WateringTimesWatering plan, $19
Populus trichocarpa (Populus balsamifera), WUCOLS IV

How often to water black cottonwood in California

In July in the Central Valley, established black cottonwood in loam needs a deep watering about every 5 days: about 55 gallons each time under a 10 foot canopy, about 76 gallons a week. Black cottonwood is the largest American poplar, a fast, tall, deciduous native of stream banks and river bottoms. Its leaves turn yellow in fall, and female trees release cottony seed in late spring. It has low drought tolerance and aggressive roots that hunt for water, often under lawns, drains and foundations.

TreeCalifornia nativeRated for 5 of 6 zones

How often to water black cottonwood, 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 cottonwood 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 a foot down under the canopy in summer signals that black cottonwood needs a deep soak
  • Leaves yellowing and dropping in midsummer, long before fall color, can indicate a dry root zone
  • Leaves wilting at the crown top on hot afternoons can follow a dry spell
  • Pale, sparse leaves with standing water around the trunk for days point to stagnant, airless soil
  • Roots surfacing in a nearby lawn or clogging a drain show the tree is chasing extra water

How to water it

  • Water the whole area under the canopy and beyond with soakers, not just near the trunk
  • Soak the root zone 2 feet deep or more, then recheck before the deeper soil dries
  • Plant far from sewer lines, drains and foundations, since its roots seek out moisture
  • Avoid low spots where water pools and stagnates, though it tolerates brief flooding
  • Move soakers outward each year as the canopy widens on this fast grower

Through the seasons

Give a young black cottonwood frequent deep soaks through its first summers while roots chase moisture downward. Established trees still need deep water through the dry season, since the species has low drought tolerance. It drops its leaves in fall and stays dormant through winter, when it is very frost hardy. Winter rain covers the cool months on the coast and in the Central Valley. WUCOLS rates black cottonwood high water on the north-central coast, in the Central Valley and in the high desert. It rates moderate water on the south coast and in South Inland. WUCOLS marks it as not suited to the low desert.

In pots

Black cottonwood is a poor long-term container plant because it grows fast and gets very large. A young tree or cutting can spend a season or two in a deep pot before planting out. Check the mix daily in summer heat and water whenever the top inch is dry. Water until it drains through, then empty the saucer so roots are not left in stale water. In winter, while it is leafless, water only enough to keep the mix from drying out.

1 Your climate zone six WUCOLS zones, each from its own reference city
4.27gal per sq ft in JulyHigh water use in the Central Valley zone
A year
27.38 gal per sq ft
Reference
Sacramento climate table
Counts
Plant need, before sprinkler losses
1.22.43.64.90.64JAN1.04FEB1.74MAR2.43APR3.32MAY3.97JUN4.27JUL3.82AUG2.88SEP1.88OCT0.89NOV0.55DEC1.22.43.64.90.64JFM2.43AMJ4.27JAS1.88OND
Gallons per square foot per month, Central Valley. Peak 4.27 in Jul, 27.38 a year.

Monthly water need by climate zone

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

MonthN. CoastC. ValleyS. CoastS. InlandHi DesertLo Desert
Jan0.60.640.780.781.19–
Feb0.841.040.810.871.49–
Mar1.391.741.151.32.33–
Apr1.882.431.41.613.03–
May2.333.321.611.953.82–
Jun2.583.971.742.294.56–
Jul2.584.271.982.514.86–
Aug2.383.821.862.334.46–
Sep1.982.881.491.863.42–
Oct1.391.881.151.332.33–
Nov0.740.890.840.91.49–
Dec0.50.550.650.681.04–
Year19.227.3815.518.4134.03–

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 high water in the Central Valley, like black cottonwood. Plants that drink alike can share one valve and one schedule.

Big Leaf MapleAcer macrophyllumhigh water, Central Valley4.27 gal/sq ft in JulyWhite AlderAlnus rhombifoliahigh water, Central Valley4.27 gal/sq ft in JulyRed AlderAlnus rubra (Alnus oregona)high water, Central Valley4.27 gal/sq ft in JulyRiver BirchBetula nigrahigh water, Central Valley4.27 gal/sq ft in JulyWater BirchBetula occidentalishigh water, Central Valley4.27 gal/sq ft in JulyEuropean White BirchBetula pendulahigh water, Central Valley4.27 gal/sq ft in JulyWhite-Barked Himalayan BirchBetula utilis var. jacquemontiihigh water, Central Valley4.27 gal/sq ft in JulyEuropean BeechFagus sylvaticahigh water, Central Valley4.27 gal/sq ft in JulyCarolina PoplarPopulus x canadensishigh water, Central Valley4.27 gal/sq ft in JulyWeeping WillowSalix babylonicahigh water, Central Valley4.27 gal/sq ft in JulyPeking WillowSalix matsudanahigh water, Central Valley4.27 gal/sq ft in JulyCoast RedwoodSequoia sempervirenshigh water, Central Valley4.27 gal/sq ft in JulyVine MapleAcer circinatumhigh water, Central Valley4.27 gal/sq ft in JulyHydrangeaHydrangea macrophylla & cvs.high water, Central Valley4.27 gal/sq ft in JulyBlueberriesVaccinium hybrids and cvs (CA native and non- native spp.)high water, Central Valley4.27 gal/sq ft in JulyShell GingerAlpinia zerumbethigh water, Central Valley4.27 gal/sq ft in July

Similar plants that need less water 16

Rated lower than black cottonwood in the Central Valley. Worth a look for a new bed or a lawn replacement.

Citrus TreesCitrus spp.moderate water, Central Valley2.67 gal/sq ft in JulyJapanese MapleAcer palmatummoderate water, Central Valley2.67 gal/sq ft in JulyAvocado TreesPersea americanamoderate water, Central Valley2.67 gal/sq ft in JulyMagnolia TreesMagnolia grandiflora ssp. & cvs.moderate water, Central Valley2.67 gal/sq ft in JulyPalm TreesSyagrus romanzoffiana (Arecastrum romanzoffianum)moderate water, Central Valley2.67 gal/sq ft in JulyApple TreesMalus spp. (edible) e.g, 'Anna' (low chill variety for low desert)moderate water, Central Valley2.67 gal/sq ft in JulyPeach TreesPrunus spp. ediblemoderate water, Central Valley2.67 gal/sq ft in JulyPersimmon TreesDiospyros kakimoderate water, Central Valley2.67 gal/sq ft in JulyAmur MapleAcer ginnalamoderate water, Central Valley2.67 gal/sq ft in JulyFernleaf MapleAcer japonicummoderate water, Central Valley2.67 gal/sq ft in JulyBox ElderAcer negundomoderate water, Central Valley2.67 gal/sq ft in JulyNorway MapleAcer platanoidesmoderate water, Central Valley2.67 gal/sq ft in JulyRed MapleAcer rubrummoderate water, Central Valley2.67 gal/sq ft in JulyChinese MapleAcer truncatummoderate water, Central Valley2.67 gal/sq ft in JulyRed BuckeyeAesculus paviamoderate water, Central Valley2.67 gal/sq ft in JulyRed HorsechestnutAesculus x carneamoderate water, Central Valley2.67 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 cottonwood, 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 cottonwood in summer?

In July, established black cottonwood in loam needs a deep watering about every 5 days in the Central Valley, about every 11 days on the south coast and about every 9 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 cottonwood need per day?

In July in the Central Valley, established black cottonwood needs about 11 gallons a day under a 10 foot canopy and about 43 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 cottonwood change across California?

Yes. WUCOLS rates black cottonwood separately for each of the six California climate zones: high water in the North-Central Coastal zone, high water in the Central Valley zone, moderate water in the South Coastal zone, moderate water in the South Inland zone, high water 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.

Will black cottonwood roots damage my house?

They can. Its aggressive roots can invade drains and pull moisture from gardens and foundations, especially in clay. Plant it well away from structures and pipes.

Why is my cottonwood dropping leaves in summer?

Check the soil a foot down, since dry soil is the usual cause in a tree with low drought tolerance. Give it a slow, deep soak across the root zone. Then recheck before the deep soil dries again.

Can I stop the cotton from my tree?

Only female trees make the cottony seed, released in late spring and early summer. Male trees produce none. Choose a male tree if the fluff would be a nuisance.

Why are shoots coming up from the roots?

Black cottonwood sprouts readily from its roots, and suckers are a common maintenance chore. Cut them off at the source. Do not let suckers grow into a thicket.

Is black cottonwood safe in strong wind?

Its shallow roots, especially in wet soil, make it prone to wind and storm damage. Deep, spaced soaks encourage deeper roots than frequent light watering. Keep it away from where a falling limb would land on the house.

More plant watering guides

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Sources