230 Science of Plant Life 



In the ordinary sense the term " weed " applies to any un- 

 desirable plant that reproduces abundantly and that adjusts 

 itself to diverse habitats. In its broadest sense the term is 

 applied to any plant growing out of place. Rye may become 

 a weed in wheatfields. Red clover is very desirable in a field 

 on the farm, but it becomes a weed when it springs up in a 

 lawn. The most pernicious weeds, like the dandehon, cockle 

 bur, Canada thistle, bindweed, and sand bur, are not desir- 

 able plants anywhere. 



Weeds interfere with crop production. Weeds are un- 

 desirable and harmful, (i) because they use soil water needed 

 by crop plants, (2) because they interfere with the growth of 

 crops by shading them and by occupying the land, (3) be- 

 cause some weeds promote the spread of diseases and injurious 

 insects, and (4) because some of the species found in pastures 

 and grazing lands are poisonous, or harmful to cattle, or 

 spoil the quality of milk, butter, and cheese by their persistent 

 and unpleasant flavors. It is estimated that the average 

 American farmer loses annually one dollar on every acre of 

 his land through the damage done by weeds. 



Weeds spread rapidly. How rapidly a weed may spread 

 is illustrated by the history of the Russian thistle. It was 

 introduced into South Dakota in imported flaxseed, in 1874, 

 and it had become a common weed by 1888. In 1894 it had 

 spread as far as Chicago. In 1900 it was reported in all the 

 states and provinces east of the Rockies, from the Gulf of 

 Mexico and Atlantic Ocean to Saskatchewan. 



The control of weeds. In crops that are cultivated at 

 intervals, most weeds can be controlled by turning under the 

 seedlings as fast as they develop. In fields of wheat and 

 other small grains the problem of weed control is more dif- 



