Any farmer or gardener can tell you that a hard winter, with long periods of freezing temperatures, is damaging to perennial plants and to some seeds in the soil. When the plants or seeds are invasive weed species, this isn’t such a bad thing. Using thermal methods of weed control, such as cryogenic freezing, offers herbicide-free and less labor-intensive ways of managing invasive species.
A study reported in the current issue of the journal Weed Technology tested the use of cryogens in farm fields. Experiments were conducted for two years at Hokkaido University in Sapporo, Japan. Soil containing a number of weed seeds was cryogenically treated during the winter months, and the growth of weeds among oat crops was recorded in the spring. The results show that while this method of weed control is possible, it is not yet practical.
Freezing temperatures under dry conditions can be beneficial to seeds—cryopreservation is used as a method of preserving plant genetic resources. But adding water produces a different result. Plant or seed tissues that contain high amounts of cellular water will be severely damaged or destroyed when intracellular water crystallizes into ice.
With sufficient water, freezing seeds could effectively reduce weed establishment. Researchers placed bags of various cryogenic salts, including sodium chloride, ammonium nitrate, and magnesium chloride-6-hydrate, on bare soil before snowfalls. Contact between the snow and the cryogens further decreased the soil temperature, negatively affecting seed survival.
One invasive species, corn spurry, was significantly reduced through the cryogenic effect. Overall weed establishment was reduced as well, in both years of the experiment. However, the effects of cryogen application in the field were smaller than researchers expected. Some weed seeds have a high freezing tolerance, making cold thermal control difficult. Therefore, it is not yet possible to use this method alone for effective weed control.
Full text of the article, “Possible Weed Establishment Control by Applying Cryogens to Fields Before Snowfalls,” Weed Technology, Vol. 25, No. 3, July-September 2011, is available at http://www.wssajournals.org/.