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Watering natives: myths, soil & water-repellent soil
Natives do need water, at least while they establish. How much depends on your soil, your site and the plant. This guide covers how water moves through North Shore soils and what to do when soil won't wet.
On this page: myths · your soil · how water moves · pots · soil that won't wet · new vs established plants · too much or too little?
Three watering myths
"Natives never need water."
Not quite. ANPSA says natives are usually drought tolerant only if they come from dry areas, and only after they've made it through their first 12 months in the groundANPSA. Rainforest and gully plants, plants in pots, and plants in fast-draining sand need more. A lot of young natives die in their first summer.
"One watering schedule fits all."
How much water a soil can take in and hold depends mostly on its mix of sand, silt and clay, plus its structure and organic matter (old plant material)DPI. A sunny, sandy ridge-top garden and a shady clay gully need different watering. Slope, tree roots and plant choice change it again. From the field: set the schedule for the site, don't copy it from a label.
"Mulch always helps."
Mulch cuts evaporation, but the type matters. A thick layer of fine mulch can soak up a lot of water before any reaches the soil: APS NSW says up to 20 mm of rainAPS. Fine mulches that haven't been composted can also pack down and start repelling waterANPSA-MULCH. Coarse mulch, laid on moist soil and kept off stems, lets water through more easilyAPS. From the field: mulch isn't automatic. Whether a bed needs it, what kind and how deep is worked out on a site visit, because too much can keep soil soggy. Where it's used: coarse mulch only, usually 50–75 mm, kept off stems. No fine mulch, and no fine layers underneath. More in mulch and your soil.
Your soil: sandy or clay?
The North Shore sits on two very different rocks, often in the same suburb. Sandstone breaks down into sandy soil. Shale breaks down into clay.
| Sandy soil (from sandstone) | Clay soil (from shale) | |
|---|---|---|
| Where | Ridges, plateaus, slopes and gullies on Hawkesbury Sandstone (much of Mosman, Lane Cove, Willoughby and the sandstone parts of Ku-ring-gai) | Higher ridges on Wianamatta Shale (e.g. Blue Gum High Forest and Turpentine-Ironbark country on the upper North Shore) |
| Water gets in | Fast, if the soil isn't water-repellent | Slowly; it can run off or pool on top |
| Water drains away | Fast; often shallow over rock | Slowly; can get waterlogged in wet spells |
| Water held for plants | Low | Higher, but some is held too tightly for roots to useDPI |
| Risk of water-repellent soil | High (sandy soils are the most proneSQ) | Lower, though still possible in crumbly, well-structured soilsSQ |
Under the surface. Dig a hole and you'll usually see layers: mulch and leaf litter, a darker topsoil, then subsoil (often paler sand or heavier clay), and sometimes rock just a spade's depth down. Water behaves differently in each layer. Sandy topsoil over clay can hold water sitting on top of the clay. Sand over sandstone drains down to the rock and then runs sideways. On clay, APS NSW recommends raising beds where drainage is poorAPS-CLAY.
The sausage test. Wet a small handful of soil and roll it between your palms. If it makes a bendy sausage, you have clay. If it falls apart, it's sandy.
How water moves: soaking in, draining, and holding on
- Soaking in (soil scientists call it infiltration) is how fast water gets into the soil. It depends on the bigger gaps between soil crumbs, and on channels left by roots and soil crittersNSWSP. Trampling, crusting and loss of organic matter slow it downDPI.
- Draining is how fast water moves on, down past the roots. Sand drains fast; clay drains slowly.
- Holding on is what stays behind for the plant. Small gaps store water plants can use, but the very tiniest gaps in clay hold water so tightly roots can't get itNSWSP. Sandy, packed-down and heavy clay soils generally have less water available to plantsDPI.
Try the trowel test. Water a bed as usual, wait an hour, then dig a small hole and see whether the soil is moist 15–20 cm down, where the roots are, or only on top. From the field: this tells you more than any schedule.
Pots vs the ground
A plant in a pot can't send roots off looking for water, so it's relying on you. In hot or windy weather, check pots every day with a finger: if the mix is moist, wait another dayWAV. Terracotta and dark pots dry out and heat up faster, and pots shouldn't sit in saucers of waterWAV. Use a native potting mix (low phosphorus) and a thin layer of mulch on topWAV. Potting mix that dries right out can be hard to re-wet: dunk the whole pot in a bucket until the bubbles stop.
Soil that won't get wet (water-repellent soil)
If you water a bed and the soil underneath stays dry, it's often water-repellent (also called "hydrophobic"). Water beads up on the surface, runs off, or slips down a few channels and leaves dry pocketsCSIRO.
What causes it
- Waxy coatings on soil grains. As leaves, roots and other plant material break down, waxy and oily stuff from plants, roots and some soil microbes coats the soil grainsSQKBCSIRO.
- Sandy soil. Sand grains have little surface, so it only takes a little of that coating to make them repel water. Soils with less than about 5% clay are most at riskSQ.
- Dry spells. It's worst when the soil dries out. Once properly wet again, the soil soaks up water normallySQKB.
- Gum trees and their relatives. From the field: in North Shore native beds, I often see it under leaf litter from eucalypts, paperbarks, bottlebrushes and tea-trees, whose leaves are full of oils. Research backs up a link with eucalypts: soil got more water-repellent in the years after paddocks were planted with themEUC. But it isn't the only cause. Any plant litter, fungi and fine mulch can add to it, and it's the type of plant material, not the amount, that matters mostSQ.
Test it: the water-drop test
- Pick a dry day. Scrape away any mulch and the top few millimetres of soil.
- Put a few drops of water on the soil with an eye-dropper or spray bottle.
- Time how long they take to soak in.
If the drops soak in straight away, the soil is fine. If they soak in within a minute, it's slightly water-repellent. If they're still sitting there after a minute, it's severely water-repellentDPIRD. Test a few spots, because it's often patchy.
How to fix it
- Soil wetting agents. These work a bit like detergent, helping water get into soil that repels itSQKB. ANPSA describes them as very effective for sandy soils that won't wetANPSA-MULCH. Follow the label, apply to the soil (not on top of the mulch), and water in. They wear off, so expect to re-apply, usually in spring before the dry.
- Gentle loosening. Loosening or mixing the top layer breaks up the water-repellent crust and dilutes the coated layerDPIRD. In a garden, use a fork, and stay away from the roots of trees and established shrubs.
- Compost, chosen well. Well-made compost improves soil structure and water-holding over timeDPI. But because water-repellence comes from the type of plant material, not the amountSQ, don't expect more raw leaf litter or fine mulch to fix it. If you do mulch, keep it coarse (usually 50–75 mm) and skip fine mulch altogether.
- Water in pulses. From the field: give a short watering, wait 10–20 minutes, then water again. The first pass starts wetting the surface; the second soaks in instead of running off.
- Basins and wells. From the field: a shallow soil basin around a new plant holds water in place while it soaks in. ANPSA suggests sinking a pot or tin with holes in it beside each new plant, so hand-watering goes straight to the rootsANPSA.
- Don't let it dry right out. It gets worse as soil driesSQKB, so for new plants it's easier to stop the soil drying completely than to re-wet it.
✓ Do this
- Water deeply, then let the top dry a little
- Check the soil with a trowel before watering
- Water before 10am or after 4pm
- Use a wetting agent on sandy soil that beads
✗ Not that
- Little sprinkles every day
- Water on a fixed schedule from a label
- Pile fine mulch on to 'hold water'
- Let new plants' soil dry right out
New plants vs settled-in plants
New plants (first 12 months)
Water deeply rather than a little and often, especially through the first summerANPSA. Sydney Water's rules let you water new gardens at any time for 28 days after planting, following the installer's instructionsSW. After that, do the trowel test and water when the root zone is drying out. How often depends on your soil, which way the garden faces, and the weather.
Established plants
Local natives that suit the site often need little or no extra water in a normal year. But sandy, shallow or water-repellent soils, plants under big trees (competing for water), pots, and rainforest or gully plants may still need help in dry spells and heatwaves. Water before 10am or after 4pm by trigger hose, sprinkler or watering system; drip systems may run any timeSW.
Sydney Water rules apply to drinking water and can change; check the current Water Wise GuidelinesSW. Rainwater from a tank not topped up from the mains is exempt.
Signs of too much or too little water
| Too little water (or water not reaching roots) | Too much water (or poor drainage) |
|---|---|
| Wilting in the heat of the day that doesn't recover by evening; soft new growth drooping first | Wilting even though the soil is wet |
| Crispy, brown leaf tips and edges; leaves dropping in summer | Older leaves turning yellow; plant generally going downhill |
| Dry soil a trowel's depth down, even after watering (test for water-repellent soil) | Soggy or sour-smelling soil; water pooling for hours |
| Potting mix shrunk away from the side of the pot | Rotting at the base of the stem (keep mulch off stemsAPS) |
From the field: some signs look the same either way, which is why the trowel test matters. Dig before you water more.
Sources
- SQ Soil Quality Pty Ltd / GRDC, "Water Repellency" fact sheet Link
- SQKB Soil Quality Knowledge Base, "Soil Water Repellence" Link
- CSIRO CSIRO, "Water repellent soils" Link
- EUC Soil Research 53(2), "Eucalyptus reforestation induces soil water repellency" Link
- DPIRD WA Department of Agriculture, "Managing water-repellent soils" (Journal of Agriculture) Link
- DPI NSW DPI, "Improving soil moisture" (Agnote DPI-494) Link
- NSWSP NSW Government, "Soil physics" Link
- ANPSA Australian Native Plants Society (Australia), Frequently Asked Questions Link
- ANPSA-MULCH ANPSA, "Mulch and Native Plants" Link
- APS Australian Plants Society NSW, "All about mulch" Link
- APS-CLAY Australian Plants Society NSW, "Growing native plants in clay soils" Link
- SW Sydney Water, Water Wise Guidelines Link
- WAV Waverley Council (Second Nature), "Tips for growing natives in pots" Link
- From the field My own professional observations and practice, not taken from a published source.
Much of the published research on water-repellent soil comes from farming on sandy soils in southern and Western Australia; the same principles apply to sandy garden soils. This guide is general information, not advice for a specific site.
Get in touch about your soil
Every garden drains differently. If you're planning a native garden, I can check your soil, test for water-repellent patches, and look at drainage, slope and tree roots as part of native garden advice. For fixing beds or regular care, ask for a care quote.
DIY it
What you'll need:
- A trowel or spade to dig a test hole after watering
- A spray bottle or eye-dropper for the water drop test
- A soil wetting agent, and coarse mulch if your soil needs it
Get in touch for a quote
Every site drains differently. I can test your soil on site, find where the water is going (or not going), and set a watering approach for your garden on a Native Garden Walk.
Paid on-site advice for native gardens. Garden care and installation are quoted separately. No travel fee on the North Shore.