Are Organic Soils More Effective at Keeping Weeds from Coming Back?

Every year, I know that the plants that will grow best in my yard are the weeds. And I’m always right. In fact, the weeds eventually choke out the flowers that I’ve purchased and planted. No matter what I do, how much I pull, mow or spray the weeds, they seem to come right back and even multiply faster than I can keep up with them. I know I’m not alone in my plight; nearly everyone I know has his or her own monthly battles with the weeds.

Why do weeds multiply so quickly? Most weeds have millions of tiny seeds that they disperse regularly. Dandelions, for instance, have the parachute-like seeds that fly throughout the yard when they’re caught by a wind. Mimosas carry seed packets on their branches that fall on the ground or get dispersed by the birds. Once the seeds hit the ground, the real battle begins, because some seeds can be viable—capable of sprouting new plants—for several years.

For farmers, weeds are one of the three banes of existence, along with insects and plant diseases. Weeds are especially difficult for farmers because they can shade out the developing crops, outcompete growing crops for nutrients and water, and become mixed in during harvest, causing profits to fall. So each year, farmers are especially diligent about trying to get ahead of the weeds. Farmers who use conventional methods usually spray herbicide; organic farmers usually use tillage and nutrient combinations to try to make the soil unappetizing for weeds.

Two groups of scientists—one from the Agricultural Research Center in Beltsville, Maryland and the other from the Rodale Institute in Pennsylvania—tested long-term seed viability for two particular nuisance weeds, smooth pigweed and common lambsquarters, by comparing conventional systems to organic systems. They hypothesized that rich microbial matter in organic soils would actually be more inhospitable to weed seeds and decrease the amount of time they could live in the soil.

They planted plots of smooth pigweed and lambsquarters in Maryland and Pennsylvania. Both sites had comparable organic and conventional plots of soybeans. The scientists planted seeds at various depths, to find out if planting depth had any influence on seed life. Small mesh bags contained the seeds, so that scientists could retrieve the seeds more easily later to test them for viability. Scientists dug up the seed bags each spring and fall during a 2-year period.

Although most seeds overwintered after the first year, fewer seeds persisted in both systems at the end of the second year. However, previous research has reported lambsquarters seeds persisting for over four years.

Researchers found that seed depth had little influence on seed persistence. Microbial matter, which researchers had hypothesized might have some effect on seed persistence, had uneven results. Seeds for pigweed had shorter lifespans in two out of four experiments, while lambsquarters had shorter lifespans in only one experiment. Organic plots with high microbial activity and low seed viability were all in Pennsylvania. Ultimately, scientists could not definitively prove their hypothesis based on the results.

As scientists looked further, they discovered that the weed seeds contain sugars that form a glassy coat around the seed to protect it from water and temperature changes. Heavy water content and high temperatures can cause seed death. In the future, scientists will have to figure out how cropping practices in general, for both conventional and organic systems, can break down the protective seed coating and allow microbial activity to kill the seed.

Until then, we will all continually have to deal with persistent weeds.

Source: Allen Press Publishing Services (2011, April 21). Organic and conventional farming methods compete to eliminate weed seeds in soil. ScienceDaily. Retrieved May 10, 2011, from­ /releases/2011/04/110421211238.htm

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