How to use sugar to reduce weeds in your garden

"Battling weeds is a war of attrition and we have to use every weapon in our arsenal."

That's Alexi Gilchrist's call to arms against every gardener's nightmare — weeds.

Mr Gilchrist is the restoration ecologist at Liverpool City Council in Sydney's south-west.

He was tasked with reducing the weed infestations at Wattle Grove Lake, an artificial water hole in the small suburb of Wattle Grove.

Controversy over the use of herbicides to kill weeds has grown around the world in recent years, with health and environmental concerns prompting a push for chemical-free alternatives. 

Glyphosate — the active ingredient in the weed killer Roundup — has sparked lawsuits in Australia and the US over claims it causes cancer. 

The concerns prompted the local council to trial the use of sugar as an eco-friendly alternative to keep weeds at bay.

The trial is based on a CSIRO study that found sugar reduces seed germination rates of some herbaceous weeds.

"We want to retain our native vegetation and Bidens pilosa is a common weed that is easily spread," Mayor Wendy Waller says.

"The results from the test plots in Wattle Grove Lake are promising. By reducing the use of harmful herbicides it leads to more economical means of weed control."

How does using sugar for weed control work?

After purchasing 1.2 tonnes of sugar, the team at Liverpool City Council got to work.

Two test plots were set up and both were weeded and mulched.

One test plot was treated with a thin layer of white sugar (half a kilogram per square metre), before being mulched.

Within the first three months, only a few weeds were present, and at six months none were germinating.

"Generally native plants prefer low nitrogen soils … so where they're competing with weeds it's good to bring the nitrogen down to levels that favour native plants," Mr Gilchrist told ABC Radio Sydney.

"The sugar causes the soil microbes to go through expanded growth and metabolise the available nitrogen, which is what weeds love and what causes them to grow."

Does sugar kill every weed?

Mr Gilchrist says sugar is recommended for herbaceous annual weeds — plants with soft green stems, fleshy green leaves and exotic grasses.

But people are being warned not to try sugar for weed control on certain areas around the home, including on footpaths or other paved areas.

"I'd only recommend it when they are doing formative gardens … because sugar depletes the nitrogen in the soil and it may affect other species of plants that you want in your garden," Mr Gilchrist says.

"You couldn't do it in your veggie patch, for instance. You need to be applying nitrogen in your veggie patch to promote growth."

DIY weed controls

In a bid to reduce the use of chemicals in the garden, ABC Radio Sydney listeners had some suggestions of their own:

"Pure white vinegar applied in the sunshine will rapidly kill green growth, including weeds." — Jim

"Acetic acid and sodium chloride is an effective non-toxic solution for weeds." — Callum

"I've used vinegar and it works!" — Jenny

If you're nervous about trying any of these methods, Mr Gilchrist says the time-consuming practice of pulling weeds by hand or using a pick is still just as effective.

"We also have mechanical removal such as chainsaws, brush cutters … which mulch piles of weeds as they move over them."

Boiling water is also commonly cited as a DIY treatment for weeds, but people are being advised against using it in some areas.

"I wouldn't use it in your veggie patch or other garden beds where you want other things to grow, because it will affect the soil microbes," Mr Gilchrist says.

Whatever method you choose, make sure to do your research beforehand, he advises.

Natural industrial methods (but not for home)

Steam weeding is a technique in which high-pressure steam is directly applied to plants using a machine, destroying the weeds' cell walls.

The bulky equipment needed for steam weeding means its use is limited to urban areas, including paved areas like footpaths, carparks and sporting fields.

Flame weeding involves using a backpack flame thrower, which shoots out a jet of flame that destroys cell walls, causing weeds to die.

While the devices are more portable than those used for steam weeding, there are potential risks.

"You run the danger of starting fires, so it has to be used in controlled circumstances," Mr Gilchrist says.

This story was first aired on ABC Radio Sydney.