When to Mulch, How Deep, and How Often to Top Up

Mulch is the cheapest thing you can do for a garden and one of the easiest to get slightly wrong. Too thin and weeds come straight through. Too thick and water never reaches the roots. Piled against a trunk and you rot the plant you were protecting. Here's the timing and the numbers.

The best time to mulch in Melbourne

Spring, before the heat. September through November is the main window. Mulch laid onto soil that is already moist from winter rain locks that moisture in for summer — which is the entire point. Wait until January and you are sealing dry soil, which is worse than not mulching at all.

Autumn is the second window. March and April work well too, particularly for insulating roots ahead of a cold snap and for suppressing the winter weeds that otherwise get away on you.

The rule underneath both: always mulch onto moist soil. If the beds are dry, water them well the day before.

How deep

Below 50mm, light gets through and weeds germinate. Above about 100mm, light rain never reaches the soil at all — it wets the mulch and evaporates.

The three mistakes

• Volcano mulching. Mulch heaped against a trunk or stem holds moisture against bark that is not built for it, and invites collar rot. Leave a clear ring of 50–100mm around every trunk.
• Mulching dry soil. A mulch layer is as good at keeping water out as it is at keeping it in. Water first.
• Never turning it. Fine mulches can knit into a water-repellent mat over a couple of years. A light fork-over once a year keeps it porous.

How often to top up

This is where the mulch you chose matters. Landscapers Mulch is coarse and slow to break down — chosen specifically for large areas and commercial landscaping where fewer top-ups are worth paying for. Pine Garden Mulch breaks down faster, which is the trade-off you accept in exchange for it enriching the soil as it goes. The dyed mulches, Black and Red, hold their colour well but the layer still thins as it composts down.

As a rough guide, expect to top up garden beds every 12 to 24 months. You are not re-mulching from scratch — a 25–30mm top-up over a thinned layer brings it back to depth.

Doing the sums

Area in square metres × depth in metres. A 4m × 2m bed at 75mm needs 0.6m³. A whole front garden of 40m² at 75mm is 3m³ — comfortably one truck load, since a load carries up to 8m³ of a single material.

If you're mulching two areas in different colours, remember they travel as two loads and are charged as two deliveries. Doing the whole property in one mulch is meaningfully cheaper than doing it in two.

Browse the Mulch collection, compare the five types in Five Mulches, Five Jobs, or work through quantities with our calculation guide.