oak need leaf litter?

Yes, its leaves can build a litter layer if the ground under it stays weed free. Leave fallen leaves in place. Weeds slow that buildup.

More plant watering guides

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Sources