WateringTimesWatering plan, $19
Eucalyptus robusta, WUCOLS IV

How often to water swamp mahogany in California

In July in the Central Valley, established swamp mahogany 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. Swamp mahogany is a large, fast-growing eucalyptus from the edges of swamps, lagoons and rivers in southeastern Australia. Its thick, spongy, deeply fissured bark is reddish brown, and its creamy white flowers carry showy stamens. Despite its wet origins, an established tree copes with dry summers, though its root crown should never sit in soggy ground.

TreeRated for 4 of 6 zones

How often to water swamp mahogany, 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 swamp mahogany 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 6 inches down at the edge of a young swamp mahogany's root ball means it is ready for a slow, deep soak
  • Small holes in the bark with stains or oozing below them mean borers have found a stressed tree, though this species resists them well
  • Discolored, wilting foliage and dying limbs can follow borer damage or a long drought, so check both the bark and the deep soil
  • Aerial roots forming on the trunk show that the ground is staying wet, a cue to cut back irrigation and let it dry
  • Thick, spongy, deeply furrowed bark is normal for swamp mahogany and is not a sign of rot or disease

How to water it

  • Soak a young swamp mahogany over its root ball and the soil just beyond it, then move the wet zone outward as the canopy grows
  • Run drip emitters slowly beneath the outer canopy so water reaches a foot or more deep, keeping the trunk dry
  • Give a lawn-side tree its own slow soak, since the frequent, shallow watering used for turf does not suit eucalyptus
  • Never let irrigation water pool around the root crown, where eucalyptus are open to Armillaria and Phytophthora root rots
  • Plant in sand, loam or clay, since this tree takes both poorly drained and fast-draining ground

Through the seasons

Give a young swamp mahogany slow, deep soakings through its first summers while the roots spread. Though it comes from swamp edges, an established tree needs only occasional deep soaks in long summer and fall dry spells. Drought stress in those months draws borers. The tree is evergreen. Winter rain covers the cool months on the coast and in the Central Valley. WUCOLS rates swamp mahogany low water on the north-central coast, 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

Swamp mahogany grows into a very large tree, so a pot is only a short stop for a seedling on its way into the ground. Choose a deep container with drainage holes and a well-drained mix. During summer heat, test the mix 2 to 3 inches down every few days and water once it has dried there. Let water flow out the holes, then tip out the saucer so the young root crown is never left standing in water. Plant it out while small, before the roots wind around the pot wall. In winter, rain can keep a pot wet for days, so feel the mix before adding more.

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
0.270.540.801.10.16JAN0.26FEB0.43MAR0.61APR0.83MAY0.99JUN1.07JUL0.95AUG0.72SEP0.47OCT0.22NOV0.14DEC0.270.540.801.10.16JFM0.61AMJ1.07JAS0.47OND
Gallons per square foot per month, Central Valley. Peak 1.07 in Jul, 6.84 a year.

Monthly water need by climate zone

Swamp Mahogany: gallons per square foot per month, by WUCOLS climate zone

MonthN. CoastC. ValleyS. CoastS. InlandHi DesertLo Desert
Jan0.150.160.310.31––
Feb0.210.260.320.35––
Mar0.350.430.460.52––
Apr0.470.610.560.64––
May0.580.830.640.78––
Jun0.640.990.690.92––
Jul0.641.070.791––
Aug0.60.950.740.93––
Sep0.50.720.60.74––
Oct0.350.470.460.53––
Nov0.190.220.330.36––
Dec0.120.140.260.27––
Year4.86.846.27.37––

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 low water in the Central Valley, like swamp mahogany. 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 swamp mahogany, 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 swamp mahogany in summer?

In July, established swamp mahogany 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 swamp mahogany need per day?

In July in the Central Valley, established swamp mahogany 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 swamp mahogany change across California?

Yes. WUCOLS rates swamp mahogany 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, 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.

Do borers attack swamp mahogany?

Swamp mahogany is listed among the eucalyptus species more resistant to longhorned borers. Any eucalyptus can still be attacked when it is badly stressed, for example by poor soil or too little water. A well-watered tree is the best protection.

Do lerp psyllids infest swamp mahogany?

It is rated among the less susceptible species for redgum lerp psyllid, the sticky pest that favors river red gum. If honeydew coats the ground beneath a swamp mahogany, check whether a more susceptible tree overhangs the spot.

Can swamp mahogany grow in poorly drained soil?

Yes. It grows wild beside swamps and lagoons, and it takes both poorly drained and fast-draining soil. Still, keep irrigation from leaving the topsoil soggy around the root crown, since eucalyptus are prone to root rot there.

Where should I plant swamp mahogany?

Give it open ground well away from buildings, paving and power lines. It becomes a large tree, its roots have moderate potential to damage hardscape, and its wind resistance is rated in the lowest class. Plan the watering zone around the mature canopy.

Is swamp mahogany salt tolerant?

Yes. It is rated highly tolerant of salt and is suited to the southern California coast as well as inland valleys. Near the shore, a young tree still needs deep soaks until its roots spread.

More plant watering guides

Mother of ThymeSweet AcaciaChaste TreeRoger's Red GrapeWhite Rain LilyDouble Knock Out RoseCat ThymeWhite Ironbark

Sources