Flood Prevention Until the 20th century, pioneers and farmers used cannabis to clear fields for planting, as a fallow year crop, and after forest fires to prevent mud slides and loss of watershed. Hemp seeds put down a 10 to 12 inch root put in 30 days, unlike the one inch root put down by the rye and barley grass presently used by the U.S. Government. Southern California, Utah, and other states routinely used cannabis until about 1915 to prevent floods. It also can be used to break up compact, overworked soil.