During 2017 and 2018, researchers from Spaunhorst’s team researched the effects of heat on weed seeds in the lab. After collecting seeds, they applied temperatures of 100, 150 and 200°C to various groups. Times of heat exposure varied as well.
One of the tricks of weed control by burning is creating the right conditions. Burning can kill the actual weed plant, and it can also kill weed seeds retained on the soil surface. “Once weed seeds are buried below the soil surface, killing seed using heat is difficult,” says Spaunhorst. “The soil acts as an insulator to protect seeds, similar to a heat shield on a space shuttle that protects astronauts as they reenter into Earth’s atmosphere. But, the temperature, length of time of exposure, and other variables need to be determined for each weed species.”
A more important variable for heat tolerance is that the seeds of the plant have different structures. “Itchgrass seed is protected by an outer coating, similar to a husk,” says Spaunhorst. “However, divine nightshade seeds are located inside a fluid-filled berry. The fluid inside the berry seems to insulate the seed from high temperatures for short periods.”
Now that the team has collected lab and greenhouse results, the next step will be to apply these conditions in the field. It’s easy to control temperatures in an oven, but care will need to be taken to get the temperatures just right in the field. Wet crop residue in the field may not completely burn and produce temperatures too low to kill weed seeds. The burns will be started just like prairie burns — a little fuel, some wind, and a match.
These two weeds are a growing problem in Louisiana, where nearly half of the United States’ sugarcane is grown. Itchgrass competition can reduce the sugar yield in cane by 7-17%. And, the longer it competes with sugarcane, the more the sugar yield is reduced. Although divine nightshade is a relative newcomer to Louisiana, it can reduce sugar production by up to 43%. Therefore, researchers are looking at the effect of heat to control itchgrass and divine nightshade seed before it emerges in sugarcane fields.
Once the seeds had been heat-treated, they were planted to see if they would grow in a greenhouse. This is where the seeds from the two weedy species differed in how they reacted to heat. This makes sense, because each is from a different family of plants.
“Integrated weed management strategies have become more common in U.S. agriculture,” says Spaunhorst, who is based at the USDA-ARS Sugarcane Research Unit in Houma, Louisiana. “Mechanical types of weeding, like cultivation, burning, and seed crushers, show a lot of promise.”
Contamination with soil or uncomposted residues, especially after the active phase of composting has finished, can lead to the reintroduction of weed seeds or plant pathogens. Avoid adding fresh material after the active phase.
In California, Downer et al. (2008) found that unturned piles of fresh and aged green waste (note that these piles would not have satisfied organic certification requirements) did not uniformly expose pathogens to lethal temperatures. They recommended that green waste stockpiles should be turned intermittently to mix pile contents and move propagules to a part of the pile where they would be more likely to be killed by heat, microbial attack, or chemical degradation that occurs during active aerobic composting.
Susceptibility of weed seeds to thermal mortality, however, is influenced by the moisture content of the compost; weed seeds in a dry environment are able to survive higher temperatures for longer times than seeds in a moist environment. Some (Egley, 1990; Thompson et al., 1997) have suggested that thermal mortality may be greatest for fully imbibed seeds—seeds that have absorbed water and split their seed coat in the process of germination. In Nebraska, Eghball and Lesoing (2000) showed that adding water to beef manure compost greatly enhanced weed seed destruction; moist compost was faster and more effective at killing cocklebur, morningglory, pigweed, sunflower, velvetleaf, foxtail, smooth brome, and shattercane than dry compost, in part due to higher compost temperatures.
Other factors are thought to contribute to weed seed mortality during composting. Larney and Blackshaw (2003) observed considerable variability in the relationship between temperature exposure in windrows and seed viability for a number of weeds, and concluded that additional factors, such as germination into lethal conditions or pathogen infestation, were contributing to weed seed mortality. Others have implicated plant-toxic compounds that accumulate to sufficiently high concentrations during composting (phenols, ammonium, and acetic acid, for example) in weed seed mortality and suppression of germination (Eghball and Lesoing, 2000; Shiralipour and Mcconnell, 1991).
Under these conditions, populations of microorganisms will thrive and organic residues will be decomposed, consuming oxygen and releasing intermediate breakdown products, carbon dioxide, and heat. As the temperature of the pile rises, the community of microorganisms will go through a succession, culminating in thermophilic (heat-loving) organisms at temperatures above 113 °F (45 °C). If the mass of the compost pile is large enough to be self-insulating, temperatures within the pile during this active phase of composting may reach 131–170 °F (
Dry heat is less effective than moist heat at killing weed seeds. Ensure that moisture content of the pile or windrow is maintained at 40–60%.
See the related article, Making and Using Compost in Organic Agriculture, for more information about composting.