WateringTimesWatering plan, $19
Robinia pseudoacacia, WUCOLS IV

How often to water black locust in California

In July in the Central Valley, established black locust 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. Black locust is a fast-growing deciduous tree with thorny branches and drooping clusters of fragrant white flowers in spring. Its roots fix their own nitrogen and spread by suckering, often forming thickets beyond the original planting spot. An established tree shrugs off long rainless stretches across most of the state, needing more help only in the hottest desert zones.

TreeRated for 6 of 6 zones

How often to water black locust, 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 black locust 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 young black locust with leaflets drooping in afternoon heat over dry soil needs a deep soak
  • Leaves yellowing and dropping during a long dry spell is this tree's normal drought response, so check the soil before assuming disease
  • Wilting paired with small holes or sawdust-like debris on the trunk can indicate borer damage rather than dry soil
  • Desert plantings with pale, drooping leaflets through peak summer heat can need a deeper soak than a coastal tree would
  • Leaves yellowing across the canopy where soil stays saturated for days can indicate root rot

How to water it

  • Soak a young black locust's root ball and the ground just past it while thorny new growth establishes
  • Let the soil dry out well between soakings on coastal and valley plantings once the tree is established
  • Give desert plantings a deeper soak through peak summer heat, matching their higher water rating there
  • Plant in well-drained ground, since this tree manages dry soil far better than standing water
  • Cut unwanted root suckers off at ground level rather than withholding water from the main tree to control them

Through the seasons

A newly planted black locust needs steady moisture while its roots establish and begin fixing their own nitrogen. Established trees are deciduous, flowering in spring or early summer and dropping their leaves each fall before winter dormancy. This species tolerates dry soil well across most of the state, though the hotter desert zones call for more attention than the coast. WUCOLS rates black locust very low water on the north-central coast. It rates low water in the Central Valley, on the south coast and in South Inland. It rates moderate water in the high desert and the low desert. Winter rain usually covers the dormant season on the coast and in the valley, while desert plantings depend more on direct watering.

In pots

Black locust grows fast even in a container, and that vigor means it outgrows a pot sooner than most young trees. Check the mix several inches down in summer heat, since the thorny top growth draws more moisture than a small pot can easily hold. Let the water run fully through the drainage holes, then tip out the saucer so the thorny young tree's roots do not sit wet. Size up the container as growth accelerates rather than waiting for roots to crowd it. In winter, let the mix dry further between waterings while the thorny branches stand bare.

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
0.761.52.33.00.16JAN0.26FEB0.43MAR0.61APR0.83MAY0.99JUN1.07JUL0.95AUG0.72SEP0.47OCT0.22NOV0.14DEC0.761.52.33.00.16JFM0.61AMJ1.07JAS0.47OND
Gallons per square foot per month, Central Valley. Peak 1.07 in Jul, 6.84 a year.

Monthly water need by climate zone

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

MonthN. CoastC. ValleyS. CoastS. InlandHi DesertLo Desert
Jan0.040.160.310.310.740.78
Feb0.050.260.320.350.930.93
Mar0.090.430.460.521.461.52
Apr0.120.610.560.641.891.95
May0.150.830.640.782.392.45
Jun0.160.990.690.922.852.79
Jul0.161.070.7913.042.85
Aug0.150.950.740.932.792.6
Sep0.120.720.60.742.142.08
Oct0.090.470.460.531.461.49
Nov0.050.220.330.360.930.96
Dec0.030.140.260.270.650.68
Year1.26.846.27.3721.2721.08

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 black locust. 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 black locust, 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 black locust in summer?

In July, established black locust 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 black locust need per day?

In July in the Central Valley, established black locust 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 black locust change across California?

Yes. WUCOLS rates black locust separately for each of the six California climate zones: very 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, moderate water in the High and Intermediate Desert zone, moderate water in the Low Desert zone. The monthly table above shows how the same rating translates into different gallon amounts as reference evapotranspiration changes.

Why does black locust need more water in the desert than near the coast?

WUCOLS rates it moderate water in the high and low desert but very low to low water elsewhere in the state. Intense desert heat raises demand on even a drought-tolerant tree like this one. A coastal planting can go much longer between soakings.

Why are suckers coming up far from my black locust's trunk?

This tree spreads by root suckers that can emerge several feet from the main trunk and form thickets. Removing unwanted suckers at the base controls the spread. This habit is separate from how the main tree is watered.

What causes dieback and sawdust near the base of my locust?

The locust borer can tunnel into the trunk and branches, sometimes causing serious dieback or tree failure. Check for small holes or sawdust-like frass as a sign of this pest. A heavily infested tree may need removal rather than more water.

Is black locust considered invasive?

It spreads readily by suckering and self-sown seed and is considered weedy outside its native eastern and central United States range. Established trees need little water to keep spreading this way. Consider that spreading habit before planting it in a small yard.

Can black locust handle poor, rocky soil?

Yes, its nitrogen-fixing roots let it grow in poor soils that would starve many other trees. It still prefers well-drained ground over heavy, waterlogged clay. Good drainage matters more here than soil richness.

More plant watering guides

SalviaIrisesAloeSucculentsPomegranate TreesEucalyptusPine TreesJunipers

Sources