The farmer himself often introduces weeds into his cultivated fields. This occurs through the seeds of weeds that are mixed with grass seed and grain that he has bought for planting. In using hay for packing purposes, seeds are often shipped great distances to find their way into foreign soil. Animals, inadvertently carry seeds on their fur. Seeds are carried in the intestinal tracts of animals and deposited intact in new areas through their manure. Stones and gravel used for ballast in the holds of ships have often been a means of depositing in the United States foreign weeds from across the oceans. Farm tools are frequently vehicles for carrying seeds from one locality to another. Other methods by which seeds of obnoxious weeds are transferred from one place to another include railroad trains, automobiles, airplanes, wind, water and birds.
The interesting aspect about the spread of weeds is the ironical manner in which the civilizing devices of man has served to spread and propagate these undesirable plants over the face of the earth. Formerly seeds relied upon wind, water, and animals for movement, but now they travel in modern fashion by train, automobile, boat and airplane. These vehicles make it much more difficult to confine or eradicate our pernicious weeds.
Weeds may be compared with our undesirable elements in human society. Whenever the weeds are considered, it is with a view toward correction of some one or more of their wrong doings. When they really get out of hand they make front page news. Like criminals they achieve notoriety and world wide recognition only when they become a serious menace to society. Such distinction has been achieved by the entire family of hay fever weeds.