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.”
“We know that high temperatures can kill itchgrass and divine nightshade seed,” says Spaunhorst. “Now we will experiment with temperature probes in the soil — both at the surface and just below. We expect that residue density and moisture will be significant factors in the next results. After we do the field burns, we will collect the seed and attempt to grow plants in the greenhouse again.”
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.
“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.”
Weeds are thieves. They steal nutrients, sunlight and water from our food crops. In the case of sugarcane, yield refers to the amount of biomass and the sucrose concentration of the cane, which ultimately determines the amount of sugar produced. Two weedy culprits, namely itchgrass and divine nightshade, reduce cane biomass and sucrose yield.
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.
Burning sugarcane fields is common after harvest to reduce crop residue on the fields. This helps promote the growth of the next year’s crop, as the residue has been found to reduce cane biomass. Research on flame weeding and other forms of heat to control weeds is of interest in the U.S., largely because of herbicide-resistant weeds. So, Douglas Spaunhorst and his research team looked to find the right temperature to control weed seed during the normal sugarcane residue burns.
After a week at 130 ° F (55 ° C)*, most weed seeds will be dead, but it takes a month at 145° F (63 ° C) or more to kill the most resistant ones. Curiously, most common weeds actually produce seeds that are fairly easy to kill and they’ll die at relatively low temperatures. That’s the case with dandelions (Taraxacum officinale) and Canada thistle (Cirsium arvense), for example.
In general, the bigger the compost pile, the more heat it produces … and heat kills seeds, even weed seeds.
*Note that such temperatures will also kill any weed roots and rhizomes placed in the compost. Two birds with one stone!
The compost bins commonly sold generally can’t hold enough material to ensure high temperatures. If you’re using one, you’ll have to resort to other methods if you want to kill weed seeds in your compost.
To find out if your compost pile heats up enough to kill weed seeds, simply insert a compost thermometer into it and note the temperature. If you don’t have a compost thermometer, try sinking your hand into the pile. If it’s so hot you to feel uncomfortable, it’s heating up enough.
It’s important to understand is that weed seeds* can only germinate when exposed to light. If you are concerned that your compost might contain viable weed seeds, simply bury it when you use it, covering it with soil or, if you apply it to the surface, cover the compost with mulch. Problem solved!