Irrigation Water 
19 
As a rule there is a greater nmn))er of weed species on a 
new ditch hank tlian on an old one. As vegetative develop- 
ment proceeds there is a reduction in the number of species 
and an increase in the number of individuals of the successful 
species. 
Fig-. 3 — An irrigation ditch heavily overgrown with tall weeds. 
One may find on a section of an old ditch bank almost a 
pure stand of one or two weeds such as sweet clover (MeliJo- 
tus alba), tall ragweed (A mbrosia trifida), Rocky Mountain bee- 
plant (Cleome serrulata), tall marsh elder (Iva xanthifoUa), and 
fire-ball (Kochia scoparia). The above species often form rather 
pure stands along ditches. A few species of sedges, chiefly 
Carex nebraskensis, and the grasses, such as Fanlcularia grandis, 
become established at the water’s edge along old ditches, and 
occupy the soil to the exclusion of other plants. These two 
species are not weeds in fields and their growth along ditch 
banks is to be encouraged. Furthermore, the grasses and 
sedges are excellent soil binders. 
Ordinarily, there is little opportunity for a ditch bank to 
pass through a normal vegetative development. Almost an- 
nually there is disturbance of the hal)itat by the washing of 
water, by the cleaning of tlie ditch, and by the mowing of the 
