Add fertiliser to fight weeds

Fresh Science

Feeding weeds fertiliser sounds like exactly the wrong thing, if you want to get rid of them, but Jennifer Firn of CSIRO Sustainable Ecosystems has been doing just that—to control African lovegrass, an invasive species of rangelands in every Australian state.

Her method works by making the weed tastier to grazing animals. It illustrates that we need to be smarter in dealing with weeds, not just reaching for the Round Up, Jennifer says. Her work was published in the Journal of Applied Ecology earlier this year and is being presented for the first time in public through Fresh Science, a communication boot camp for early career scientists held at the Melbourne Museum last week. She was one of 16 winners from across Australia.

Australia spends about A$1.4 billion a year in controlling weeds, yet most continue to spread. For decades, Jennifer says, the methods used against weeds have centred on killing the invaders with herbicides, slashing and bulldozers.

Fertilisation caused a marked drop in weed growth (photo: Jennifer Firn)

“But these measures create harsh disturbances, the very conditions that favour invasive species. Consequently, one weed may be removed from an area only to have the same or another one take its place. I found a better approach was to determine what environmental conditions were favourable to invasive species, and then change them to favour the growth of more desirable native species.”

In her work on African lovegrass, Jennifer evaluated 24 different ways of controlling it in a large field experiment. Then she monitored the abundance of lovegrass and native species over multiple growing seasons.

“I found a key reason why the lovegrass dominated was that it is unpalatable to grazing animals such as cattle and kangaroos. So the most effective control measure was to keep grazing but make lovegrass ‘tastier’ using a low application rate of fertiliser. This method decreased lovegrass abundance without using herbicides and labour-intensive slashing,” Jennifer says.

A tussock of African Lovegrass (photo: Jennifer Firn)

In addition, the native grasses became more abundant because they were grazed less and had access to more nutrients in the soil. In turn, that meant that the abundance of another weed, Mayne’s pest, was kept at low levels because of increased competition from the natives.

“At first these findings appeared counterintuitive to me,” Jennifer says, “because grazing and fertilizers generally don’t favour Australian native plants. This strategy worked because lovegrass responded very quickly to the added nutrients but grazing pressure kept it from producing seed.

Jennifer Firn took detailed measurements across 192 plots

The recommendation from this study is not to use fertilizer and grazing for all invasive species. Instead my findings point to a need for a broader approach where we understand how the invasives grow, what the natives need and then change the conditions to return our native species. ”

Jennifer Firn is one of 16 early-career scientists releasing their research to the public for the first time thanks to Fresh Science, a national program sponsored by the Australian Government. Her challenges so far have included presenting her discoveries in verse at a Melbourne pub.

For further information, contact Jennifer Firn at

Jennifer Firn (photo: Mike Coulson)

Jennifer Firn pitching her story (photo: Mike Coulson)

Grazing animals on delicious weeds fostered native growth (photo: Jennifer Firn)