Mulch is the rare garden material where the depth is the whole point: too thin and it does nothing, too thick and it keeps the water out and the weeds in regardless. The usual recommendation is fifty to a hundred millimetres, which over a forty-square-metre border is two to four cubic metres — a number that means very little until it is turned into bags, because mulch is sold in fifty-litre bags and a cubic metre is twenty of them. This converts area and depth into volume, volume into bags, and bags into money, and draws the depth on a scale so you can see whether the figure you typed is a real mulch or a light dusting.
Area, depth and bag size
Mulch is sold by the litre and applied by the millimetre. This connects the two.
The bed area, not the whole garden. Subtract paths and lawn.
50 mm to suppress weeds, 75 mm is the working default, 100 mm on a hungry bed.
Bagged mulch is usually 50 L or 70 L; bulk is sold by the cubic metre.
Result
Depth is the decision, area is just the multiplier
Everything mulch does — holding moisture, suppressing weeds, feeding the soil as it breaks down, buffering temperature — depends on depth, and none of it happens below about forty millimetres. A thin layer lets light through to weed seeds, dries out within days because it has no thermal mass, and blows about. It is the most common way to waste money on mulch: buying the right volume and spreading it over twice the area it should have covered.
Above about a hundred and twenty millimetres the returns flatten and then reverse. A very thick layer of fine material sheds water rather than passing it, because the surface crusts over; a very thick layer of coarse bark can tie up nitrogen as it decomposes, which shows up as yellowing on shallow-rooted plants. Seventy-five millimetres is the working default for a border, a hundred for a bed that has not been mulched before, and fifty as a maintenance top-up over existing material.
Why the bag count is higher than the volume suggests
A cubic metre is a thousand litres and a fifty-litre bag is a twentieth of that, so three cubic metres is sixty bags, and sixty bags is a number that surprises people who were thinking in cubic metres. It is also why bulk delivery becomes worth considering somewhere around two cubic metres: the same material by the lorry load is typically a third to a half of the bagged price per litre, and it arrives in one pile rather than sixty bags to dispose of.
The counter-argument is handling. A cubic metre of bark mulch weighs somewhere between two hundred and four hundred kilograms depending on how wet it is, and three cubic metres is a genuine trailer load. Sixty bags can be moved by hand over an afternoon; three cubic metres delivered in a pile has to be shifted by barrow, and it needs somewhere to be dumped.
Mulch piled against a trunk or a woody stem keeps the bark wet and rots it, and it gives voles somewhere to live against the bark over winter.
Leave a clear collar of fifty to a hundred millimetres around every stem. The mulch should look like a ring, not a mound.
Choosing the material
Bark is slow to break down, looks tidy and lasts two or three years, which makes it the right choice for ornamental borders where the point is appearance and weed suppression. Wood chip is cheaper, coarser and breaks down faster, which suits vegetable beds and paths. Compost and well-rotted manure feed the soil but suppress weeds for only a season, so they are a soil improvement that happens to mulch rather than a mulch.
The particle size matters as much as the material. Fine material packs down, crusts and sheds water; coarse material lets water through but lets light through too, so it needs to be deeper to suppress the same weeds. A mixed grade with fines and coarse pieces together is usually the best compromise, because the fines fill the gaps in the coarse fraction.
| Material | Depth | Lasts | Best for |
|---|---|---|---|
| Bark, mixed grade | 75 mm | 2 to 3 years | Ornamental borders |
| Wood chip | 100 mm | 1 to 2 years | Vegetable beds and paths |
| Garden compost | 50 mm | 1 season | Feeding, not suppressing |
| Well-rotted manure | 50 mm | 1 season | Vegetable beds in autumn |
| Cocoa shell | 50 mm | 1 season | Smells good, slugs love it |
| Gravel or stone | 50 mm | Indefinitely | Dry gardens; does not feed |
A worked example
- Area and depthForty square metres of border at seventy-five millimetres.
- Convert to volumeForty times 0.075 is 3.0 cubic metres, or three thousand litres.
- Convert to bagsAt fifty litres a bag that is 60 bags — a number worth knowing before you load the car.
- Price itSixty bags at $6.50 is $390, which is $9.75 per square metre of bed.
- Sanity checkSeventy-five millimetres is halfway up the scale and in the range where mulch works, so the figure is a real mulch rather than a dusting.
Nine dollars and seventy-five cents a square metre, lasting two to three years, is roughly three to five dollars a square metre a year for weed suppression and a large reduction in watering. Against hand-weeding, it is one of the best-value things you can do to a border.
Measuring the area, and what to leave out
Measure the bed, not the garden. Paths, lawn, paving and anything already covered do not get mulched, and a border around a lawn is a ring rather than a rectangle, so it is usually easiest to measure the outer rectangle and subtract the inner one. The garden area figure handles the common shapes including the L and the circle and lets you subtract what is not being mulched.
Do not mulch over the crowns of perennials. They need to see light and air at the centre, and a dormant crown buried under seventy-five millimetres of wet bark is a crown that rots. This reduces the effective area slightly on a densely planted border, and it is a good reason to mulch before the plants emerge rather than after.
Water first. Mulch laid over dry ground locks the dryness in, because it does not wet through easily once it has dried. Soak the bed, then mulch, then leave it alone — which is the sequence that makes the irrigation run time worth reducing afterwards.
Do
- Measure the bed area, not the whole garden.
- Use 75 mm as the working depth, 100 mm on a bed new to mulch.
- Water thoroughly before laying, never after.
- Leave a clear collar around every stem and trunk.
- Consider bulk delivery from about two cubic metres upwards.
Avoid
- Spreading too thin and topping up every season instead.
- Piling it against stems, where it rots the bark.
- Mulching over the crowns of herbaceous perennials.
- Laying it on dry ground, which locks the dryness in.
- Forgetting that three cubic metres is sixty bags to shift.
Where the numbers stop being true
The volume is computed at the depth you enter and the material will settle: a loose bark mulch compacts by ten to twenty percent within a few months, so a bed mulched at seventy-five millimetres measures closer to sixty by the end of the first season. If you want seventy-five millimetres to be there in a year's time, enter ninety.
Bags are not filled to the brim and the contents settle in transit, so a nominal fifty-litre bag rarely delivers fifty litres of settled material. Buying by volume from a bulk supplier avoids this entirely; buying by the bag means the true figure is somewhat higher than the calculation.
Nothing here accounts for slope. On a sloping bed, material migrates downhill, and the effective depth at the top falls while the bottom gets deeper than intended. Nor does it account for what the mulch is being laid on: over a weed membrane the required depth is lower, because the membrane is doing part of the work.
How the figures are produced
Area multiplied by depth in metres gives cubic metres, and multiplying by a thousand gives litres. The number of bags is litres divided by bag size, rounded up, and the cost is bags times price. The scale on the side of the diagram runs to a hundred and fifty millimetres so the depth you entered can be read against the range where mulch is actually effective.
Questions people ask
How deep should mulch be?
Seventy-five millimetres as a default, a hundred on a bed being mulched for the first time, fifty as a top-up over existing material. Below forty it does not suppress weeds.
How many bags are in a cubic metre?
Twenty bags of fifty litres, or about fourteen of seventy. That is why three cubic metres is sixty bags and a full car load.
Should I use a weed membrane underneath?
It is a trade-off: it suppresses perennial weeds better, but it stops the mulch feeding the soil, it eventually silts up, and it makes future planting awkward. Most borders are better without.
Will it take nitrogen from the soil?
Fresh wood chip can, because the organisms breaking it down take nitrogen from somewhere. Use composted or aged material, or add a nitrogen feed alongside fresh chip.
Can I mulch over bulbs?
Yes, and it helps them. Lay it after the foliage has died back rather than before, so the leaves can feed the bulb first.
How often should I top it up?
Every year or two, at about half the original depth. The material underneath has broken down into the soil, which is the point.