NEW WEEDS INTRODUCED 151 



interferes with the growing of the crop in which we are inter- 

 ested. It may be a valuable crop if grown separately, but 

 we do not want it to interfere with the crop under cultivation. 

 The weed may be an excellent crop in one part of the world and 

 a pest in another part. The value of the plant depends upon 

 the use we can make of it and the ease and profit with which 

 it can be grown. 



How Weeds are Injurious. Weeds are injurious for many 

 reasons. (1) They take a certain amount of the food and 

 water which the crop plant should have for its growth. The 

 farmer cannot afford to have his crop plants weakened and the 

 amount of the harvest reduced by the weeds and he cannot 

 afford to buy fertilizers to feed useless plants. (2) Weeds may 

 prove injurious to the crop by shading and if too numerous 

 may completely choke out the crop plants. (3) The weeds 

 may harbor certain insects and fungi which attack the crops. 

 (4) Thistles, nettles, and other thorny or prickly plants may 

 interfere with using the growing crop for pasture or interfere 

 with tillage. (5) Weeds in the meadow reduce the feed value 

 of hay. ( 6 ) The presence of weed seeds in the harvest crop may 

 reduce its value. (Figs. 93 and 94.) 



Weeds Sometimes Useful. It is sometimes said that weeds 

 are useful, especially on unused lands, because they serve as a 

 cover and prevent the escape of nitrogen and can be plowed 

 under and thus used as a fertilizer. This is an old idea that has 

 come to us from the time when the land was rested by being 

 left uncultivated for a period of one or more years. But we 

 now know that in most cases it is much better to use a good 

 agricultural cover crop. Rye, certain clovers, vetches and other 

 plants are much more valuable than a miscellaneous lot of 

 weeds. 



'New Weeds Introduced. When a new country is settled 

 and brought under cultivation we find certain native weeds, 



