WateringTimesWatering plan, $19
Grevillea robusta, WUCOLS IV

How often to water silk oak in California

In July in the Central Valley, established silk 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. Silk oak is a fast-growing evergreen tree from the rainforests of eastern Australia, reaching 50 to 75 feet. Its fernlike, dark green leaves are silvery gray beneath, and brushlike clusters of yellow-orange flowers open in spring on mature trees. It needs sun and well-drained, acid to neutral soil, and it gets by on little water once established in most of California.

TreeRated for 5 of 6 zones

How often to water silk 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 silk 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

  • Ground dry a foot down under the canopy of a young silk oak in summer means it needs a slow, deep watering
  • Leaves yellowing over soil that stays wet point to too much water, since grevilleas want little
  • Seedlings and root suckers popping up nearby come from the tree's own spread, not from extra irrigation
  • Cottony white clumps or bumps on twigs point to cottony cushion scale or other scales, not thirst
  • No flowers on a young tree is normal, since only mature trees bloom

How to water it

  • Run drip lines or a slow soaker under the full canopy of a young tree
  • Soak the soil 12 to 18 inches deep, then let it dry well before the next round
  • Plant in sun in well-drained, acid to neutral soil without much salt
  • Feed lightly, if at all, with a fertilizer low in phosphorus
  • Taper watering once the tree is established, and widen the wetted area only as the canopy grows

Through the seasons

Give a young tree deep, regular watering through its first summers while roots spread, then taper off. It is evergreen, and mature trees bloom in spring. Established trees need little summer water in the coastal zones and the Central Valley, where winter rain does most of the work. Silk oak carries a low water rating from WUCOLS in four zones: the north-central coast, the Central Valley, the south coast and South Inland. Its low desert rating is moderate water. WUCOLS marks it as not suited to the high desert.

In pots

Silk oak becomes a large tree, but young plants are often grown in containers for their fernlike foliage. Use a gritty, lime-free mix and a pot with drainage holes. In summer heat, poke a finger into the mix every few days and water once it is dry two knuckles deep. Water until it drains, then empty the saucer. Trim to keep it in scale. In winter, keep the mix on the dry side and protect the pot from frost.

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
0.711.42.12.90.16JAN0.26FEB0.43MAR0.61APR0.83MAY0.99JUN1.07JUL0.95AUG0.72SEP0.47OCT0.22NOV0.14DEC0.711.42.12.90.16JFM0.61AMJ1.07JAS0.47OND
Gallons per square foot per month, Central Valley. Peak 1.07 in Jul, 6.84 a year.

Monthly water need by climate zone

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

MonthN. CoastC. ValleyS. CoastS. InlandHi DesertLo Desert
Jan0.150.160.310.31–0.78
Feb0.210.260.320.35–0.93
Mar0.350.430.460.52–1.52
Apr0.470.610.560.64–1.95
May0.580.830.640.78–2.45
Jun0.640.990.690.92–2.79
Jul0.641.070.791–2.85
Aug0.60.950.740.93–2.6
Sep0.50.720.60.74–2.08
Oct0.350.470.460.53–1.49
Nov0.190.220.330.36–0.96
Dec0.120.140.260.27–0.68
Year4.86.846.27.37–21.08

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 silk 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 silk 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 silk oak in summer?

In July, established silk 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 silk oak need per day?

In July in the Central Valley, established silk 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 silk oak change across California?

Yes. WUCOLS rates silk 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 suited to this region 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.

Is silk oak invasive in California?

It is on the watch list for invasive plants in California. It spreads by wind-blown winged seed and by root suckers. It has escaped into grasslands and stream bottoms in the central and south coast ranges. Pull seedlings and suckers while they are small.

Can silk oak irritate skin?

Yes. The plant is a skin irritant and allergen for people and pets. Wear gloves and long sleeves when pruning or handling the leaves and flowers.

Why isn't my silk oak flowering?

Only mature trees bloom, so young trees and potted plants grown for foliage rarely flower. Give it full sun and patience, not extra water.

Can you overwater silk oak?

Yes. Grevilleas mostly need little or no water once established, and soggy soil causes problems. Water deeply but rarely, and let the soil dry well between soakings.

How fast does silk oak grow?

It grows fast, eventually reaching 50 to 75 feet tall and 30 to 40 feet wide. Plant it well away from buildings and power lines.

More plant watering guides

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Sources