Rumors float around the net saying that sugar can control weeds in your lawn. Guys like me say "no, it really can't." Internet responds with "you're in the pocket of the big chemical companies."
I dug in to search around the internet for any sign that someone had done some testing on this that would have set off the stampede to the grocery store's baking goods section.I finally tracked down a scientist who actually has done the work, and I think may have inadvertently created the data that so many people misinterpreted and misused. Dr. Suzanne Prober was kind enough to write back to my inquiries and clarify her studies.
"I’m not convinced sugar would be useful for home lawns. Use of sugar for weed control capitalizes on some particular ecological differences between weeds and target plants in certain common situations, as follows:
1. The weeds are nitrophilic, ie they don’t grow well at low available nitrogen
2. The plants that you do want grow well at very low available nitrogen (<2.5 mg/kg nitrate)
3. The plants that you do want have the capacity to maintain these low nitrogen levels once established (otherwise it would be necessary to add 0.5 kg/m2 sugar every three months)
We have found for instance, than in grassy eucalypt woodlands, native herbaceous species thrive at low nitrate, but many exotic annuals like wild oats do not. We use the sugar to temporarily control the weeds, to give sown native grasses a chance to establish well. The native grasses, especially Themeda australis, then keep the soil nitrate levels low in the longer term, so that the weeds are suppressed in the long term."
"Thank you very much for your response. Just to be clear that I understand correctly, does sugar have the effect of actually lowering the available nitrogen? If this were the case, logic would lead me to believe that the result in a home lawn care setting would be the opposite of what is claimed. The lower nitrogen levels would not benefit grass growth, but might allow for weeds like clover to get a foothold. Am I understanding correctly?"
She responded again:
"Yes it’s correct that the sugar temporarily lowers available nitrogen, which I imagine wouldn’t be good for lawn grasses. This could favour some types of weeds."
So there you go folks. The perfect example of a little knowledge leading to bad practice. Stick to fertilizers as needed and save the sugar for your coffee.