Weeds -- when are they a good thing?

B. Frick, E. Johnson - Scott Research Farm

Problem
Weeds are often defined as plants growing where they are unwanted. The definition reflects our attitude, but doesn't help us understand the roles these plants play in managed ecosystems.


Background
In natural environments, weeds are the first species to colonize disturbed habitats. Many weeds are well adapted to survive and reproduce where other plants can not – often in conditions of low fertility or frequent disturbance. Weeds can modify these habitats in ways that make them more hospitable for other species. Weeds shade the soil surface – reducing evaporation and the sun’s harmful effects. They can reduce wind speeds at the soil surface. In winter, they trap snow, adding to soil moisture. Weeds can be important agents of soil conservation. Weed roots can stabilize erodible soil and provide channels for the movement of water and air in the soil. They contribute to soil tilth.

Some weed roots penetrate so deeply that they tap nutrients unavailable to crop plants. The weeds bring these nutrients to the surface as the plant grows new shoots or shallow roots. When weeds die and decompose, those nutrients become available in the soil's surface layers. Weeds can also add substantial organic matter to the soil.

Weeds may indicate soil or management conditions. Redroot pigweed and wild mustard, for instance, do well only when phosphorous levels are relatively high, and thus they indicate adequate phosphorous when they are abundant. Lamb’s quarters, on the other hand, is more common in phosphorous deficient soils, and thus may indicate a problem. Dandelions do poorly in soils low in potassium. Other weeds, such as foxtail barley, may indicate salt accumulations. Thus weeds can signal the astute manager that management changes are required.

Some weeds are highly nutritious, as human food, or animal feed. They can be harvested, cut for feed, grazed, or left for wildlife. Lamb’s quarters, dandelion, common chickweed, and redroot pigweed can be used in salad or as cooked greens. Stinkweed seeds add flavour to salad dressings. Dandelion roots can be used as a coffee substitute. Wild oats, kochia and quack grass make nutritious forage if cut early. Weeds that emerge late in the season may cause little crop loss in that year. If the field is fenced, they might provide suitable grazing after harvest.

Weeds can harbour beneficial insects, mychorrizae, birds, etc. Weed seeds at soil surface may be an important food source for insects. Weeds with a shallow nectar source are particularly important as food sources for predatory wasps, hoverflies and other desirable predatory insects.

Even in a crop, weeds are not always a problem. Volunteer pea in a wheat field may be of more value than the wheat crop if it can be separated after harvest. A few weeds in a bean or pea field may reduce wind damage and help raise the pods higher off the ground, making them easier to harvest. In a wet year, weeds in a lentil field might stress the crop into flowering, rather than producing only vegetative matter.


Conclusions
Weeds are signs of life in otherwise inhospitable habitats. They play a role in both natural and managed ecosystems. They alter the environment in ways that can be beneficial.


Funding
Provided by the Canada-Saskatchewan Agri-Food Innovation Fund


Contact information
Brenda Frick, Ph.D., P.Ag.
Prairie Coordinator
Organic Agriculture Centre of Canada
c/o Department of Plant Sciences
University of Saskatchewan
51 Campus Drive, Saskatoon
Saskatchewan, Canada S7N 5A8
Tel: (306) 966-4975
Fax: (306) 966-5015
Email: brenda.frick@usask.ca


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