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killing weed seeds with moist heat

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.

“Evaluation of non-chemical strategies for itchgrass and divine nightshade management is limited,” says Spaunhorst. Chemical-alternative weed control can help slow herbicide-resistance evolution. “In fields historically under sugarcane cultivation, no chemical-resistant divine nightshade or itchgrass has been reported,” says Spaunhorst. “However, a few examples have been found where resistant itchgrass became established in soybean fields.”

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.”

“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.

A compost pile that gives off water vapor is working hard to kill weed seeds. Source: Anatomy of Living, http://www.youtube.com

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.

*Note that such temperatures will also kill any weed roots and rhizomes placed in the compost. Two birds with one stone!

Heat-resistant weed seeds requiring treatment at 45° F (63 ° C) include:

Note too it may be necessary to water your compost pile from time to time. Compost heats most efficiently when it is neither dry nor wet, but moderately moist.

Ideally, you wouldn’t add weeds that are in seed or even in the late part of their blooming cycle to the compost pile. Thus you can avoid the problem of their seeds germinating in the garden when you later use the compost you produced. But sometimes, you have little choice: perhaps the most easily available compostable material (horse manure, hay, etc.) contains seeds or else the endless sorting of weeds according to their “seediness” would just be too complicated. Or, like me, you just feel that everything organic should be composted.