Weeds ‑ when are they a problem?

B. Frick, E. Johnson - Scott Research Farm


Weeds are often defined as plants growing where they are unwanted. What makes these plants so undesirable? What sort of problems can weeds cause?


Farmers are often concerned that weeds may reduce crop yields. Weeds use the same nutrients that crop plants use, often in very similar proportions. They also use resources such as water, sunshine and space that might have gone to crops. The more similar the weed and crop requirements, the more they will compete for those resources. Weeds that compete aggressively with crops reduce their yield. Weeds are most damaging to crop yields if they have some advantage over the crop. Four factors are especially important: density, timing, size and chemistry.

More weeds are generally a larger problem than few weeds, but weed density is not the only concern. For instance, at very high densities, green foxtail plants tend to compete strongly with each other and thus remain very small. These small plants probably have little competitive effect on the crop even when there are many of them. At medium densities, green foxtail plants grow larger and can severely reduce crop yields. In this example, a reduction in weed numbers may actually increase the weed problem.

Timing of weed-crop competition is important. Ecologists have defined a critical period of weed competition. This is the time when the weed reduces crop yield. Weeds that are removed before the critical period, or that emerge after the critical period do not cause any appreciable yield loss. The exact timing of this period is not an “inherent property of the crop” and varies for different crops, for different weed species, and under different conditions such as year or location. In general, weeds should be removed at early crop growth stages. Early weed removal was found necessary to protect field pea yield.

Relative timing of crop and weed emergence is very important in determining the magnitude of yield loss from weed competition. When it comes to plant competition, generally the first one out of the ground wins. Competition from wild oat resulted in a 17% yield loss in barley when it emerged five days before the crop compared to a 3% yield loss when wild oat emerged five days after crop emergence.

Weed size is partly a matter of timing. Weeds that emerge before the crop are generally larger and better established than those that emerge after the crop. This gives them greater access to soil and spatial resources, and thus they do more damage to crop yield. Size also varies among species. For instance, three Canada thistle plants are naturally much larger, and likely to cause more yield loss, than three thyme-leaved spurge plants. Size also depends on plant nutrition, disease, and pests.

Some weeds may limit crop development through chemical means, or allelopathy, either while they are alive, or as they decompose. Some weeds, for example Canada thistle or quack grass, release chemicals that inhibit their neighbors. This affects their competitive relationships.

Weeds can cause problems other than crop yield loss. Some weeds are poisonous and can taint food and feed crops. For example, wild mustard seed cannot readily be removed from canola, and can flavor the resulting canola oil if crushed with the crop seed. Stinkweed in feed for dairy cattle produces off-flavors in milk.

Weeds that remain green at harvest, especially those with fibrous stems, can interfere with harvest. The problem varies with both the crop and the weed. A low-growing weed like wild tomato causes very little problem in a cereal crop because most of the plants are below swath height. In a crop like lentil, chickpea, or bean, severe harvest difficulties may occur. The low cut means that wild tomatoes are harvested with the crop, and they can stain the pulse and clog the machinery. Weeds like wild buckwheat, that twine through a crop can also be problematic.

Weeds can harbour problem insects and crop diseases. For instance, mustard-family weeds can carry over canola diseases, making rotation a less effective tool for disease management.

Immature weeds can interfere with harvesting operations. Weed seeds in harvested crops cause dockage and increase risk of spoilage. This can reduce crop value, or increase shipping costs. Weeds in grasslands are generally those that are less palatable. They increase with grazing, because the livestock graze them less than the more palatable plants. Over time, this reduces range productivity for livestock. Weeds such as smooth brome or purple loosestrife can compete aggressively with native vegetation, and replace it.


Weeds cause many problems. Most importantly, weeds can reduce crop yield. Weeds cause greater crop losses if they occur in large numbers, if they get a head start on the crop, if they are especially vigorous, or if they produce allelopathic substances. Other problems weeds cause include dockage, tainted products such as feed or food, increased numbers of harmful insects or diseases, and more difficult harvest.


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

Originally published in Research Report 2002, Canada-Saskatchewan Agri-Food Innovation Fund