Perth Soil Types and How to Water Them
· Thibaut Holl
Two houses, two streets apart, can run the same sprinklers for the same twelve minutes and get completely different lawns. The usual assumption is that one of them has a retic fault. Often neither does. They are watering two different soils on the same schedule.
Greater Perth is not one soil. It is seven, and they behave differently enough that the same run time is generous on one and useless on the next.
Need a hand rather than a how-to? Call us on (08) 6263 4644 — repairs are booked, not quoted, and most faults are fixed in the same visit.
The seven soils under greater Perth
These are the soil-landscape units of the Swan Coastal Plain and the Darling Scarp behind it, mapped by the Department of Primary Industries and Regional Development. The counts are the suburbs we cover that sit on each one.
| Soil | Suburbs | What it does with water |
|---|---|---|
| Spearwood sands | 158 | Yellow-brown sand over Tamala limestone. Drains fast, but holds more than it looks. The best of Perth's sands for nutrients and moisture. |
| Bassendean sands | 73 | Pale grey sand, washed clean of nutrients over thousands of years. Perth's most leached sand, poorest for nutrients, and the most likely to turn water-repellent. |
| Pinjarra Plain clay-loams | 54 | Holds water well. The problem is getting it in — sprinklers apply faster than clay absorbs. |
| Darling Scarp gravels | 38 | Gravel over clay pockets, usually on a slope. Sheds water sideways before it soaks. |
| Quindalup coastal sands | 28 | Young dune sand, still full of shell and lime fragments. Almost no reserve moisture at all. |
| Foothills gravels and clay-loams | 12 | Gravel, sand and clay mixed within a single block. Drainage varies station to station. |
| Estuarine flats (Vasse soils) | 3 | River-deposited silt and clay. Holds moisture well, so it needs less water than Perth's sands, applied more patiently. |
Two things follow from that table. On sands the water leaves too fast. On clays and gravels it never gets in. Both end up as a dry lawn, and both get misdiagnosed as a broken system.
Water repellency: why one long run makes it worse
Perth sand does something that surprises people. When it dries out completely, it stops accepting water.
Organic matter in the soil breaks down into waxy compounds that coat each sand grain. The coating is hydrophobic. Once the sand is dry enough, water beads on the surface and runs off sideways instead of soaking down. Dig a spade in after a long sprinkler run on badly repellent sand and you will find dry sand two centimetres below wet sand.
This is worst on Bassendean sands, because they are the most leached and hold the least moisture to begin with. It shows up on Spearwood sands too, especially in beds that have been mulched for years.
The instinct is to water for longer. That is the wrong lever. A longer run on repellent sand mostly waters your path, because the water is running off, not soaking in.
Cycle and soak: the fix that costs nothing
The fix is to split one watering into two shorter bursts with a gap between them — both inside the same window, so both before 9am or both after 6pm.
The first burst wets the surface and breaks the repellency. The gap lets it soak. The second burst goes in properly, because the surface is no longer shedding it.
Two eight-minute cycles beat one sixteen-minute run on the same soil, using the same amount of water. The compliant way to do it is your controller's Cycle & Soak function (Hunter, Rain Bird and most modern controllers have one), which builds the soak pause into a single programme run. Do not add a second start time in the evening: Water Corporation's rule is one watering per rostered day, and the by-laws allow reticulation in the morning window or the evening window, not both.
One important limit: this means two cycles inside one watering window on your rostered day — not extra days, and not a second watering that evening. Perth is on a two-day sprinkler roster set by Water Corporation, and watering more often is neither permitted nor helpful. Water Corporation's own guidance is roughly 10mm on each of your two rostered days. Splitting the run changes how that 10mm is delivered, not how much or how often.
If you are establishing a brand-new lawn and genuinely need more than the roster allows, that is what a watering exemption is for. Apply on the day the turf goes down, and wind the controller back to your rostered days when it expires.
The same fix, a different reason, on clay and gravel
Cycle-and-soak also works on Pinjarra Plain clay-loams and the Darling Scarp gravels. The reason is different, and the difference matters if you want to get the timing right.
On sand, the problem is repellency — the surface refuses the water.
On clay and gravel, the problem is how fast the ground can take water in — its infiltration rate. These soils accept water only slowly. A sprinkler that puts down 15mm an hour onto a soil that can absorb 5mm an hour will produce runoff no matter how wet the surface already is. On a Scarp block with any slope, that water simply travels downhill.
So on sand you split the run to break repellency. On clay and gravel you split it to stay under the absorption rate. Same instruction, different cause, and on the Scarp you usually want a longer gap between the two cycles.
Matched precipitation, in plain terms
The other half of the job is making every sprinkler on a station apply water at the same rate.
Sprinklers throw different distances, and a nozzle covering a quarter-circle pushes out a quarter of the water of one covering a full circle — or it should. If the nozzles on a station are mismatched, some of the lawn gets double what the rest gets. You then set the run time for the driest patch and drown everything else, or set it for the average and keep a dry patch forever.
Matched-precipitation nozzles fix this. Every head on the station applies at the same rate, so one run time suits the whole station. It is the single most common thing we change on an underperforming system, and it usually costs less than adding a station.
MP Rotators go a step further by applying water slowly — around 10mm an hour. On clay and gravel that matters because the soil genuinely cannot keep up with a faster nozzle. On sand the benefit is different: matched precipitation across every arc, and large droplets that resist wind drift in a Perth easterly rather than misting away. We have compared them against standard spray nozzles in detail.
Wetting agents: useful, and not a cure
A soil wetter is a surfactant: it lowers water's surface tension so water can wet the waxy coating on the sand grains instead of beading on it and running off. On Bassendean sand it makes a real difference, and it is worth doing at the start of the season rather than in January when the lawn is already stressed.
Two honest caveats. It washes through sandy soil and needs reapplying — usually a couple of times a season, not once a decade. And it does little for clay, which is slow rather than repellent — though WA's lateritic sandy gravels can be water-repellent too, so the Hills are not a blanket exception. Applying wetter to a Scarp block is money spent on the wrong problem.
For Bassendean sand specifically, working composted organic matter and a clay such as bentonite into the top 30cm does more for water retention than any change you can make at the sprinkler. The City of Bayswater recommends exactly this in its verge guidelines, and its suburbs sit squarely on that sand.
What we actually change on site
When a system underperforms and nothing is broken, this is the order we work through:
- Check coverage first. Run each station and watch it. Mismatched or blocked nozzles explain more dry patches than soil ever does.
- Identify the soil. A spade and a look, on the block, at the depth the roots are actually in.
- Set the run time to deliver about 10mm on each rostered day, for the nozzles actually fitted — not a default number.
- Split the run where the soil needs it, with the gap suited to sand or to clay.
- Clear the unused programs. Most "random" watering is an old start time left in a program nobody checks.
None of that is exotic. It is the difference between a system that works and one that runs.
Which soil is under your place?
Every suburb page on this site states the soil under that suburb, what it means for scheduling, and what we most often get called out for there — drawn from the jobs we have actually completed in that suburb rather than a general rule.
Some typical examples of each:
- Spearwood sands (158 suburbs) — Innaloo, Nollamara, Doubleview
- Bassendean sands (73) — Dianella, Bayswater, Maylands
- Pinjarra Plain clay-loams (54) — Bassendean, Bedford, Midland
- Darling Scarp gravels (38) — Kalamunda, Lesmurdie, Roleystone
- Quindalup coastal sands (28) — Quinns Rocks, Hillarys, Sorrento
- Foothills gravels and clay-loams (12) — Swan View, Maida Vale, Koongamia
- Estuarine flats (Vasse soils) (3) — Wellard, South Yunderup, Burswood
Find yours on our service areas page.
Sources: soil-landscape units from the Department of Primary Industries and Regional Development, Bulletin 4359 — Soils of the Swan Coastal Plain. Watering roster, application rates and exemptions from Water Corporation.
