WateringTimesWatering plan, $19
Juglans californica, WUCOLS IV

How often to water California black walnut in California

In July on the north-central coast, established California black walnut in loam needs a deep watering about every 13 days: about 55 gallons each time under a 10 foot canopy, about 29 gallons a week. California black walnut, also called Southern California black walnut, is a winter-deciduous native tree, often with several trunks. It grows wild on slopes, in canyons and near stream beds. Once settled it tolerates heat and drought, yet it does best with some moisture and can take garden watering. Toxins from the tree keep most plants from growing beneath it.

TreeCalifornia nativeRated for 3 of 6 zones

How often to water California black walnut, 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 California black walnut 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

  • Soil dry 4 to 6 inches down at the edge of a young walnut's root ball means it is time for a deep soak
  • Drooping leaflets on a young tree during a heat spell, with dry soil below, point to a thirsty root zone
  • Bare branches from fall through winter are normal for this winter-deciduous native and are not a sign of thirst
  • Leaves that wilt and yellow while the soil stays wet, and do not recover, point to root rot from soggy ground
  • Plants failing under the canopy usually reflect the walnut's toxins rather than a shortage of water

How to water it

  • Water a young walnut over its root ball and a band of soil beyond it, widening the band as the crown spreads
  • Soak slowly until water reaches about 18 inches down, then let the soil dry well before the next soak
  • Set drip emitters or a slow hose toward the canopy edge, and keep spray and pooled water off the trunk
  • Plant it on a slope or bank where it can root deeply, in any soil from fast to slow draining
  • Keep lawn and thirsty beds out from under the canopy, so the area near the trunk is not kept wet in summer

Through the seasons

Give a new California black walnut deep soakings through its first summers, widening the wet area as roots spread. Once established it tolerates heat and drought, and an occasional deep summer soak is the most it needs. Leaves and yellow flowers appear in spring. Leaves drop in fall, and the bare tree handles frost down to about 20 to 25 degrees Fahrenheit. Winter rain covers the cool months on the north-central coast and on the south coast. WUCOLS rates it low water on the south coast and in South Inland. It rates moderate water on the north-central coast. WUCOLS marks it as not suited to the Central Valley, the high desert or the low desert.

In pots

A California black walnut only suits a container while it is young, before it goes in the ground. Grown from a nut, the seed needs months of cold, moist storage before it sprouts. Use a deep pot, check the mix 2 to 3 inches down every few days in summer, and more often in South Inland heat. Water until it drains from the holes, then empty the saucer. Plant it out while small, since the tree soon outgrows a pot. In winter the leafless seedling uses little water, so let the mix dry a little more between soakings.

1 Your climate zone six WUCOLS zones, each from its own reference city
Not ratedWUCOLS does not rate California Black Walnut for the Central Valley zone
A year
no figure
Reference
Sacramento climate table
Counts
Plant need, before sprinkler losses

WUCOLS lists California Black Walnut as not suited to this region for the Central Valley zone, so no monthly figure is computed here.

Monthly water need by climate zone

California Black Walnut: gallons per square foot per month, by WUCOLS climate zone

MonthN. CoastC. ValleyS. CoastS. InlandHi DesertLo Desert
Jan0.37–0.310.31––
Feb0.53–0.320.35––
Mar0.87–0.460.52––
Apr1.18–0.560.64––
May1.46–0.640.78––
Jun1.61–0.690.92––
Jul1.61–0.791––
Aug1.49–0.740.93––
Sep1.24–0.60.74––
Oct0.87–0.460.53––
Nov0.47–0.330.36––
Dec0.31–0.260.27––
Year12–6.27.37––

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 not suited to this region in the Central Valley, like California black walnut. Plants that drink alike can share one valve and one schedule.

Sugar MapleAcer saccharumnot rated, Central Valleyno Central Valley ratingQueensland KauriAgathis robustanot rated, Central Valleyno Central Valley ratingCherimoyaAnnona cherimolanot rated, Central Valleyno Central Valley ratingPurple Orchid TreeBauhinia variegata (purpurea)not rated, Central Valleyno Central Valley ratingToogBischofia javanicanot rated, Central Valleyno Central Valley ratingClustered Fishtail PalmCaryota mitisnot rated, Central Valleyno Central Valley ratingWhite SapoteCasimiroa edulisnot rated, Central Valleyno Central Valley ratingFilbertCorylus maximanot rated, Central Valleyno Central Valley ratingSpotted GumCorymbia maculata (Eucalyptus maculata)not rated, Central Valleyno Central Valley ratingDove TreeDavidia involucratanot rated, Central Valleyno Central Valley ratingRoyal PoincianaDelonix regianot rated, Central Valleyno Central Valley ratingTriangle PalmDypsis decaryinot rated, Central Valleyno Central Valley ratingMountain White GumEucalyptus dalrympleananot rated, Central Valleyno Central Valley ratingRainbow EucalyptusEucalyptus degluptanot rated, Central Valleyno Central Valley ratingRoxburgh FigFicus auriculatanot rated, Central Valleyno Central Valley ratingIndian BanyanFicus benghalensisnot rated, Central Valleyno Central Valley rating
Minutes for your sprinklers

Turn this into minutes on your water days

Every city page has a calculator that takes California black walnut, 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 California black walnut in summer?

In July, established California black walnut in loam needs a deep watering 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 California black walnut need per day?

In July on the north-central coast, established California black walnut needs about 4.1 gallons a day under a 10 foot canopy and about 16 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 California black walnut change across California?

Yes. WUCOLS rates California black walnut separately for each of the six California climate zones: moderate water in the North-Central Coastal zone, not suited to this region in the Central Valley zone, 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.

Can I grow plants under a California black walnut?

Few plants manage it. The walnut is one of the strongest allelopathic plants, and toxins from the tree keep most others from growing beneath it. Set new plantings well beyond the canopy instead of crowding the trunk.

Are California black walnut nuts edible?

Yes, the small nuts are edible. The meat is hard to get out of the shell, so the tree is not grown for crops. Birds eat the nuts and like to nest in the branches.

Can California black walnut grow on a slope or in clay?

Yes. It is well suited to slopes and is used for bank stabilization. It adapts to soils that drain fast, slowly or in between, and it tolerates seasonal flooding.

Does an established California black walnut need summer water?

Very little once its roots are settled, since it tolerates drought. A rare deep soak in a long dry summer is plenty. WUCOLS rates it low water in South Inland and along the south coast.

Is California black walnut rare?

It is now endangered in large parts of the southern portion of its natural range. Development has removed much of its habitat there. A garden tree also gives birds food and nesting sites.

Will deer eat California black walnut?

It is listed as deer resistant. That makes it a good choice for wild edges and hillside gardens. Young trees still need deep soakings until their roots are established.

More plant watering guides

Candelabra TreeMilk BushBoojumOcotilloSilver TreeNew Zealand Christmas TreeTanbark OakPygmy Date Palm

Sources