Weeds on organic farms

B. Frick, E. Johnson - Scott Research Farm


Are the weeds on organic farms different from those on neighboring farms?


Weeds, like all plants, live in the context of their environment, the history of the land and the random workings of chance. Some of the most important factors that make up the environment are beyond a farmer’s control, such as climate. Some factors important to weed development, like soil temperature and moisture, reflect both the local climate and specific farm management. Farm management on organic farms is often different from neighboring farms, and thus weed communities might be expected to be different.

One study in Saskatchewan indicated that organic systems had more weed species and more individual weed plants than conventional systems. Wild mustard, lamb’s quarters and Canada thistle especially were more abundant in organic systems. Wild oats were relatively less of a problem. In that study, differences among years were greater than differences between organic and conventional systems.

A second study compared larger numbers of Saskatchewan organic and non-organic farms. It concluded that cropping system, especially if the farm had a history of perennial forage crops, had a greater influence on the weed community than whether the farm was managed organically or not. Perennial forages tended to encourage perennial and biennial weeds, while annual cropping systems encouraged annual weeds. Organic fields had greater numbers of weed species most susceptible to herbicides -- wild mustard, lamb’s quarters and bluebur (Table 1). Organic fields had more individual weeds, on average, and greater numbers of weed species than neighboring fields.

In a Manitoba study, organic producers reported problems with wild mustard, Canada thistle, red root pigweed, green foxtail and wild oats. Wild mustard was much more of a problem for organic producers than their conventional counterparts (Table 2).In experimental comparisons of organic and conventional systems in South Dakota, annual broad-leaved weeds were not consistently different in organic systems than in conventional systems. Grassy weed numbers, mostly green and yellow foxtail, were substantially higher after six years in organic cereals than they were in conventional cereals. They were not consistently higher in organic soybeans.

Table 1. Comparison of weed density (#/m2) in different cropping systems* and in organic systems relative to those with moderate levels of chemical inputs (Mod).

Head Fallow Annual crops Annual and perennial crops
Mod Organic Mod Organic Mod Organic
Stinkweed 16.2 44.1 13.4 40.3 11.9 21.2
Green foxtail 17.4 45.3 10.1 85.3 1.9 30.4
Wild buckwheat 1.7 6.1 4.7 9.3 5.0 24.1
Lamb's-quarters 0.6 75.7 1.0 52.8 4.4 24.7
Wild oats 0.9 19.3 9.5 4.6 0.3 0.7
Narrow-leaved hawk's beard 0.6 0.9 2.9 1.6 34.5 24.9
Quack grass 1.0 0.9 1.3 0.7 23.3 31.5
Russian thistle 2.6 4.3 1.3
8.5 1.1 1.2
Shepherd's purse <0.1 1.1 4.3 5.4 0.8 18.7
Canada thistle <0.1 1.5 1.6 1.4 0.3 1.5
Wild mustard   52.3 0.5 6.3 <0.1 0.1
Dandelion <0.1 1.3 0.2 0.2 16.1 4.6
Perennial sow-thistle <0.1 0.8 0.7 0.6 1.1 2.0
Field horsetail   0.5 0.3 2.8 1.1 3.0
Volunteer canola 0.9 0.7 2.3 0.2 <0.1  
Volunteer wheat 0.2 0.3 1.0 0.5 <0.1  
Kochia 1.6 3.0 <0.1 0.1 22.4 0.1
Flixweed 0.7 0.8 <0.1 0.2 6.8 1.1
Bluebur <0.1 1.5 <0.1 1.8 0.4 1.3
Cleavers   <0.1 2.1 <0.1    
* Fallow systems inlcude fallow; annual and perennial cropping systems include perennial crops; annual cropping systems do neither of the above.

Table 2. Comparison of weed problems reported by Manitoba organic producers and weed numbers found on Manitoba farms (mostly not farmed organically)

Weed species Organic producers
Relative importance
Other producers
Relative importance
Wild mustard 21 4
Canada thistle 18 6
Redroot pigweed 14 4
Green foxtail 13 28
Wild oats 12 11
Wild buckwheat 5 9
Quack grass 4 3
Perennial sow thistle 4 3
Lamb's-quarters 3 3
Curled dock 2 0
* Importance is equal to the number of reports for a given species, expressed as a percentage of all species reports (i.e. larher numbers indicated greater importance)


Weed communities vary because of many factors. Factors such as cropping system, year and region can be more important than whether farms are managed organically or not. Organic farms will likely have more wild mustard, lamb’s quarters, Canada thistle, bluebur and redroot pigweed than will conventional farms.


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

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