How much mulch do you need for a Pakenham garden? Size it first, then price it

How to measure your beds properly, with a tape and a phone

Almost nobody has a rectangle. The trick is to stop trying to measure the curve and break the bed into shapes you can multiply. Walk the bed and split it into rectangles, and where a bed sweeps around a corner, measure the width at three points and use the average. For a triangle, base times height divided by two. Write each shape into your phone as you go rather than trying to hold six numbers in your head.

If you have no tape, pace it. An adult stride is close to 0.75 metres, so pace the length, multiply by 0.75 and you will be within about 10 per cent, which is inside the allowance you were going to add anyway. For a long boundary strip, the satellite measure tool in a maps app on your phone is genuinely accurate enough to order from, and it is much faster than a tape on a windy day.

Then take photos. Stand at each corner of the yard, get the whole bed in frame with the tape lying in it if you can, and keep them. If you are asking two or three businesses to price the work, those photos plus your square metre figure will get you comparable quotes without anyone needing to visit, which usually gets you a faster answer.

Finally, add up all your shapes and write down one number: total square metres of bed. That single figure is what a supply yard, a landscaper and this calculator all need. Everything else is arithmetic.

Rules of thumb, and the allowance to add

• 75mm is the working depth. It is the point at which mulch actually blocks light to weed seed. Thinner looks fine on day one and grows weeds by November.
• 50mm for a top up over an existing layer that is still in reasonable shape, and 100mm for a new bed or one with a weed history you want to break.
• Never past 100mm on shallow rooted natives such as grevilleas and banksias. Deep mulch over their surface roots does more harm than the weeds did.
• Keep 50 to 75mm clear of trunks and stems. Mulch piled against bark holds moisture on it and rots the collar. It is the most common way a nice new mulch job kills a tree slowly.
• Keep it below the weep holes. On a brick veneer estate house, mulch must stay clear of the weep holes and the bottom of the cladding: it is both a damp problem and a termite pathway.
• Add 10 per cent and round up to the next half metre. You will lose some on the driveway, some in the barrow and some to settling.
• Order for delivery, not for the boot. Most yards charge one delivery fee whether it is 2 metres or 6, so if you are close to the next size up, take it.

The sizing mistake that costs the most around here

Under ordering. Somebody measures 42 square metres, orders 2 cubic metres to keep the bill under $300, spreads it at 45mm because that is all there is, and by the following spring the beds are green with wintergrass and clover. Now they buy another 2 cubic metres, pay a second delivery fee, and spend a Saturday weeding a bed they had already mulched. Ordering the right 3.5 metres the first time was cheaper than the two half jobs, and it lasted two summers instead of one.

The other one is bags. A 50 litre bag is one twentieth of a cubic metre. At $8 to $12 a bag that is $160 to $240 a cubic metre, roughly triple the bulk price, and 3.5 cubic metres is 70 bags, which is three or four car trips and a lot of plastic. Bags make sense for a courtyard bed under about half a cubic metre, for a pot display, or when you genuinely cannot get a delivery in. Above that, bulk wins every time.

If you are carting it yourself, know your trailer. A 6 by 4 with standard 300mm sides holds about 0.6 to 0.8 cubic metres level and around 1.2 heaped, and dry hardwood chip is roughly 350 to 450 kilograms a cubic metre, going over 600 when wet. Two heaped metres of wet mulch is beyond what most box trailers are plated for. Check the plate, not the optimism.

Worked example: a typical Cardinia Lakes house

Four beds, measured with a tape in about ten minutes:

• Front bed along the facade: 9 metres by 1.2 metres = 10.8 square metres.
• Side strip down the driveway: 14 metres by 0.9 metres = 12.6 square metres.
• Rear boundary bed: 8 metres by 1.5 metres = 12 square metres.
• Corner bed by the shed: 4 metres by 1.5 metres = 6 square metres.

Total is 41.4 square metres, so call it 42. At the standard 75mm that is 42 multiplied by 0.075, which is 3.15 cubic metres. Add 10 per cent and round up to the next half metre and you order 3.5 cubic metres.

Now the money. Bulk graded hardwood delivered at $65 to $95 a cubic metre is $228 to $333, plus a delivery fee of $60 to $95, so $288 to $428 in materials on your driveway. Spreading 3.5 cubic metres by barrow across four beds is 8 to 10 barrow hours for one person, more if the rear bed is through a side gate. Supplied and spread by us on the same 42 square metres is about $6 to $10 a square metre, so $312 to $510 with the delivery, the barrowing and the raking included.

That is the honest comparison: the gap between doing it yourself and having it done is often under $100 on a job this size, because the mulch costs the same either way and the labour is a morning. On a 100 square metre garden the gap widens and DIY genuinely saves money, if you have the Saturday.

Mulch, fire risk and the house wall

Fine, dry, fluffy mulch is fuel. On the bushfire exposed blocks around Pakenham Upper, Tynong and the edges of the shire, CFA advice is to keep fine flammable material away from the immediate area around the house and to use coarse composted mulch, gravel or pebble in that first band next to the walls, rather than fine pine bark. Straw and fine bark are the worst offenders for carrying embers; coarse composted hardwood is the safest of the organic options because it holds moisture and does not carry flame across a bed.

The same band matters for termites and damp. Keep the finished mulch level well below the weep holes and below the bottom edge of any cladding, and leave an inspection gap you can actually see along the footings. If you are also dealing with a fire prevention notice on a larger block, our fire prevention notice page covers what the shire usually asks for.

Honestly: when you should not be ordering mulch at all

Do not mulch dry ground. Mulch keeps water out as effectively as it keeps water in, so if the bed is bone dry going into December, water it heavily first or wait for a wet week. Do not mulch over a bed full of live couch or kikuyu runners either, because they will simply grow through and you will have paid to feed them; kill or dig those out first and mulch a fortnight later.

Do not keep stacking layers. If the existing mulch is already 100mm deep and matted into a crust that water beads on, the right job is to break it up with a fork and turn it, not to add another 50mm. And if your total bed area is under about 5 square metres, buying three bags is genuinely cheaper than paying a delivery fee. We will tell you that on the phone rather than sell you a delivery.

Best time to do it around here is late winter or early spring, after the soil has taken the winter rain and before the weeds get going, or in autumn ahead of the wet. The seasonal timing guide sets out the whole calendar. If you are choosing the mulch product rather than the quantity, the lawn variety spec page explains how the same Australian Standard logic applies to soils and turf, and the warranty page covers what to do if a load turns up contaminated. Beds that need clearing before they can be mulched are part of a garden clean up, and you can see how that day runs in what to expect on the day. For weed control after mulching, see mulching and weed control.