There are in Utah a number of weeds considered to be unusually persistent and difficult to eradicate once they have become established. The most common in this group are wild morning glory, whitetop, Canada thistle, perennial sow thistle, and Russian knapweed, the last being one of comparatively recent introduction. The weeds in this group are quite generally feared and much talked of by farmers. They are feared because, if allowed to persist, they usually take possession of the soil and hinder the growth of farm crops. The discouraging fact about these weeds is their spread. Old infested areas are increasing in size, and each year new lands are becoming infested. In time practically every farmer in Utah will be fighting one or more if these weeds unless some vigorous· measures are taken to keep them in check.
Peterson, William and Tingey, D. C., "Circular No. 71 - Weeds: Losses, Agencies in Distribution, Nature, and Control" (1928). UAES Circulars. Paper 60.