Which mulch suits a Cairns garden bed?

Trisim carries four mulches. Here is what each one is and where it earns its place in a Cairns yard.

• Aussie Bush Mulch: made entirely from recycled green waste. It is the most cost-effective of the four and the one to choose when the bed needs feeding, because it breaks down into the soil as it goes. Expect to top it up more often than bark.
• Caribbean Pine Bark: thicker, chunkier pieces that break down slowly and block weeds well. It is the one for beds that take run-off from a roof or driveway, and for anyone who wants the layer to last between top-ups.
• Premium Decorative Red Cypress wood chip: chosen for its finish. A red-toned chip reads well from the street, which is why it suits front beds, entries and homes being presented for sale.
• Tea Tree Mulch: a seasonal product, so availability changes through the year. Ask when you book and Trisim will tell you whether it is in stock.

All four are organic, so all four break down over time. Queensland Government waterwise guidance says organic mulch breaks down and improves the soil, and needs to be topped up frequently, preferably in autumn and spring. Plan on topping up rather than laying once and forgetting it.

Start with what the bed needs most, then pick the mulch. Most properties have more than one kind of bed, and it is normal to use two products on one block.

• Soil health: a bed of hungry, sandy or tired soil benefits from a mulch that breaks down. Recycled bush mulch is the practical choice. Queensland Government guidance notes that fine organic mulch can improve soil texture as it breaks down, but is more prone to compaction and needs to be loosened regularly so water gets through.
• Weed control: coarse organic mulch lets water through, keeps the soil cool and is good for reducing weeds, although the same guidance notes it will not improve the soil's texture because it does not readily break down. Pine bark is the coarse option of the four, and it needs to go on deep enough.
• Decorative finish: red cypress for beds that people see first, such as the front entry, a pool fence line or a courtyard.
• Slopes and heavy rain: on a bed that sits below a downpipe, a driveway or a slope, Trisim's approach is to lay a coarser, slower breaking product such as pine bark rather than a fine mulch that can wash onto the lawn or down the driveway.

If you are getting a house ready for sale, a fresh layer is one of the quickest ways to make beds look cared for in listing photos. Trisim includes bed work in its pre-sale garden presentation service.

Queensland Government guidance sets the depth by the size of the pieces. Large chunks such as pine bark need a deeper layer of more than 5 cm. Mulch made of fine particles is more prone to compaction, so it goes on in a thinner layer of around 2 cm and needs loosening regularly so water can get through.

To work out how much to order, use this simple calculation:

1. 1Measure the bed's length and width in metres and multiply them to get the area in square metres. For odd shapes, break the bed into rectangles and add them up.
2. 2Multiply the area by the depth you want in centimetres, then divide by 100. The answer is the volume in cubic metres.
3. 3If the mulch is sold in bags, check the volume printed on the bag. One cubic metre is 1,000 litres.

Worked example: a front bed 5 m long and 2 m wide is 10 square metres. At 5 cm deep that is 10 x 5 / 100 = 0.5 cubic metres. At 7.5 cm deep it is 0.75 cubic metres, or 750 litres. Round up rather than down, because mulch settles after the first rain and a short order leaves a thin patch where the weeds get in.

Before any mulch goes down, the guidance is to remove the weeds, rake or dig the surface, and water the remaining plants. Once it is spread, keep the mulch about 6 to 7 cm clear of plant stems so they do not rot. A layer of newspaper under the mulch helps deter weeds, but keep it thin so air still reaches the soil.

The waterwise guidance says organic mulch should be topped up frequently, preferably in autumn and spring. Mapped onto the Cairns climate, those two windows do different jobs.

• Spring, September to November: this is the end of the dry. Bureau of Meteorology averages for Cairns Aero show just 34.1 mm of rain in September and 47.2 mm in October, and October has the lowest average 9am humidity of the year. A fresh layer now holds moisture through the hottest dry weeks and is in place before the storms and the wet season rain arrive.
• Autumn, April to May: the heavy rain eases (201.8 mm in April, 92.0 mm in May). This is the time to replace what the wet washed out or broke down, before the drier months.

Both windows sit outside the three wettest months. January, February and March each average more than 400 mm of rain at Cairns Aero, so a spring layer is down well before that rain and the autumn top-up comes after it.

After laying fresh organic mulch, watch the lower leaves of your plants. Decomposing organic mulch draws nitrogen from the soil, and the first sign is yellowing lower leaves. A nitrogen-rich fertiliser fixes it. It is a feeding issue, not a sign the mulch is harming the plants.

Sugar cane mulch is a common alternative. Queensland Government waterwise guidance lists it alongside grass clippings, leaves and pine bark as an organic mulch that breaks down over time and improves your soil, and like the others it needs topping up.

Trisim does not supply sugar cane mulch. If it is what you already use, the same guidance applies: the ideal depth depends on the particle size of the material, with a deeper layer of more than 5 cm for large chunks and around 2 cm for fine particles. Check the product you buy and lay it to suit.

Termites are a real concern in this part of the state. The Queensland Building and Construction Commission (QBCC) describes termites as particularly aggressive in North Queensland, and advises homeowners to have a licensed pest controller inspect and report on the property every year, or more often in high hazard areas.

Queensland Government and QBCC advice on mulch and garden beds focuses on where they go:

• Do not put garden beds, shrubs, trees or bark against the walls of the house.
• On a brick home, keep the weep holes (the gaps between the bricks and the concrete slab) open so moisture can escape. In practice, that means not piling soil or mulch over them.
• Building garden beds, laying turf or landscaping around the house can make an existing termite management system less effective, so check with a licensed pest controller before adding new beds against the building.
• Fix leaking outdoor taps, because damp ground attracts termites, and do not stack loose timber against or under the house.

You will see claims that particular mulches repel termites. The guidance cited here does not rely on mulch type, so treat any product as a garden choice, not a termite barrier, and keep the annual inspection. If you notice mud tunnels on walls, paths or posts, or timber that sounds hollow when tapped, call a licensed pest controller rather than disturbing them. The QBCC termite guidance explains the different termite management systems.

Laying mulch properly is mostly preparation: weeds out, surface raked, plants watered, the layer spread to depth and kept clear of stems, with beds kept off the house walls. Trisim supplies and lays all four products across Cairns, prepares the bed before the mulch goes down, pulls the layer back off stems and trunks, and quotes the job before anyone starts. Trisim also mows lawns in suburbs including Edge Hill, Whitfield, Manunda and Westcourt, so ask about the beds when you book the lawn. See the garden mulching page for the details, or request a free quote.