I've mulched more garden beds than I care to count. In Queensland's climate especially, mulch isn't optional — it's the difference between a garden that survives a dry spell and one that doesn't. But the question I hear most often from people tackling it themselves is simple: how much do I actually need?
The answer depends on three things: the size of the area, the depth you're putting down, and what type of mulch you're using. Get all three right and you'll order the right amount. Get one wrong and you're either making a second trip to the landscape yard or bagging up the excess.
The depth question — and why most people get it wrong
The most common mistake is putting down too little. A 25 mm layer of mulch looks okay on the day but within a few weeks it's done nothing useful — the sun bakes through it, weeds push through it, and the moisture retention benefit is basically zero.
For most Australian garden beds, the right depth is 75–100 mm. That's roughly the depth of your fist. At that depth, mulch does what it's supposed to:
- Suppresses weeds by blocking light to weed seeds in the soil
- Retains soil moisture by slowing evaporation — critical in Queensland summers
- Regulates soil temperature — keeping roots cooler in heat and warmer in the occasional cold snap
- Breaks down slowly to add organic matter to your soil over time
Go to 100 mm if you're in a hot, dry area or if your beds have a serious weed problem. Stay at 75 mm for established gardens with reasonable soil. Don't go much deeper than 100–125 mm for most plants — piling mulch too deep can trap moisture against stems and cause root rot, and some plants struggle to push through it.
One important tip I've learned from doing this properly: keep mulch 50–100 mm away from plant stems and tree trunks. Mulch piled against stems holds moisture and causes disease. You'll often see this done wrong in landscaped gardens — a thick donut of mulch packed right up against the base of a tree. It looks tidy but it's slowly killing the tree.
Choosing the right mulch for Australian conditions
Not all mulch is equal, and the right choice depends on what you're trying to achieve and what's available in your area.
Hardwood chip or forest mulch
The workhorse of Australian garden mulching. Coarse, chunky, slow to break down. Good weed suppression, excellent moisture retention, lasts 1–2 years before needing a top-up. What most landscape yards sell as "garden mulch" or "forest mulch." My default recommendation for most garden beds.
Pine bark
Finer than hardwood chip, looks tidy, popular for ornamental beds and front gardens. Breaks down faster than hardwood — you'll be topping up annually rather than every two years. Tends to float in heavy rain if your beds aren't contained, which is worth knowing if you have sloped garden beds or frequent downpours.
Sugar cane mulch
Very popular in Queensland and northern NSW. Lightweight, easy to handle, breaks down relatively quickly (faster than wood chip) and adds good organic matter to the soil. Excellent for vegetable gardens and food gardens where you want the mulch to break down and feed the soil. Less ideal for long-term weed suppression because of how quickly it decomposes — you'll be applying it more frequently.
Lucerne (Alfalfa) hay
Premium option for vegetable gardens. Breaks down quickly, adds significant nitrogen to the soil, and veggie gardens love it. More expensive than other options and won't last long before it's fully broken down — but that's the point. Use it where you want to feed the soil, not just suppress weeds.
Wood chip (raw)
Often available free or cheap from arborists and council mulch programs. Coarser than commercial mulch, variable quality, but perfectly fine for pathways, around trees and in utility areas. One caution: fresh wood chip can temporarily draw nitrogen from the soil as it breaks down. Let it age for a few months before using it directly in planting beds if you can.
Bulk bags vs loose delivery vs bags from Bunnings
This is where the maths matters most, because the price difference between buying right and buying wrong is significant.
Bags from Bunnings — convenient for small jobs (a single garden bed, a top-up of an existing bed). Expect to pay $8–$15 per 50L bag. For a 10 m² bed at 75 mm depth you're looking at 750 litres of mulch — that's 15 bags and potentially $150–$200 in mulch alone. The same volume bought in bulk would cost a fraction of that.
Bulk delivery from a landscape yard — the right choice for anything over about 1 cubic metre. Most landscape yards in Brisbane and surrounds sell mulch by the cubic metre delivered. Prices vary but you're typically looking at $60–$100/m³ delivered, depending on type and location. For larger jobs the savings over bags are substantial.
Bulka bags (1 m³ in a woven sack) — the middle ground. Delivered to your driveway, you work through it at your own pace. No rush to unload before a truck leaves. Good option if you don't have easy truck access to your yard.
Trailer load — if you have access to a trailer, picking up direct from a landscape yard is often the cheapest option. A standard 6×4 trailer holds roughly 0.7–0.9 m³ of mulch. Know your volume before you get there so you can tell them exactly what you need.
How to calculate the right amount
The formula is straightforward: length × width × depth (in metres) = cubic metres. For a 5 m × 3 m garden bed at 75 mm deep: 5 × 3 × 0.075 = 1.125 m³.
In practice, always add 10–15% to your calculated volume. Mulch settles, gets kicked around edges, and the ground is never perfectly level. Ordering a little more than the maths says is always cheaper than ordering a second load.
Rather than doing this by hand, use the free mulch calculator — enter your dimensions, choose your depth, and it'll tell you exactly how many cubic metres, tonnes and bags you need with the waste margin already built in.
Free mulch calculator
Enter your garden bed dimensions and get cubic metres, tonnes and bag count — with waste margin included.
Use the mulch calculator →When to mulch — and when not to
The best time to mulch is after rain or after watering, when the soil is already moist. Putting mulch on dry soil traps the dryness in — the mulch acts as a barrier to rainfall and you end up with dry soil under a thick layer of mulch that sheds water rather than letting it through.
In Queensland, I typically mulch twice a year — once in spring before the wet season heat kicks in, and once in autumn to prepare beds for the dry season. One good 75–100 mm application in spring usually sees me through to autumn with a top-up rather than a full reapplication.
Don't mulch right after planting seedlings unless you're careful about keeping mulch away from stems. New seedlings can struggle to push through a thick mulch layer. Wait until plants are established and have some stem height before pulling the mulch in close.
The bottom line
Mulching properly is one of the highest-return things you can do for an Australian garden. It saves water, reduces weeding time, and improves your soil over time. The key is getting the depth right (75–100 mm), choosing the right type for your situation, and ordering the right volume so you're not making two trips or bagging up the excess.
Use the calculator, order in bulk for anything over a cubic metre, and keep the mulch away from your stems. Everything else is detail.