Terra Australis Landscapes

Gardening facts and fiction

Is mulch a pile of dung?

A pile of mulch

The summer heat is just about upon us – not to mention on our gardens. If I was on the telly now as a gardening presenter – and I think that would be a great job – I'd be telling you how important it is to mulch this and mulch that, promoting as many of the mulches that the gardening superstore sponsors can sell you. 'It will save water,' I'd say. 'It can reduce your weeds, make your kids behave etc, etc, etc.'

But if you mulch, mulch and mulch again, will you end up with the Botanic Gardens in your backyard? Is mulching the be-all and end-all of solving your garden's problems?

Mulch is generally worthwhile investing in – besides as something the birds can spread out over the path or to give the family pet somewhere to bomb in and dig into. Mulch will conserve water, reduce weed growth, improve the soil structure and keep the soil surface cooler in summer. In fact, it will do most of the things that everyone says it does. But as much as we like to simplify nature's work, nature also likes to work without regard to our wishes or expectations. So there are some pitfalls to be aware of when it comes to mulching.

For example, mulch won't kill weeds. It may even promote weed growth – weeds are plants after all. No depth of mulch will stop the growth of Kikuyu grass or any other creeping turf species. Oxalis and many other weeds will regularly find their way through mulch. So before mulching, make sure you remove as many weeds as possible to stop them finding their way back through later.

Mulch can also act as an impenetrable barrier to the soil underneath and inhibit the penetration of water into the soil. The smaller the particles in the mulch, the tighter it will settle over time. This goes for both organic and inorganic mulches. Don't suffocate your garden. Larger mulch particles of a more regular shape – that is, half-inch pine bark or half-inch pebble – will not settle and will still allow water and gases to penetrate through the soil. We generally use a 50-75 mm thick layer for organic mulches, and 25-50 mm thick for inorganic mulch.

Something else to remember – gravels and stone mulches are buggers to clean, and generally can't be raked over. Use a blower.

Mulches often absorb any rainfall or water before it gets to the soil underneath. It's very difficult to water soil through a mulch topping. See for yourself – as an experiment, water your mulched garden, then scratch away the surface and check whether the soil is wet or not. You'll probably find that you've only watered your mulch! When watering your garden, you may need to water for a bit longer than you thought. Which means another few mozzie bites. And you'll probably have to top up your drink…

Mulch may break down and improve your soil, but how long does it take? Five years? Ten? Longer? Don't expect to turn heavy clay or sandy soils into a veggie patch; it won't happen no matter what the person on the telly says. You're better off choosing plants according to your soil type rather than trying to change your soil. Work with what you've got.

Lovely, moist, composting mulch also provides a healthy germination bed for weed seeds that settle in your garden. Keep an eye out for weeds growing through – one year seeding means seven years weeding, so the saying goes.

Don't put some crappy mulch from the local tip on your garden, buy it from a garden centre. Green recycling from your local council is a great idea, but importing uncomposted mulch full of weed seeds will kill just about anyone's passion for gardening!

Don't buy mulch by the bag from the TV sponsors' megastores, either. Your local garden centre will deliver quantities as small as half a cubic millimetre to your door for a minimal charge. Bagged mulches can work out ten times more expensive than buying in bulk, not to mention you'll be helping to support a local business.

But in the end, let's face it – mulch is a dry topic for a dry summer. The next post should be wetter.