How often to water blackwood acacia in California
In July in the Central Valley, established blackwood acacia 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. Blackwood acacia is a fast-growing evergreen tree from coastal southeastern Australia, 30 to 50 feet tall, with rough dark gray bark. Mature branches carry flat, lance-shaped phyllodes instead of true leaves, and creamy flower puffs appear around winter's end. It needs very little water once established, but its roots, brittle limbs and spreading habit make it a problem tree in many gardens.
How often to water blackwood acacia, month by month
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.
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.
How to tell it needs water
- On a young tree, soil dry 6 inches down in a long dry spell calls for a deep soak
- Root suckers sprouting across the yard show its spreading habit, not a watering need
- Yellowing leaves and decline in soil kept wet point to poor drainage, since it needs well-drained ground
- Branches snapping in wind reflect brittle wood rather than thirst
- Seedlings popping up in nearby beds come from its pods, which self-sow readily
How to water it
- Water a young tree deeply until it is established, then rely mostly on winter rain
- On the north-central coast, give an established tree water only in a very dry year
- Plant in sun to light shade in well-drained soil, away from paving, pipes and foundations
- Avoid sites near wildlands, streams or open space, where it spreads
- Remove suckers and seedlings promptly, digging out root pieces so they cannot resprout
Through the seasons
Water a newly planted tree through its first summer or two, then taper off. It is evergreen, and flowers open in late winter and early spring, followed by beanlike pods. Winter rain carries established trees on the coast and in the Central Valley, with little or no summer water needed. WUCOLS rates blackwood acacia very low water on the north-central coast. It rates low water in the Central Valley, on the south coast and in South Inland. WUCOLS marks it as not suited to the high desert or the low desert.
In pots
Blackwood acacia is a large, fast tree with aggressive roots, so it is a poor long-term container plant. A nursery seedling can wait in a deep pot with drainage holes and a gritty, well-drained mix until planting. Water when the top 2 inches of mix are dry, which may be every few days in summer heat. Let water drain, then empty the saucer, and plant it out within a season.
- A year
- 6.84 gal per sq ft
- Reference
- Sacramento climate table
- Counts
- Plant need, before sprinkler losses
Monthly water need by climate zone
Blackwood Acacia: gallons per square foot per month, by WUCOLS climate zone
| Month | N. Coast | C. Valley | S. Coast | S. Inland | Hi Desert | Lo Desert |
|---|---|---|---|---|---|---|
| Jan | 0.04 | 0.16 | 0.31 | 0.31 | – | – |
| Feb | 0.05 | 0.26 | 0.32 | 0.35 | – | – |
| Mar | 0.09 | 0.43 | 0.46 | 0.52 | – | – |
| Apr | 0.12 | 0.61 | 0.56 | 0.64 | – | – |
| May | 0.15 | 0.83 | 0.64 | 0.78 | – | – |
| Jun | 0.16 | 0.99 | 0.69 | 0.92 | – | – |
| Jul | 0.16 | 1.07 | 0.79 | 1 | – | – |
| Aug | 0.15 | 0.95 | 0.74 | 0.93 | – | – |
| Sep | 0.12 | 0.72 | 0.6 | 0.74 | – | – |
| Oct | 0.09 | 0.47 | 0.46 | 0.53 | – | – |
| Nov | 0.05 | 0.22 | 0.33 | 0.36 | – | – |
| Dec | 0.03 | 0.14 | 0.26 | 0.27 | – | – |
| Year | 1.2 | 6.84 | 6.2 | 7.37 | – | – |
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 blackwood acacia. 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 July
Fig TreesFicus carica ssp. & cvs.low water, Central Valley1.07 gal/sq ft in July
Crape MyrtleLagerstroemia spp., hybrids & cvs.low water, Central Valley1.07 gal/sq ft in July
AloeAloe spp. & cvs.low water, Central Valley1.07 gal/sq ft in July
Pomegranate TreesPunica granatumlow water, Central Valley1.07 gal/sq ft in July
EucalyptusEucalyptus polyanthemoslow water, Central Valley1.07 gal/sq ft in July
Pine TreesPinus canariensislow water, Central Valley1.07 gal/sq ft in July
JunipersJuniperus spp. & cvs.low water, Central Valley1.07 gal/sq ft in July
Texas Red OakQuercus buckleyilow water, Central Valley1.07 gal/sq ft in July
Portuguese OakQuercus faginealow water, Central Valley1.07 gal/sq ft in July
Soapbark TreeQuillaja saponarialow water, Central Valley1.07 gal/sq ft in July
Necklace TreeSophora affinislow water, Central Valley1.07 gal/sq ft in July
Bailey AcaciaAcacia baileyanalow water, Central Valley1.07 gal/sq ft in July
Silver WattleAcacia dealbatalow water, Central Valley1.07 gal/sq ft in July
Green WattleAcacia decurrenslow water, Central Valley1.07 gal/sq ft in July
Sydney Golden WattleAcacia longifolialow water, Central Valley1.07 gal/sq ft in JulyTurn this into minutes on your water days
Every city page has a calculator that takes blackwood acacia, 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 blackwood acacia in summer?
In July, established blackwood acacia 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 blackwood acacia need per day?
In July in the Central Valley, established blackwood acacia 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 blackwood acacia change across California?
Yes. WUCOLS rates blackwood acacia 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, 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.
Is blackwood acacia invasive in California?
It has escaped cultivation along the coast, in the coast ranges and around the Bay Area. Root suckers grow into large clonal stands, and water and people spread its seeds. Avoid planting it near wildlands.
Will blackwood acacia roots damage paving?
Yes. Its shallow, spreading roots can lift and heave pavement. Plant it well away from walks, driveways and foundations.
Why are blackwood acacia branches breaking?
The wood is brittle, and the tree does not handle strong wind well. It is also relatively short-lived, so check older trees for weak limbs.
How do I get rid of blackwood acacia?
Cut trees resprout from roots, so most root fragments must be removed to stop regrowth. Pull seedlings and suckers while they are small.
Can you overwater blackwood acacia?
Yes. It needs well-drained soil and very little water once established. Regular summer irrigation is unnecessary in its rated zones.
More plant watering guides
Sources
- WUCOLS IV plant water use classification for Acacia melanoxylon, the University of California survey California agencies use for landscape plan reviewRead August 23, 2026 on wucols.ucdavis.edu
- Model Water Efficient Landscape Ordinance, adopted text and appendices, effective 2025-01-02 · monthly reference evapotranspiration per city, Appendix CRead August 23, 2026 on water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/Water-Use-And-Efficiency/MWELO-Rulemaking/Final-MWELO-Text-and-Appendices_20250103.pdf
- UC Statewide IPM Program, irrigation scheduling using evapotranspirationRead September 30, 2026 on ipm.ucanr.edu/PMG/GARDEN/ENVIRON/evap.html
- Shaw and Pittenger, Landscape Irrigation System Evaluation and Management, UC Cooperative Extension, 2009Read September 30, 2026 on ucanr.edu/sites/default/files/2013-02/160836.pdf
- Pittenger, Water Management in Gardens and Landscapes, UC Riverside, 2017Read September 30, 2026 on ucanr.edu/sites/default/files/2017-03/258022.pdf