Weeds are the most prominent plants of the settled areas of the northern United States and Canada. They grow all around us, and affect us in many different ways. They are the plants that humans, for a number of quite different reasons, consider undesirable. Weeds can cause economic losses, damage to native vegetation, injuries, a dermatitis, hay fever, or be unsightly or be poisonous. The most successful weedy colonizers are the introduced plants that have had a long history of association with human activities elsewhere. These weeds were already well adapted for survival in the many disturbed habitats that became available with the destruction of much of our native vegetation by settlers from other Continents, starting with the arrival of Europeans about 500 years ago. Weeds are now the unwelcome inhabitants of our lawns, gardens, pastures, cultivated fields, roadsides and many other types of disturbed habitats. Some introduced plants have become so invasive that they have virtually eliminated all of the native vegetation in some habitats. Possibly because of the recent global warming, some weeds have recently extended their ranges, from warmer areas further south, northward into and within the northern United States and Canada; some now appearing for the first time in northern Canada and Alaska.

Our most successful weeds are introduced plants, from similar hardiness zones, that were already adapted for survival in the many disturbed habitats created by humans. Primitive people gathered plants from the wild for food and for other purposes and deposited the residue in nearby campsite rubbish piles. Plants that volunteered in these piles became camp followers and, over the course of many years, selection by humans resulted in the formation of our cultivated plants and of our weeds. In some cases both cultivated and weedy types arose in the same species (for example: wild and cultivate carrots, Daucus carrota L.; wild and cultivated parsnips, Pastinaca sativa L.; and wild marijuana and cannabis cultigens, Cannabis sativa L.).

The popular impression is that weeds are ubiquitous. However, many weeds have distribution patterns as clearly marked as those of our native plants. These patterns indicate a close response to climate and other environmental condition. Smooth hawk's-beard (Crepis capillaris (L.) Wallr.) is abundant only in the maritime west, whereas wild radish (Raphanus raphanistrum L.), bird rape (Brassica rapa L.), and corn spurry (Spergula arvensis L.) are common not only in that area but also in our eastern maritime region. Bull thistle (Cirsium vulgare (Savi) Ten.), St. John's-wort (Hypericum perforatum L.), and chicory (Cichorium intybus L.) are agricultural weeds in the eastern and western parts of the northern United States and Canada, but are rare or nonexistent in the prairie regions. Narrow-leaved hawk's-beard (Crepis tectorum L.), tall hedge mustard (Sisymbrium loeselii L.), and Russian knapweed (Acroptilon repens (L.) DC.) are common only in our dry prairies. Scentless chamomile (Matricaria perforata Mérat.) is comprised of two populations that are widely separated geographically. It has two chromosome races, a tetraploid one that is an aggressive colonizer on our dry prairies, and a less weedy diploid population that occurs in our eastern maritime region. Although the two chromosome races appear to have quite different physiological requirements, no morphological differences between them have been detected.

An understanding of the life durations, growth habits, methods of seed dispersal, special seed germination properties, the potential for vegetative spread or dispersal, the breeding systems, and any floral characteristics that will attract pollen vectors to flowers, is useful in trying to determine how and why introduced plants are able to quickly invade and become established, in constantly changing disturbed habitats, far from their original homes. Hayfield weeds are usually perennial, whereas pasture weeds are biennial, perennial, or very rarely annual. Row crop and grainfield weeds are nearly all annual, but if they are perennial they usually flower in the first year from seed or survive cultivation by the means of vigorous underground rootstocks. The habit of growth of plants with similar life durations also varies with different specialized habitats. Grainfield annuals and hayfield perennials are usually tall and erect. Row crop annuals and pasture perennials can be prostrate or erect. Many normally erect perennial colonizers of pastures, assume a low compact form of growth when grazed by livestock. Some perennials are not grazed because of spines or dense hairs, or because of an unpleasant odor or taste. Germination requirements vary in different weeds. Discontinuous germination may be advantageous in row crops, because of the repeated cultivations normally used in this habitat, whereas a precise germination time is essential for annual weeds to become established in a grainfield. The dormant seed of wild oats (Avena fatua L.), a common weed of cereals in our prairies, can survive fall and spring cultivations as dormant seed, and then germinate the following year about the same time as the cereal crop.

Many annual, biennial, and short-lived perennial weeds are self-compatible (autogamous), have small flower targets, and are rarely visited by pollinating insects. They can build up new colonies, great distances from existing populations, by the chance dispersal of one or a few propagules. Nevertheless, although self-incompatible, some annual, biennial and short-lived perennial weeds are widespread. These weeds are usually dispersed as impurities in crop seeds or as cultivated plants. Wild mustard (Sinapis arvensis L.), a self-incompatible annual is an example of the former strategy. Large amounts of wild mustard seed are frequently introduced into cultivated fields as impurities in the crop seed. Long-lived perennial weeds are usually allogamous or apomictic. Allogamous species are self-incompatible, only producing viable seed when pollen is transferred from a flower on one plant of a species to the stigma of a flower of another plant of the same species. These long-lived perennial out-crossers usually have large flower targets (individual flowers or flower-clusters) whose outer parts reflect ultraviolet or blue wavelengths and whose central parts absorb those wavelengths. Insects can see markings at wavelengths that are invisible to humans. These markings are apparently of importance in the close-range orientation of insects to their food sources and to the sex organs of the plants. The flowers invariably also have odors or rewards that further attract insects. Most self-incompatible weeds are rhizomatous or stoloniferous, ensuring that they can become established and spread after the introduction of only a few fragments of their rhizomes or stolons, even when suitable pollinators or genotypes of the same species are absent. A few long-lived perennial out-crossers become established and spread almost exclusively by means of fragments of rhizomes or stolons. Creeping yellow cress (Rorripa sylvestris (L.) Besser), rarely produces viable seed in nature, spreading almost exclusively by fragments of its rhizomes. Ground-ivy (Glechoma hederacea L.), a garden escape, with creeping stems that root at the nodes, frequently becomes established in lawns, and is very difficult to eradicate. It rarely, if ever, produces seed in nature. Some other long-lived perennial weeds are apomictic (able to produce viable seeds without their ovaries being fertilized). Our dandelion (Taraxacum officinale Weber), a long-lived perennial, is an example of a very successful apomictic weed. Although it is a triploid, its asexual means of seed production, its long distance dispersal of seeds attached to a parachute-like pappus, and its long tap root that reaches to a considerable depth in the soil, makes our dandelion a very effective colonizer.

Presented below, are short descriptions and pertinent information on the economic and other impacts caused by most of the common weeds of the northern United States and Canada. Also included, are their life durations, breeding systems, means of seed production and/or vegetative spread, methods of seed dispersal and/or vegetative establishment, growth habits, distributions, habitats, and any special properties. Each of these accounts is accompanied by one, and sometimes many, colored photographs. All of the weeds discussed above are included. For the convenience of readers, the colored photographs are keyed to the English and French names that have been standardized by the weed organizations of the United States and Canada. These common names are often more stable than are the scientific names. The most generally accepted scientific names and any recent synonyms are also included.

Most of the photos are part of my personal collection. The photos of herbarium plants are of specimens in the Agriculture Canada herbarium in Ottawa (DAO). My photos of herbarium specimens of seedlings of a number of grass species grown and collected by Faith Fyles (1875-1961) are followed by her initials (FF). I have photographed, and included on this website, many of the colored drawings of weeds by Norman Criddle (1875-1933) that appeared in the 1951 publication Farm Weeds, Illustrated in colour (Canada Department of Agriculture, Ottawa). Norman Criddle's initials (NC) accompany each photograph. The initials (F&M) appear after the many illustrations taken from Clarence Frankton & Gerald A. Mulligan, 1970, Weeds of Canada, Agriculture Canada Publication 948, Ottawa, Ontario, 217 pages.

There is also a page that covers Common plants of the northern United States and Canada reported to have caused poisonings, a dermatitis or hay fever in humans ("Poisonous plants" link at top left).