The Effects of Fresh Water Storage on the Germination of Certain Weed Seeds: I. White Top, Russian Knapweed, Canada Thistle, Morning Glory, and Poverty Weed

Abstract

ATER, especially water used for irrigation, has been adjudged for vIe many years an important agent in the dissemination of weed seeds. Egginton and Robbins (7) found a total of 81 different species of weeds in 156 weed seed catches from three different ditches during 1918 and 1919 and determined that the number of weed seeds passing a given point on a ditch which averaged 12 feet in width during a period of 24 hours may reach several millions. Hope (11) analyzed a considerable number of weed seed catches from irrigation channels during 1924 and 1925. He computed that 1,674,030 seeds of 13 different weed species which occurred on the surface and to a depth of 1.57 inches in one 10-foot ditch passed during a 24-hour period on September 2, 1925. Further computations illustrated that the flow of weed seeds would amount to a deposition of 170,800 seeds per acre of land in one 6-inch irrigation. The latter computations were based on the assumption that the seeds were distributed uniformly over the cross section of the ditch at the point of catch. The extent to which weed seeds are carried to farm lands by irrigation water depends, in part, on the degree of buoyancy of such seeds. Egginton and Robbins (7) secured data which showed that weed seeds differ in the readiness with which they sink or float. The buoyancy is influenced somewhat by the condition of the water surface and by the manner in which the seeds alight upon it. Some seeds float for days no matter how they strike the water surface or how it is agitated; some float if laid on the water carefully and the surface is not disturbed, but they sink readily if the surface is agitated, or sink almost immediately if they strike the surface with sufficient force. Bruns (4) dropped 100 seeds each of morning glory (Convolvulus arvensis L.), Russian knapweed (Centaurea repens L.), white top (Cardaria draba (L.), Desv. var. repens (Schrenk) 0. E. Shulz), Canada thistle (Cirsium arvense L.), and poverty weed (Iva axillaris Pursh.) into separate 11/8-inch columns of still water 48 inches deep. Agitation to break the surface tension was supplied by vigorous stirring with a glass rod for short periods each day. This test indicated that the weed seeds were relatively buoyant and might be transported considerable distances by irrigation water.