Observations on the ecology of noxious weeds on Ganga River banks at Varanasi, India.
Ganga river bank at Varanasi consisting of sandy fresh alluvium is subjected to natural stresses of flooding, silting and erosion in the rainy season (July to October), agricultural practices in winter (November to February) and dry hot scorching sun in summer (March to June). Of the twenty-five plant species, mostly annual weeds and a few perennial herbs, recorded from the area the more characteristic ones have been ecologically studied for the mechanism of their perennation and propagation under such an unstable and stressed habitat and their effectiveness in binding the soil against erosive forces. Alhagi camelorum [= A. pseudalhagi], a small leguminous weed, survives through a network of very deep subterranean stems and roots full of vegetative buds from which new leafy shoots emerge, the more so, when exposed and/or injured due to ploughing or mechanical cultivation. In summer, when the soil is very dry and loose and no crop cultivation is possible, its dense cover (exploiting moisture from deeper soil layers) protects the soil surface from strong winds. During the rains its aerial portion completely dies and grasses such as Saccharum benghalense and Cynodon dactylon, sedges such as Cyperus rotundus, and dicotyledons such as Xanthium strumarium survive prolonged inundation through the formation of new roots full of air spaces as the old ones succumb to submergence and poor aeration. The first named species binds the soil against erosion and experiments show its conservation value to exceed 90%.