What is mulch?
Mulch is one of the gardener’s most valuable tools. It blocks weeds from resprouting, keeps roots cool in the heat of summer and provides a clean, organised aesthetic that makes it easier to navigate around your plants.
“Mulch is a bit like a protective blanket,” says horticulturalist Chloe Thomson of @beantheredugthat, and host of Bunnings podcast Staying Grounded. “It helps reduce evaporation, suppresses weeds and, as it breaks down, adds organic matter and nutrients to the soil,” she explains.
When should you mulch?
Mulch at any time but, for best results, apply it at the start of the growing season, preferably when the ground is damp.
“In the vegie garden, work the old mulch into the soil and then replace with fresh organic mulch once you have planted,” says Chloe.
In the rest of your garden, organic mulches should be reapplied every year to keep them at a depth of 3-4cm, while inorganic mulches, like pebbles, will only need topping up every few years.
How much do I need
More mulch is not always better – if applied too thickly, it can prevent rain and irrigation reaching the plant roots. The amount needed varies based on the product, but for bark mulches, aim for around 3-4cm.
“Straw can be slightly thicker – up to 7cm, as it is light and fluffy, and weeds can easily push through thin layers,” says Chloe. Grass clippings and leaf mould can gradually be built up to a 5cm layer – if you have excess, split it with the compost bin.
Natural materials such as pea straw, sugar cane, pine bark, seed-free grass clippings and leaf mould make great mulches, as they break down and help improve soil structure, according to TV gardening expert and Richgro ambassador Charlie Albone. “But keep it away from stems and trunks to avoid stem rot – instead, create a moat around newly established plants to help trap water and direct it to the root zone,” he adds.
Leaves: These need to be shredded first, or composted. If left whole, they can clump and form impenetrable mats that can suffocate the soil. An easy way to shred leaves is to mow over them. They will then break down over the season, providing nutrients to your garden in the process. The downside: if you live in a windy area, they can blow away.
Wood waste/bark: Mulches made from untreated wood waste or bark break down slowly and won’t blow away. However, woodchips are low in nitrogen, which can cause a temporary deficiency in soil. Add a conditioner, such as blood and bone, to soil before applying wood-based mulch.
Sugar cane mulch: This breaks down easily over the year to give your garden the benefit of added nutrients, but it’s not great for windy areas. Imported sugar cane mulch is not available in Western Australia, as it may contain the seeds of prohibited weeds.
Grass clippings: An excellent, free source of mulch. They’re also great for boosting and conserving soil moisture.
Pine needles: These are good for acid-loving plants and a great mulch for strawberries – of course, you’ll need access to pine trees to source a decent supply for your garden..
Coir mulch: A decorative mulch, this is light and quite easy to transport, and also typically contains fertiliser.
Stone/gravel: This is an aesthetically pleasing option, but better used to create paths through a landscape, over pots or areas where a more designer look is desired. “It adds nothing to the soil, so you will need to address the lack of organic matter by adding extra compost to the soil,” says Charlie.
Landscape fabric/weedmat: Weedmat is a porous material that suppresses weeds. While weeds can sometimes grow through the upper layers of cheaper fabrics, the more expensive products may last up to five years. It’s a little bit unattractive on its own, but can be covered with mulch so it’s less visible.
Learn how to install weed matting using our step-by-step guide.
Photo credit: Getty Images, Gap Photos/Perry Mastrovito