Practical guide
Mulching properly
Depth, particle size, clearance around stems and timing by zone — and why mulch laid on dry soil does the opposite of what it is supposed to.
By Dev Raghunathan. Published 20 September 2026. Last reviewed 12 March 2027.
Mulching is the highest-return job in an Australian garden per hour spent. It cuts evaporation from the soil surface substantially, keeps soil temperature down where the fine feeding roots are, suppresses weed germination, and slowly feeds the soil as it breaks down. It is also frequently done in a way that achieves none of that.
What mulch does, and what it does not
Mulch works by covering soil. Bare soil in an Australian summer heats to temperatures that damage surface roots, crusts over so that rain runs off instead of soaking in, and offers a clear seedbed to every weed seed that lands on it. A layer of coarse material over the top solves all three.
What mulch does not do is feed plants quickly, fix drainage, or remove established weeds. It is not a substitute for watering, and a mulched bed still needs deep watering during establishment. It also does not compensate for a plant in the wrong position.
Timing by climate zone
In temperate, cool and arid gardens, mulch in early spring — around October — while the soil is still moist from winter and before the dry months. Mulching in February over soil that has already dried out is the least useful timing available.
In the tropics, renew mulch at the end of the dry season before the storms arrive, and again in the early dry once the wet has finished. In the subtropics, mulch in early spring and top up in autumn. Both zones break organic mulch down two to three times faster than southern gardens do.
The nine steps
Weed the bed first
Mulch suppresses weed seed germination by keeping light off bare soil. It does very little to established weeds, which will simply push up through it. Remove existing weeds by hand, taking the roots, before any mulch goes down.
Do this while the soil is moist. Perennial weeds left in place under mulch grow back stronger, because the mulch conserves moisture for them as effectively as for anything else.
Water the soil thoroughly
Mulch applied to dry soil keeps it dry. It intercepts light rain and light watering, so a bed that was dry when you mulched it may stay dry for months.
Water deeply the day before, and check by pushing a trowel in that the soil is damp to at least 150 mm. This single step is what most people skip and it is the difference between mulch that helps and mulch that hurts.
Choose the right mulch for the position
Coarse bark or woodchip is the general-purpose choice for shrub and tree beds in every zone: it lets water through, does not mat, and lasts. Straw and lucerne break down quickly and suit vegetable beds where the mulch is replaced each season. Composted mulches give cover and soil improvement but need topping up within a year. Gravel and pebble suit arid gardens where organic mulch decomposes or blows away.
Avoid fine mulches — sawdust, bark fines, thickly applied grass clippings. They mat into a layer that sheds water and starves the soil beneath of air, which is worse than no mulch at all.
Work out how much you need
Mulch is sold by volume. For a settled depth of 60 mm, one cubic metre covers roughly 16 m². Measure the bed area, divide by 16, and order a little over, because loose mulch settles by around 20 per cent within a few weeks.
Order coarser grades if the bed is exposed to wind or on a slope. Fine grades migrate and end up in low corners and on paths.
Spread it to a settled 50 to 75 mm
Barrow it in, tip small loads, and spread with a rake rather than tipping one large pile and dragging it across the bed, which compacts the soil under the wheel line and damages plants.
Fifty to 75 mm settled is the target. Thinner than 50 mm and weed seed still gets light and the moisture benefit drops sharply. Thicker than about 100 mm and the mulch starts intercepting light rain before it reaches the soil, and air movement into the soil is restricted.
Keep it clear of stems and trunks
Pull the mulch back to leave 50 to 100 mm of bare soil around every stem and trunk. Mulch piled against bark holds moisture there permanently and causes collar rot, which kills the plant slowly and is almost never traced back to the cause.
The same applies to the base of hedges and to the crowns of clumping plants such as Lomandra and Dianella, which rot if buried.
Keep it off paths and away from buildings
Finish the mulch 50 mm below the level of adjoining paths and lawn edges so it stays in the bed. Keep it clear of weep holes at the base of brick walls and below the level of any damp-proof course.
Keep organic mulch away from the immediate perimeter of the house. Loose material against walls is best avoided; a gravel or paved strip is the usual arrangement.
Water it in
Water thoroughly after spreading. This settles the mulch, removes air pockets and stops the lightest material blowing away in the first breeze.
For the first few waterings after mulching, run the hose longer than usual. Dry mulch absorbs a surprising amount of water before any reaches the soil beneath.
Check it twice a year
Rake mulch level twice a year, redistributing what has washed or blown into low corners, and break up any crust that has formed on the surface. Top up to the target depth.
In the tropics and subtropics, organic mulch breaks down two to three times faster than in southern gardens; check it every few months rather than annually. In arid gardens, check after windy periods.
When mulch starts repelling water
Organic mulch that has been left for a long time can form a matted crust that sheds water rather than letting it through. The symptom is water running off the surface of the bed or pooling on top of the mulch, while the soil beneath stays dry.
Break the crust up with a rake or a fork so water has a path through. Water in several short cycles rather than one long burst, letting each cycle soften the surface for the next. Then top up with a coarser grade, because fine mulch is what mats in the first place. If the soil beneath has also become water repellent, fork the surface lightly before watering — there is more on this in soil and mulch.
Mulching around different plants
Trees. Mulch a circle at least a metre across, ideally out to the drip line, at 50 to 75 mm. Keep 100 mm clear of the trunk. A mulched circle around a tree in a lawn removes mower and whipper snipper damage to the trunk, which is a common and serious cause of young tree loss.
Shrubs. Standard depth, 50 mm clear of stems. Mulch the whole bed rather than rings around individual plants; the gaps between are where weeds establish.
Clumping grasses and strappy-leaf plants. Keep mulch out of the crown entirely. Buried crowns rot.
Vegetable beds. Use straw or lucerne at about 25 mm between established seedlings. Deeper than that smothers small plants, and it should be pulled clear of stems.
Groundcovers. Mulch lightly at 25 mm at planting so spreading stems can still reach soil to root into. Once the cover has closed, no further mulch is needed — the plants are the mulch.
What not to do
Do not lay plastic sheeting under mulch. It stops water and air reaching the soil, kills the soil life beneath, and eventually degrades into fragments that cannot be removed. Where a weed barrier is genuinely wanted, plain cardboard under mulch breaks down harmlessly.
Do not volcano mulch. A cone of mulch heaped against a trunk is the single most damaging mulching practice in circulation. Keep it flat and keep it clear.
Do not use fresh woodchip from a chipper straight onto vegetable beds. Very fresh chip draws nitrogen from the soil surface as it starts breaking down. It is fine on paths and around established trees and shrubs; let it age for a few months before it goes near seedlings.
Do not mulch over a bed you have not weeded. You will be pulling the same weeds through the mulch for a year.