ISU's student newspaper recently published a story on Dr. Ada Hayden, the first woman to earn a Ph.D. from ISU (1918). While preserving Iowa's disappearing prairie was her passion, she also documented the rapid spread of Canada thistle across Iowa. In a 1934 paper published in the American Journal of Botany, she made several statements that are especially fitting considering the current invasion of Palmer amaranth:
1) "Failure to destroy initial patches of introduced thistles . . . is accountable for unhampered spread of the thistle."
2) "The economic insecurity of agricultural pursuits from 1921 to 1932 which resulted in relaxed vigilance in farming operations has favored the occupation of new territory by Canada thistle."
3) "Introduction of Canada thistle into new communities appears to be incident primarily to industrial activities of man."
4) "The local dispersal of the plant seems to be the result largely of the efficient mechanism of the plumed fruits, combined with widespread failure of human agency to destroy the plants before fruiting occurs."
There are many parallels between the invasions of Canada thistle and Palmer amaranth. I hope we can look back in 30 years and say that we learned from the failures with Canada thistle, and that the spread of Palmer amaranth in Iowa was greatly impeded by our actions. Due to narrow margins, many farmers will be hesitant to go the extra mile needed to eradicate new introductions of Palmer amaranth, but in the long run those dollars could be one of the best investments made by a farm.
I prepared a longer article on the old weed science website that reviewed the history of Canada thistle in Iowa.
Reference: Hayden, A. 1934. Distribution and reproduction of Canada thistle in Iowa. Amer. J. Bot. 21:355-373.