WateringTimesWatering plan, $19
Quercus tomentella, WUCOLS IV

How often to water island oak in California

In July in the Central Valley, established island oak in loam needs a deep watering about every 20 days: about 55 gallons each time under a 10 foot canopy, about 19 gallons a week. Island oak is a rare evergreen oak native to canyons and ridges on the California Channel Islands and Guadalupe Island. It grows upright and nearly conical when young, then broadens to 35 to 50 feet with a pyramidal crown. Its leathery, dark green leaves have widely spaced teeth and hairy, paler undersides. It is quite drought tolerant and does best in the coastal fog belt.

TreeCalifornia nativeRated for 4 of 6 zones

How often to water island 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 island 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

  • Island oak keeping its toothed, dark leaves stiff and glossy in August means the roots are fine
  • Drooping tip growth on a newly planted island oak, with dust-dry soil a foot down, calls for a slow, deep watering
  • Leaves going yellow where a sprinkler wets the ground all summer mean it is getting too much
  • Big acorns falling in autumn are part of its yearly cycle, not distress

How to water it

  • Soak the whole root area of a young island oak slowly, then wait until the ground is dry well below the surface
  • Site it in sun or partial shade where the soil drains freely
  • Once it has settled in, give only an occasional summer soak, or none in foggy gardens
  • Pick a spot near the ocean where summer fog reaches it
  • Keep irrigation for lawns and beds well clear of the trunk

Through the seasons

Give a newly planted tree slow, deep soaks for its first few dry seasons so roots reach down. After that, coastal trees get by on occasional water or very little, and foggy summers help. Island oak keeps its leaves all year, and its large acorns ripen in fall. Rain from late fall into spring carries it near the ocean and in the Central Valley. Island oak gets a low water rating from WUCOLS on the north-central coast, in the Central Valley, on the south coast and in South Inland. Neither desert has a WUCOLS evaluation: WUCOLS has not evaluated it for the high desert or the low desert.

In pots

Island oak grows into a big tree, so a pot is only a nursery stage for a seedling or two-year-old. Fill a tall container with a gritty, free-draining mix. Soak it whenever the upper two inches feel dry, and let the excess run out. On warm days, feel the mix every few days. Pour off water from the saucer. Plant it in the ground while it is small, before the roots coil. Through the rainy season, add water only if the mix goes dry.

1 Your climate zone six WUCOLS zones, each from its own reference city
1.07gal per sq ft in JulyLow water use in the Central Valley zone
A year
6.84 gal per sq ft
Reference
Sacramento climate table
Counts
Plant need, before sprinkler losses
0.270.540.801.10.16JAN0.26FEB0.43MAR0.61APR0.83MAY0.99JUN1.07JUL0.95AUG0.72SEP0.47OCT0.22NOV0.14DEC0.270.540.801.10.16JFM0.61AMJ1.07JAS0.47OND
Gallons per square foot per month, Central Valley. Peak 1.07 in Jul, 6.84 a year.

Monthly water need by climate zone

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

MonthN. CoastC. ValleyS. CoastS. InlandHi DesertLo Desert
Jan0.150.160.310.31––
Feb0.210.260.320.35––
Mar0.350.430.460.52––
Apr0.470.610.560.64––
May0.580.830.640.78––
Jun0.640.990.690.92––
Jul0.641.070.791––
Aug0.60.950.740.93––
Sep0.50.720.60.74––
Oct0.350.470.460.53––
Nov0.190.220.330.36––
Dec0.120.140.260.27––
Year4.86.846.27.37––

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

Olive TreesOlea europaea ssp. & cvs.low water, Central Valley1.07 gal/sq ft in JulyFig TreesFicus carica ssp. & cvs.low water, Central Valley1.07 gal/sq ft in JulyCrape MyrtleLagerstroemia spp., hybrids & cvs.low water, Central Valley1.07 gal/sq ft in JulyAloeAloe spp. & cvs.low water, Central Valley1.07 gal/sq ft in JulyPomegranate TreesPunica granatumlow water, Central Valley1.07 gal/sq ft in JulyEucalyptusEucalyptus polyanthemoslow water, Central Valley1.07 gal/sq ft in JulyPine TreesPinus canariensislow water, Central Valley1.07 gal/sq ft in JulyJunipersJuniperus spp. & cvs.low water, Central Valley1.07 gal/sq ft in JulyTexas Red OakQuercus buckleyilow water, Central Valley1.07 gal/sq ft in JulyPortuguese OakQuercus faginealow water, Central Valley1.07 gal/sq ft in JulySoapbark TreeQuillaja saponarialow water, Central Valley1.07 gal/sq ft in JulyNecklace TreeSophora affinislow water, Central Valley1.07 gal/sq ft in JulyBailey AcaciaAcacia baileyanalow water, Central Valley1.07 gal/sq ft in JulySilver WattleAcacia dealbatalow water, Central Valley1.07 gal/sq ft in JulyGreen WattleAcacia decurrenslow water, Central Valley1.07 gal/sq ft in JulySydney Golden WattleAcacia longifolialow water, Central Valley1.07 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 island 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 island oak in summer?

In July, established island oak in loam needs a deep watering about every 20 days in the Central Valley, about every 27 days 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 island oak need per day?

In July in the Central Valley, established island oak needs about 2.7 gallons a day under a 10 foot canopy and about 11 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 island oak change across California?

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

Is island oak rare?

Yes, it is native only to a few California Channel Islands and Guadalupe Island. It is a relic of a once wider range. It grows in steep canyons and on ridges there.

How is island oak different from coast live oak?

It is lusher and more upright than coast live oak. It is still quite drought tolerant. Its leaves have hairy undersides.

How cold can island oak take?

It is hardy to about 10 degrees F. One tree froze to the ground near 6 to 8 degrees F and grew back from suckers. Mild coastal sites suit it best.

How big does island oak get?

It grows 35 to 50 feet tall and 20 to 30 feet wide. Young trees are narrow and conical. The crown broadens with age.

Does island oak have big acorns?

Yes, its acorns are about an inch long. They sit in thick, warty cups. They ripen in fall.

More plant watering guides

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Sources