DAMAGE OF WEEDS 495 



their seed, and die. Like annuals, they spread only from 

 seed. The bull thistle and burdock are famihar examples of 

 biennials, as are also cabbage, turnips, beets, and a number 

 of other garden vegetables. 



681. Perennials. These are plants which may live an 

 indefinite number of years. This class includes all our trees 

 and shrubs, many ornamental plants, and such garden vege- 

 tables as rhubarb and asparagus. Many of our worst weeds 

 are perennials. Some perennials spread only from their 

 seeds; others have running rootstocks or underground stems 

 which grow from year to year and new plants may spring 

 up from them. Some spread by means of both seeds and 

 running rootstocks. Perennials which spread only by seeds 

 include the dandelion, docks, and plantains. The Canada 

 thistle in many sections spreads only by its rootstocks and 

 does not produce seeds; elsewhere it seeds abundantly. 

 Other weeds which spread by both means are Johnson grass, 

 quack grass, sow thistle, and ox-eye daisy. 



THE DAMAGE DONE BY WEEDS 



682. Weeds Lower Crop Yields. Weeds occupy space 

 which is needed by crops, thus crowding them out and shad- 

 ing them. It is easy to see that an acre of wheat will yield 

 less when Canada thistle or cockle or kinghead are growing 

 in it than when the wheat occupies all the land. In the 

 same way, Johnson grass reduces the yield of cotton, and 

 weeds of many kinds prevent corn from making a full crop. 

 The greatest damage is often done early in the season, by 

 shading and stunting the crop plants before they get well 

 started. Weeds take plant food which is needed by crops. 

 It is next to useless to apply manure or fertilizer to land and 

 then allow weeds to use it. Weeds also take moisture from 

 the soil at a time when crops need it most. 



Weeds lower crop yields by harboring insects and diseases. 

 In some cases, weeds are infested with the same diseases as 



