MARGINAL WEEDS 



Weeds that grow along the margins and banks of irrigation systems 

 are the most serious problem for irrigation operators, other than 

 submersed aquatic weeds. Marginal weeds include a wide variety of 

 grass and broadleaf species. They may also grow along lake and 

 reservoir shorelines. 



Dense stands of plants such as reed canarygrass and johnsongrass 

 restrict irrigation system carrying capacities by leaning over banks and 

 growing into the water, thereby, trapping sediment. During canal 

 drawdown, marginal weeds spread throughout the bottom making it 

 essential to remove them prior to irrigation. The extensive root 

 systems, stems, and leaves make these weeds difficult to control once 

 they invade an area. Many of the species are so large that waterflow 

 is restricted and tremendous quantities of water are wasted by 

 transpiration. They also obstruct canal maintenance, thereby 

 increasing costs to water users. 



Four common marginal plants found in the water systems of the 

 western United States are explained in this section. For more 

 information, consult the bibliography on page 89. 



63 



