FLORA OF NORTH AMERICA - Volume 1, Chapter 8

Weeds in North America

Ronald L. Stuckey
Theodore M. Barkley

THE flora of North America includes a large number of conspicuous plants that are called "weeds." The concept of weed is not precisely defined, for it has both a sociological and a biological component. From the sociological perspective, a weed is simply a plant that is growing where someone wishes it were not, and therefore, a weed may be regarded casually as a "plant-out-of-place." By that definition, a rose growing in a wheat field would be a weed; a rose in a garden would not. Some plants, however, have the genetic endowment to inhabit and thrive in places of continual disturbance, most especially in areas that are repeatedly affected by the activities of humankind. These plants are biologically "weedy," and they are sometimes termed colonizing or invasive plants. These biological weeds are the focus of this essay.

Weeds have a measurable effect on the affairs of society, and therefore they have attracted much attention. Several compendia on the subject published since the mid-1960s form the basis of this essay: H.G. Baker and G.L. Stebbins 1965; J.R. Harlan 1975; J.G. Hawkes 1983; W.Holzner and M.Numata 1982; L.J. King 1966; H.J. Lorenzi and L.S. Jeffery 1987; H.A. Mooney and J.A. Drake 1986; S.R. Radosevich and J.S. Holt 1984; and H.A. Roberts 1982.

Weeds occur in all growth forms and in many lifestyles. The majority of weeds are flowering plants, and a high proportion of them share some or all of the following characteristics: short life cycle, rapid growth rate, high level of energy allocated to reproduction, efficient dispersal mechanisms, high population growth rate, wide distribution, seeds with long life spans, and flexible use of environmental resources (H.G. Baker and G.L. Stebbins 1965; F.A. Bazzaz 1986). A catalog of the attributes of weediness was outlined by H.G. Baker (1965, 1974) for "the ideal weed" (table 8.1). He noted that a plant with but few of these attributes is less likely to be successful as a weed than is a plant with all or most of them; therefore, the variation ranges from casual, local weeds to aggressive, widespread weeds.

TABLE 8.1. Ideal Weed Characteristics
  1. Germination requirements fulfilled in many environments
  2. Discontinous germination (internally controlled) and great longevity of seeds
  3. Rapid growth through vegetative phase to flowering
  4. Continuos seed production for as long as growing condions permit
  5. Self-compatible, but not completely autogamous or apomictic
  6. When cross-pollinated, unspecialized visitors or windborne
  7. Very high seed output in favorable environmental circumstances
  8. Produces some seed in wide range of environmental conditions; tolerant and plastic
  9. Has adaptations for short- and long-range dispersal
  10. If a perennial, has vigorous vegetative reproduction or regeneration from fragments.
  11. If a perennial, has brittleness, so not easily drawn from the ground
  12. Has ability to compete interspecifically by special means (rosette, choking growth, allelochemics)

After H.G. Baker 1965, 1974.

The scientific study of weeds is driven largely by economic considerations, but some interest in these plants also exists because of their biological nature. For example, why do some plants respond as colonizers, whereas others do not have that intrusive ability?


Weeds have been traditionally controlled by pulling, hoeing, burning, smothering, or otherwise damaging them, or by poisoning the soil. Studies in plant physiology, however, have led weed scientists to a good grasp of both the chemical descriptions and the modes of action of growth-regulating substances, which made possible the development of modern chemical weed control, i.e., the use of selective herbicides. These herbicides are biologically active agents that are introduced into the environment for the laudable purpose of controlling weeds, with minimal evident effects on the crop plants.

Unfortunately, herbicides may have unexpected and unwanted consequences, and they can contribute to unexpected environmental problems. Safe and use of herbicides requires both technological skill and planning, and questions must always be asked about the residual effects of applied herbicides and their degradation products in the ecosystem. The sale and use of herbicides are regulated by the appropriate branches of federal and regional governments in both Canada and the United States to help ensure the proper use of these chemicals.

Many agricultural-chemical firms manufacture and distribute herbicides, and most have research programs aimed at increasing the usefulness of their products by developing herbicides with narrow ranges of target plants, and with little or no measurable effect on other plants and the environment. The ideal herbicide would provide complete and rapid control of the target weed, while not affecting the crop plants or the environment, and it should be available at reasonable cost. Such a paragon does not exist, of course. As research progresses, however, the herbicides entering the market are more restricted in their spectra of target plants and ever more demanding of sophisticated application procedures. These factors, plus the costs of new herbicides, make it essential to have accurate identification of weeds.

Plant identification services are provided free or at nominal cost by one or more units of the governmental agricultural agencies in both the United States and Canada, chiefly through the state/provincial agricultural extension services or their regional equivalents. In addition, many regions have "user-friendly" weed manuals that are tailored to the needs of agriculturalists (e.g., T.M. Barkley 1983; Illinois Agricultural Experiment Station 1981; C.Frankton and G.A. Mulligan 1970).

The Weed Science Society of America (WSSA) is the focal group for the study of weeds in North America. It publishes two scientific journals, Weed Science and Weed Technology, and also the "Composite list of weeds" (1984), which lists 1934 species of currently or potentially important weeds as weeds in Canada and the United States. A supplement (1988) adds 58 species. The list provides accepted Latin names for each entity, plus a recommended colloquial name. A five-letter code name (the "Bayer code") is given for each species, which is used as a bibliographic shorthand in the society's publications, e.g., Cardaria draba, "hoary cress," is given the code CADDR.


A topic of endless fascination is the origin of weeds. Reason suggests that weeds may be chiefly by-products of the disturbances of the earth at the hands of humankind. Areas of continual natural disturbance, however, have occurred throughout geohistorical time. Common causes of both local and regional disturbances include fire, flood, volcanism, and wave and ice action, among others. Some of these events are seasonal and predictable, and some are not. The activities of animals, particularly the abundant large grazers, have maintained areas of continual disturbance that are also comparatively rich in available nitrogen. It is in such continually disturbed sites that colonizing plant species are thought to have evolved.

Weeds that thrive in areas of continual disturbance may be treated as two intergrading components, ruderals and agrestals. Ruderals are those plants that occur in areas of irregular or inadvertent disturbance and are not intentionally manipulated for agriculture. Ruderals such as pepper grass, Lepidium virginianum, are associated with the early stages in natural succession. Agrestals are plants adapted to agricultural practices and are often associated with a particular crop. For example, goatgrass, Aegilops cylindrica, often grows with wheat, Triticum aestivum; gold-of-pleasure or false-flax, Camelina sativa, grows with flax, Linum usitatissimum. Agrestals are the weeds of evident economic importance in that they lower agricultural productivity by competing with crop plants for available resources and for standing room, and they obstruct tillage and harvest. As a consequence, agrestals are the subjects of most of the efforts in scientific weed management.

Agriculture in North America functions on models that were imported from Europe, which, in turn, developed from ancient practices. Even crops of New World ancestry (among them, corn and potatoes) have been adapted to European farming techniques in temperate North America. This style of farming involves regular tilling of the soil with the crops grown in monoculture, and it provides the proverbial plenitude of pleasant places for a plethora of pliant plants.

For over a thousand years, beginning with travel between Scandinavia and Greenland, regular traffic has flowed between the Old World and the New World. Therefore, the plants that are adapted to the activities of humankind have had ample opportunity and time to be transported from place to place. North America now has about 2000 species of plants that are variously called "weeds." Most of them are ruderals, but some, perhaps 200, are agrestals associated with crops. The majority of these weeds are clearly the same species that are associated with human activities in the Old World.

Furthermore, the weeds of the various climatic regions of North America are associated with the weeds of similar climatic regions of the Old World. For example, most weeds in eastern North America are those of western and central Europe; the weeds of California are those of the Mediterranean region; the weeds of the Intermountain region are those of southeastern Europe and southern Asia. And, as one would expect, the agricultural crops and practices of these North American regions parallel those of the climatically similar Old World regions.

The implication seems clear and is held as conventional wisdom, namely that most of our North American weeds originated in close proximity to human activities, particularly to those of early agriculturalists. As a sedentary agricultural lifestyle developed, and plants and animals were domesticated, the continual disturbance created by humans provided a reliable niche in which colonizing species evolved and found a permanent home. It is difficult to prove this simple explanation of the origin of weeds, but it fits the information at hand.


The most troublesome and aggressive weeds are those foreign or alien species that have invaded the North American continent from regions elsewhere in the world. By comparison, fewer and less aggressive weeds are native species. Analysis of the geographical components of a large number of weeds usually shows over 60% to be foreign species. The distinction between foreign species and native species is not always clear, and it is not easy to measure the impact of those foreign or alien plants on the native vegetation. Several factors contribute to this lack of precision.

Botanists assume that species have a "place of origin," where at some time the species are differentiated from the ancestral entities. As time passes, a newly formed species migrates into new areas and/or expands its range, through the routine mechanisms of seed dispersal, seedling establishment, and other factors. Undoubtedly, some botanical traffic has occurred between North America and other continents since antiquity, but clearly colonization following Columbus's voyages to America initiated a significant number of invasions. Some of the historical aspects of plant migration at the hands of humankind are reviewed by V.Muhlenbach (1979).

Foreign or alien species are usually regarded as those that have been brought to North America by human activities in post-Columbian times, while native species either originated in North America or had arrived by various means in pre-Columbian times. Although botanists frequently use the term "introduced" for these foreign or alien species, in this chapter the term has a more restricted meaning and refers to those species deliberately brought by people into a new region, where the plants grow without cultivation (M.L. Fernald 1950, p. viii; G. H. M. Lawrence 1951, p.279). How many species have been transported from their putative places of nativity to North America in post-Columbian times is, of course, unknown. The historical documentation for these plant movements is often not well known or not yet researched, and many times what is known is based on circumstance and inference. A species is regarded as foreign or introduced if the following conditions apply:

1. The species in question has been known and specimens have been deposited in herbaria repeatedly from areas elsewhere in the world, where it is presumed to be native.

2. The species was not known in North America to the earliest naturalists and botanists who maintained herbaria, and/or it was known only as a recent immigrant.

3. The species has become known and was seen or obtained from its new location only after a given date (usually in an approximate decade), when it presumably arrived in the new area.

Herbarium records are the reliable sources of data on plant invasions and establishment of these foreign species or weeds. These records are by no means complete, for a species may be present in an area for a while before naturalists, botanists, or agriculturalists notice it and collect specimens. The time-lag, however, between the invasion of a species and its collection from the field is not likely to be great. Detective work in the herbarium can provide information as to when a plant had invaded or was introduced, how rapidly it spread into new regions, and in some cases, when it began either to migrate and expand or to decline in abundance or range, or both.

The invasion and spread of several foreign species, for example, have been documented by herbarium sources in papers by R.L. Stuckey and collaborators. Examples are the invasion and establishment of the alien species, Epilobium hirsutum, great-hairy-willow-herb; Lycopus europaeus, European water horehound; Lythrum salicaria, purple loosestrife; Potamogeton crispus, curly pondweed; Rorippa sylvestris, creeping yellow cress; and Veronica beccabunga, European brooklime. These European species all entered seaport cities along the east coast of North America and gradually moved from east to west across the continent. Lythrum salicara is the most prominent aggressive wetland weed known in the northern United States and sometimes Canada (R. L. Stuckey 1980). This pattern of invasion is also illustrated by Veronica beccabunga (fig. 8.1; D.H. Les and R.L. Stuckey 1985; see for references to other studies).

Various estimates and counts are available for the numbers of foreign species that have invaded North America and that now persist outside of intentional cultivation. Many, but not all, of these alien species persist as ruderal or agrestal weeds.

In Canada and the northeastern United States, the percentage of foreign species ranges from 20% to 30% (table8.2). In the United States, those states in the northeast that have been occupied the longest by European colonists have percentages near or over 30%, the highest percentage known being 36% from the state of New York. A percentage range of between 20% and 30% occurs in those midwestern states most extensively involved in agriculture, with outlying, more northern and western states having values below 20%. These noted variations reflect the history of migration and settlement of the European people as they moved westward across the North American continent, and of the agricultural, industrial, and recreational practices that have been developed since then.

TABLE 8.2. Percentage Comparison of Numbers of Foreign Species to Total Numbers of Species since 1950 for Canada, Northeastern United States, and Various Other States of the United States

Geographic Unit Total Number Species Number Foreign Species Percentage Reference
Canada 4153 884 27 H.J. Scoggan 1978-1979
Canada 3990 930 23 B. Boivin 1968
Northeastern United States and Southeastern 5523 1098 20 M.L. Fernald 1950
New England 2882 877 30.4 F.C. Seymour 1979
States and Provinces
New York 3022 1082 36 R.S. Mitchell 1986
Maine 2137 699 32.6 R.C. Bean et al. 1966
Illinois 3100 871 28.7 R.D. Henry, A.R. Scott 1980
Vermont 1927 533 27.6 F.C. Seymour 1969
Ohio 2518 599 23.8 C. Weishaupt 1971
Missouri 2438 557 23 J. Steyermark 1963
Iowa 1943 434 22.3 L. Eilers, pers. comm.
Minnesota 1618 392 19.5 G.G. Ownbey, T. Morley 1991
California 6400 1160 18 M. Rejmanek et al. 1991
Kansas 1872 326 17.4 F.C. Gates 1940
Colorado 3091 492 16 A. Weber, pers. comm.
Manitoba 1224 193 16 R.D. Henry, A.R. Scott 1980
North Dakota 1143 171 15 O.A. Stevens 1950
Washington, se, and adjacent Idaho --- --- 14 R.D Henry, A.R. Scott 1980
Arizona 3438 231 7 R.D Henry, A.R. Scott 1980

A reference for foreign species of continental scope is the National List of Scientific Plant Names (NLSPN), which was prepared by the Soil Conservation Service of the U.S. Department of Agriculture (1982). This publication is a compilation from many sources of those taxa that occur in North America north of Mexico (including Greenland), with the nomenclature brought into accord with current usage. The NLSPN serves the useful purpose of offering Latin names and relevant synonymies for governmental publications. The compilers of the NLSPN have noted each taxon as native or foreign to the flora of North America, and the sources of their information are cited.


Foreign species are among the most conspicuous and abundant species in places that are heavily affected by the workings of humankind. In the farming regions of the American heartland, the crops are nearly all derived from elsewhere, an exception being the native sunflowers, which are grown for their seeds. The agrestal weeds often come from the places of development of the crops, and the roadside and waste-ground ruderal weeds are mostly of Old World origin. For example, in certain regions the various species of Bromus and other grasses may form the bulk of ruderal biomass. Moreover, most of the ornamental species in parks and gardens are alien, e.g., lawn grasses, rose bushes, lilacs. Therefore, with as many as one-fifth to one-third of the species in the flora foreign, they dominate the visual impact of the flora in much of middle North America. In other words, the showiest and most conspicuous plants in the flora are frequently those whose evolutionary development occurred elsewhere in the world, but whose arrival in North America was instigated by the activities of humankind and whose presence in the flora is maintained by continuing human activities.

Completion of the Flora of North America will provide a database that includes, among other things, information on each taxon's distribution, habitat preference, and provenance. For all taxa, and particularly for weeds, this database will change as more information is accumulated. The ranges for many weeds will expand (or contract). Through continuing analysis of herbarium materials and field studies, we will gain new insights on the provenance of weeds and their role in the natural system.