101 Questions answered
Summer Weed Control: How Much Moisture Can You Really Save?
Short answer: quite a lot. Up to 86 mm, according to one study.
Weeds are plants and plants use water. All other things being equal, more weeds equals more water use. Controlling weeds, especially summer weeds with deep taproots, can lead to significant savings in stored soil moisture, particularly deep soil moisture which is extremely valuable to crops during a dry finish to the growing season.
Exactly how much water is saved from controlling weeds is tricky to specify as it depends on weed species, density, soil moisture status, summer rainfall, and temperature to name a few parameters. There are, however, a few studies around we can use to gauge how much water might be saved.
- One trial in central NSW found that over two years an extra 86 mm (2011) and 50 mm (2012) of rainfall was stored in the profile when weeds were controlled compared to the control sites.
- Research at Hopetoun in the Victorian Mallee found that in March 2011 there was an extra 19 mm of water in weed-free, stubble retained plots compared to weedy (also stubble retained) plots. On bare earth plots, the difference was 34 mm of soil water.
- SAGIT trials on the Eyre Peninsula looked at soil moisture more broadly with soil moisture probes, but did capture some weed water use data. At one site in early February 2017, 36 mm of rain fell. Two weeks later, all of that water had been removed by summer weeds.Note that in some locations in the Mallee, controlling deep rooted summer weeds has been one of the drivers behind an increase in the number and size of Mallee Seeps. Historically, summer weeds used excess soil water and prevented seeps from developing. See Question 73. ‘Why are Mallee seeps appearing in my paddock?’
