Free guide

Mulch and your soil

Whether a bed needs mulch, and which kind, depends on the soil underneath and how water moves through it. This is what the research says, what is still unsettled, and how I decide.

From the field. There's no one mulch that suits every garden. What matters most is the soil, how it's layered and how water moves through it, so I look and test before recommending anything.

How I mulch

No mulching by the calendar. Whether a bed needs mulch, what kind and how deep is worked out on a site visit. Too much mulch, or the wrong kind, can keep soil soggy, starve new plants of nitrogen and rot stems. From the field
Coarse mulch only (chunky bits, not fine and powdery), usually 50–75 mm deep, in line with the NSW Government's 5–7 cm for coarse mulch^NSW, and kept clear of stems and trunks^APS.
No fine mulch, and no fine layers underneath. In a trial, fine mulch let more water escape than bare soil did^SGA, and fine or thick layers can keep heavy clay wet^DECC. From the field

What good mulch does

Keeps soil moist. Less water dries off the surface than from bare soil.
Evens out soil temperature, so roots don't cook on hot days.
Keeps weeds down, as long as it's deep enough.
Feeds soil life as it slowly breaks down.

In trials, the difference between mulch and no mulch is usually bigger than the difference between one mulch and another^MAG. But more isn't always better, which is why your soil comes first.

"The thicker the mulch, the better."

It depends. The NSW Government suggests 5–7 cm for coarse mulch^NSW. In a trial, fine mulch let more water escape than bare soil did^SGA, and fine or thick layers can keep heavy clay wet^DECC.

What we know, and what we don't

We know

Mulch beats bare soil
Coarse mulch lets water in more easily than fine
Organic mulch can burn
Banksias and grevilleas dislike extra phosphorus^HAN

Still debated

The best depth
Whether mulch helps or worsens soil that won't wet
Whether "toxic" mulches harm grown plants

Not known yet

How Sydney's common mulches behave on our sandstone and shale soils
Fire tests of Australian garden mulches

Start with the soil

	Sandy (sandstone) soil	Clay (shale) soil
Where	Slopes, gullies, near rock	Ridgetops and upper slopes
Water	Soaks in fast, unless the surface has turned water-repellent. Dries out quickly.	Soaks in slowly. Can stay waterlogged.
Mulch goal	Hold moisture, let water through, fix water-repellent soil first	Protect the soil. Avoid fine or thick layers that keep clay soggy.

Water-repellent soil in native beds. Gum, banksia and sheoak leaves contain waxes and oils. As they break down they can coat sand grains, so water beads up and runs off instead of soaking in. One study found banksia leaves caused the strongest effect of the four species tested^DAO. So on sandy soil I test before choosing a mulch. The watering natives guide shows you how.

Mulch types at a glance

Mulch	Good for	Watch out for
Coarse wood chip	Holds moisture, lets water through, good weed control	Burns, so keep it back from the house
Coarse pine bark	Long-lasting, tidy	Floats off slopes. I don't use fine bark: it's hard to re-wet once dry.
Coarse eucalypt or forest blends	Good weed control	Fine, stringy grades can mat and shed water, so I use coarse grades only
Native leaf litter	The natural mulch of our bushland, and home for skinks and insects	Can make sandy soil water-repellent. It's bushfire fuel near the house.
Straws (sugar cane, lucerne, pea)	Cheap, good for veggies	Very flammable, breaks down fast, and lucerne and pea straw add nutrients banksias don't want
Compost	Improves soil	Higher in phosphorus. Where it suits the plants I dig it in at planting, not spread it on top or underneath mulch.
Gravel and pebbles	Won't burn, so it's the best choice right next to the house	Soil underneath runs hotter
Rubber, plastic, weed mat	Short-term weed control	Avoid in garden beds

Near the house

On bushfire-prone land, keep the zone right next to the house free of organic mulch: use gravel, pebbles or paving^CSIRO. Organic mulches can burn, and in overseas tests uncomposted ones flamed more readily^LIN. Keep all soil and mulch below the slab edge and clear of weep holes (the little gaps in the bottom row of bricks), so termite inspections can see the edge^TERM.

Depth

Sources disagree. The NSW Government suggests 5–7 cm for coarse mulch and 2.5–5 cm for wood chips^NSW, APS NSW suggests 2–6 cm for water saving^APS, and a US university guide suggests 10–15 cm of wood chips for weed control^WSU. Deeper layers stop more weeds, thinner layers let light rain through. From the field: there's no one answer, so depth is set on site. Where I mulch, it's coarse and usually 50–75 mm.

✓ Do this

Check your soil first
Use coarse mulch, usually 50–75 mm
Lay it on moist soil
Keep it clear of stems and trunks

✗ Not that

Mulch on a schedule "just because"
Use fine mulch, or a fine layer underneath
Heap it up against stems (they rot)
Pile organic mulch right against the house

What I check before recommending a mulch

Your soil, with a small test hole in each area
Whether the soil sheds water (a quick drop test)
Where water goes after rain
Slope, sun and wind
Bushfire and termite zones near the house
Your plants and what you want from the garden

Sources

APS Australian Plants Society NSW, "All about mulch" and "Mulch and native plants: a few thoughts" Link
SGA Sustainable Gardening Australia, "Choosing mulch for your garden" (Yates coarse vs fine trial) Link
NSW NSW Government (DCCEEW Water), "Saving water in the garden" (2025) Link
DECC NSW DECC, "Conserving water using compost" (2007) Link
WSU L. Chalker-Scott, "Using arborist wood chips as a landscape mulch", WSU Extension FS160E (2015) Link
DAO Dao et al. 2022, Plant and Soil 478: banksia, eucalypt and sheoak leaves and water repellence Link
MAG Maggard et al. 2012, HortTechnology 22(3): few differences among tree-based mulches Link
LIN Lin et al. 2026, International Journal of Wildland Fire: how mulches burn Link
CSIRO CSIRO, Bushfire best practice guide: mulches Link
TERM CSIRO, "Managing termites" (2021) Link
HAN Handreck 1997, Australian Journal of Soil Research 35: phosphorus and Australian native plants Link
From the field My own professional practice, not taken from a published source.

Much mulch research comes from overseas. This guide is general information, not advice for a specific site. Printable version: Choosing Mulch for Your Soil (PDF).

Get in touch about your soil

Get in touch for a quote. I'll check your soil on site, test for water-repellent patches and look at drainage, then tell you whether your beds need mulch at all, what kind and how deep. If mulch isn't the answer, I'll say so. For a whole native garden, see native garden advice.

Get in touch for a quote Call 0481 106 188

DIY it

What you'll need:

A spade to dig a small test hole
A spray bottle or eye-dropper for the water drop test
Coarse mulch from a supplier who can tell you what is in it (only if your bed needs it)

Get in touch for a quote

Not sure whether your beds need mulch, or how much? I can check your soil on site and tell you what suits it on a Native Garden Walk.

See native garden advice Call 0481 106 188

Paid on-site advice for native gardens. Garden care and installation are quoted separately. No travel fee on the North Shore.