“It won’t be 100 per cent effective but it will obviously reduce the number of weed seeds.”
– GARY MARTENS
A new study has cast a fresh light on the method of controlling weeds through burning – not the plants themselves but the seeds they leave behind.
“Fire is capable of sharply reducing the probability that soil surface-deposited seeds will germinate and emerge,” says the study published in a recent issue of the journal Weed Science.
Using fire to destroy weed seeds could “has ten and improve restorative efforts” for damaged ecosystems, the study suggests.
It could also give farmers a new tool in the ongoing struggle to control the growth and spread of weeds on agricultural land.
The study used crested wheat grass as a fuel source for burning seeds from various invasive plant species. It found 95 to 98 per cent of weed seeds at the surface of the soil failed to emerge after being burned with as little as 200 grams of wheat grass per square metre.
This gives fire an advantage over herbicides, which kill only weed plants and not the seeds themselves. And a weed seed-bank is the ultimate menace for a farmer trying to grow crops, said Gary Martens, a University of Manitoba plant scientist.
Martens said a field usually contains a million weed seeds on average. Of these, only one million will normally germinate and grow in a year. That leaves nine million seeds in the soil to germinate another time.
If these seeds are within easy reach of the flames, fire could go some way toward cleaning up a field heavily infested with weeds, he said. “It won’t be 100 per cent effective but it will obviously reduce the number of weed seeds.”
A zero-till field would be the best candidate for burning because most seeds are on the surface or in the swath. Conventional tillage buries the majority of seeds in the soil and the fire won’t get at them, Martens said.
Burning is a management technique as old as agriculture itself. The most common method is slash and burn, in which fire is used to prepare fields for cultivation. It’s estimated between 200 million and 500 million people still use this ancient practice today.
But slash and burn involves clearing fields of trees, woody plants and undergrowth. Weed seed control would be a side-effect.
Burning for the sole purpose of destroying seeds is not
a major weed control practice because disadvantages can outweigh advantages, said Martens. For one thing, burning destroys organic matter. For another, it leaves the soil without cover and vulnerable to wind and water erosion. There’s also the smoke pollution factor, which is why Manitoba strictly controls burning crop residue at certain times of the year.
That said, there are situations in which burning to control weed seeds has its place, Martens added.
Burning can be an agronomically beneficial practice in forage seed production to clean fields of unwanted seeds and pathogens.
Burning used to be a common practice in Oregon, a centre of forage seed production in the United States. But it is now restricted because of smoke nuisance problems for nearby populations.
In Australia, burning is used to control glyphosate-resistant ryegrass, a weed pest equivalent to wild oats in Canada. Farmers burn ryegrass in windrows as a weed control tool. Studies show that windrows burn hotter and longer than standing stubble does, significantly increasing the potential for destroying weed seeds, according to the University of Western Australia.
In the United States, a U. S. Department of Agriculture research and demonstration program from 1998 to 2004 found that burning in combination with herbicides reduced leafy spurge seed germination by over 95 per cent.
Kim Brown-Livingston, a Manitoba Agriculture, Food and Rural Initiatives farm production adviser in Carman, said controlled burning could be one tool in an integrated pest management program to control invasive species.
But its use by itself would be limited, she said.
“It’s not a tool that a lot of people have used before and it probably has quite specific applications.” [email protected]