Sugar for weed control

althea_gwJanuary 3, 2006

FYI -- This is from my 12/05 Soil Food Web Ezine that arrived in my mailbox just now.


Scientific trials show tying up nitrate by adding bacterial foods suppresses weeds.


by Margrit Beemster

Sugar has the potential to control annual weeds according to recent research trials conducted by researchers from Charles Sturt University.

The researchers, ecologists Dr Suzanne Prober, Dr Ian Lunt and Dr Kevin Thiele, have applied sugar to trial plots for a project funded by the NSW Environmental Trust on how to restore understorey species in endangered Grassy White Box Woodlands.

Trials on a private property "Windermere", and a travelling stock reserve "Green Gully" near Young in central NSW have provided dramatic results, with Patersons Curse and Wild Oats flourishing in untreated plots whilst plots treated with sugar had far fewer annual weeds.

The researchers have found that sugar provides a good, short-term non-chemical and ecologically friendly method of weed control.

"It appears sugar is a tool we can use to help change a system back to one dominated by native species rather than weeds," says Dr Suzanne Prober who has been working to conserve and restore grassy white box woodlands for the past 15 years. Nearly all of the woodland belt, from southern Queensland to north-east Victoria is now used for agricultural purposes, principally wheat and sheep.

So why does the sugar work? Because it is one of the fastest ways of reducing soil nitrate levels.

Dr ProberÂs compared soil nutrients in undisturbed woodlands and disturbed, degraded sites. She found the most striking difference between the two was in nitrate levels, which were extremely low in undisturbed remnants and high in degraded remnants.

"It seems that many of our weed problems are due to high nutrient levels", says Dr Prober. "There is an enormous amount of information on how to increase soil nitrogen to improve crop growth, but very little on doing the reverse. However there has been some research done overseas where sugar was used to tie up nitrogen levels for a short time."

The researchers, who spread half a kilogram of refined white sugar to each square metre of soil every three months, found this inhibited weed growth of most annual weeds giving the native plants the opportunity to become well-established. However more research is required to work out the optimum rate of application.

"We realise that the sugar levels we used in our trials would not be economic to use over broad scales", said Dr Prober, "but at the moment we donÂt know if we would get similar results if we used less sugar or if we used cheaper alternatives such as molasses or sawdust".

So how does sugar reduce soil nutrients?

"When sugar is spread on the soil, it feeds soil micro-organisms, which then absorb lots of soil nutrients as they grow," explains Dr Ian Lunt from CSUÂs Institute for Land, Water and Society.

"The micro-organisms then hold these nutrients so the weeds canÂt gobble them up. In effect we are Âstarving the weed species that require lots of nutrients to grow."

The lack of nutrients stopped the weeds from growing large, allowing the native plants, which can grow well in low nutrient levels, to grow bigger and faster.

The trial plots are now in their fourth year and the researchers believe that as the native grasses they have sown grow large enough, they will be able to lock-up the nutrients in their roots which will keep the weeds in check in the long run. Early results have indicated that nitrate levels are starting to drop in the plots with well established kangaroo grass.

"We see what we have done so far as only part of the picture," says Dr Prober. "There are a number of directions we would like to go. One of our Honours students, Lisa Smallbone, is looking at whether sugar helps us to re-introduce native wildflowers into degraded sites. If the wildflowers establish well, we want to find out if they contribute to weed control and soil nitrogen cycling later on. Our long term goal is to get the native diversity back into the understorey by working out the best method to re-establish a native ecosystem that is self-sustaining and resistant to invasion by weed species."

Using sugar as an organic weed control, to help to restore endangered woodlands and native grasslands, is an innovative alternative to using herbicides.

"Herbicides are difficult to use in many remnants because they kill the native plants you are trying to save as well as the weeds," says Dr Lunt. "Sugar does not have this undesirable effect. Herbicides also donÂt reduce the soil nitrate as sugar does, which is the underlying reason for the flourishing weeds  they control the symptoms, not the cause. Sugar may also be a useful way to control weeds that grow near other endangered native plants."

While the researchers are primarily interested in using sugar as a tool to help restore the understorey species in grassy box woodlands, they are aware their research could be the basis for other more agriculturally driven research.

"Broad leaf weeds such as Patersons Curse are the bane of every farmerÂs life. Once infestations get very bad, it gets very difficult to control them," said Dr Lunt. "Sugar may help land managers to control broad-lead weeds and to re-introduce perennial grasses in many places across the region. In particular, it could be a really helpful tool in organic farming or in places where herbicides are difficult to apply."


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I read this and my first thought was why can't they fool and starve the weeds--like the bees at zoo trash cans who think they're drinking sugar-rich cola but starve because they're drinking diet coke.

My second thought is that a LOT of native wildflowers ARE reseeding annuals. How can they differentiate a weed from a wildflower? That's man's differentiation too--not Mother Nature's--and she doesn't read the latest lists.

It sounds like throwing out the baby with the bathwater so a whole new matrix can be established. Tsk. Silly.

    Bookmark   January 4, 2006 at 10:43AM
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Althea Thanks, but I'll pass on sugar

    Bookmark   January 10, 2006 at 11:36AM
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Good observation Trudi. I wish they would publish more details about the research.

This topic reminded me of Dchall's advice to put sugar on the dog spots on your lawn to counteract the high nitrogen.

sensibly_og! Where have you been? Good pun.

    Bookmark   January 10, 2006 at 12:17PM
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Will this work in a vegetable garden without harming seeds or established plants?

    Bookmark   June 11, 2008 at 9:48AM
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The article claims sugar works by reducing nitrates in the soil. Nitrate=nitrogen which is an essential plant nutrient.

Don't see how this would kill weeds while not also affecting the growth of the desired plants.

    Bookmark   June 11, 2008 at 11:06AM
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"When sugar is spread on the soil, it feeds soil micro-organisms, which then absorb lots of soil nutrients as they grow," And these soil microorganisms, the Soil Food Web, digest any organic matter in your soil faster which then depletes that faster and starves all the plants growing there.

    Bookmark   June 11, 2008 at 1:19PM
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This post really caught my attention! For the past ten years I have been working with customers unable to grow lawns/plants due to serious southern root-knot nematode infestations. In all cases I have found that heavy applications of granulated sugar controls the nematodes and also grows healthy plants. At no time have I noticed that the sugar prevented weed growth or killed lawns/plants. Based on my observations I'm with Trudy on this one.

    Bookmark   June 11, 2008 at 3:30PM
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sharbear50(6a Bella Vista)

I say, "it is worth a try." My lawn looks terrible and my hibiscus don't look so good either. I am going to go for it.

    Bookmark   April 3, 2010 at 8:30AM
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I think the ants will be well feed.

    Bookmark   April 4, 2010 at 11:21PM
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Interesting article thanks for posting. I've been wondering about sugaring my lawn and garden for some time. Tried it in one place and the following year got a rare mushroom called a phallus on the spot - coincidence? I wanted to quickly build my sometimes sandy sometimes calcified clay into a living soil and thought of the dark soil under the wild apple trees. This story reveals what the sugar is actually doing. I thought sugar would increase nitrogen and humus. That it would indirectly feed the worms but I worried about a yeasty over bloom. At one pound per yard without damage I see I can be very generous with the sugar.
It strikes me as very useful. The microorganisms that fix nitrogen do so best in a low nitrogen environment. Therefore soil once treated can help nitrogen fixing plants and their fellow bacteria get back to work sequestering carbon.
For myself I think I will try sweeting some compost tea + nitrogen because I want to get life deep into my ground.
The real down side is sugar is the energy and inputs and subsidies that went into producing and transporting it.
As far as weeds go - its the grass that is the weed. I have a grass that grows roots down more than a foot and infiltrates my perennials. With the exception of dandelions I like all the broad leaf plants in my lawn: johny jump ups, lupin, clover and crocus please.

    Bookmark   May 7, 2010 at 1:15AM
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