›What is a weed? Although we all have a general concept as to what a weed is, it is sometimes hard to determine whether a particular plant is a weed or not. Some crop plants, for example, can become weeds when they appear where they are not wanted. On the other hand, a number of plants usually thought of as weeds may actually be useful under some conditions or in some areas. They may help to control soil erosion or may serve as foods for wild animals and birds. Sometimes certain weeds are used as forages for farm animals.
›With these points in mind, we can define a weed as a plant not intentionally sown, whose undesirable qualities outweigh its good points. This definition eliminates the many plants--often native--that grow uncultivated in every locality but seldom have weedy tendencies. They are not aggressive enough to be troublesome in cropland or pastures. Since they do not interfere with agricultural production they should be allowed to grow undisturbed. In fact, many of these plants have such colorful flowers and interesting habits that they are well worth preserving.
›How weeds are troublesome. Weeds reduce crop yields by depriving the crops of the water, light, and soil nutrients they need. Weeds may also produce allelopathic substances that are toxic to crop plants. The work of controlling weeds in crops and pastures increases production costs. If weeds are present at crop maturity, they may cause harvesting problems.
›Weeds often serve as hosts for crop diseases, and they may provide a place for insects attacking crop plants to overwinter. Some weeds detract from the quality of crops and of animal products; wild garlic, for example, reduces the value of wheat and taints the milk of cows that graze it.
›The harmful effects of some common weeds on the health of animals and people are well known. Farm animals become ill and sometimes die from eating poisonous weeds. Thousands of people who suffer from hay fever can attest to the annoyance caused by the pollen from many plants, especially ragweed.
Control is difficult. The seeds of many weeds remain dormant in the soil for years and then germinate when conditions are favorable. Some weeds have extensive root systems and underground stems that help the weed to spread and persist. Perennial weeds store reserve foods in their root systems and continue to sprout again and again after the tops are destroyed. These characteristics of weeds make control a problem.
Even though control is often difficult, the proper selection and use of herbicides can give excellent results with most of the common weeds. For best results, however, it is necessary to identify the weed problem correctly before selecting herbicides. Many herbicides are rather specific in their action and may control only one or a few species of weeds. The same thing is often true of other control measures. If control measures are applied haphazardly, they may not only fail to control weeds, but they may also injure the crop.
›Recommendations for the control of weeds vary from locality to locality, and may change fairly rapidly as new herbicides and new combinations become available. Thus no attempt is made in this publication to describe control measures. Precise, up-to-date recommendations for your situation are best obtained locally. This type of information is available from agricultural experiment stations, county agents or extension advisors, agricultural teachers, and other agricultural leaders.
How to identify weeds. A particular weed may be identified in several ways. Always, however, you need a reasonably good specimen. Identification will be easier if you have the entire plant with leaves, roots, and flowers. Fresh plants are always more easily identified than wilted or dried ones.
›If you believe the unidentified weed belongs to a particular family, such as the grass or mustard family, you have a head start in identifying it. Since plants are arranged by families in this bulletin, you can compare your plant with the different illustrations and descriptions in the family until you find what you're looking for. Check the flower color, flower shape, leaf arrangement and shape, type of growth, and other characteristics, to make sure the identification is correct.
›Sometimes several closely related weeds are grouped together on a page, with only one being described in detail. If the weed you are trying to identify is in such a group, it may not correspond exactly with the one described. It may differ in size, leaf shape, leaf color, and other minor characteristics. However, the description usually mentions if these related forms of a weed are likely to be found.
›Many plants that are occasionally found in the region are not illustrated in this publication. These may be identified by consulting local or state authorities or botanical manuals.
›Maps show distribution. With each weed illustration there is
a small outline map that indicates the general prevalence of the weed.
The maps are based on a study of herbarium samples and, perhaps more importantly,
on the opinions of weed scientists from throughout the region. Changes
are required from time to time as species advance to new areas or decline
in others as the result of changes in cropping practices, tillage methods,
and herbicide use. The following illustration gives the significance of
the shadings used on the maps.
›Scientific names of plants are more uniform than common names, but they sometimes vary according to the authority. Moreover, the names of some plants may be changed as the result of continuing taxonomic studies. In general, the scientific names used in this book follow those used in the Composite List of Weeds mentioned above, as well as in the following publications.
›Manual of the Grasses of the United States, by A. S. Hitchcock, revised by Agnes Chase. U.S. Department of Agriculture, Miscellaneous Publication 200. 1950.
›Gray's Manual of Botany, eighth edition, revised by M. L. Fernald. American Book Company. 1950.
›Manual of Vascular Plants of Northeastern United States and Adjacent Canada, by H. A. Gleason and A. Cronquist. D. Van Nostrand Co., Inc. 1963.
›Atlas of the Flora of the Great Plains, edited by T. M. Barkley. Iowa State University Press. 1977.
›In some instances, if authorities differed or if recent taxonomic studies indicated the need for a changed name, decisions were made to use different scientific names than those given in previous editions of Weeds of the North Central States.
Suggestions will be welcome. The subcommittee responsible for
this publication would appreciate comments and suggestions, so that they
may be considered for future editions. New weed problems are continuously
developing in the region while others seem to diminish. Thus information
about needed changes in distribution maps would be especially helpful.
We would also welcome suggestions as to possible changes in names of plants.