|The greater plantain, but the Algonquin called it the White Man's Foot or the Englishman's foot because it appeared everywhere the Europeans went.|
Alfred W.Crosby's 'Ecological Imperialism, the biological expansion of Europe, 900-1900 ' (Cambridge University Press, 1986) reads as if Jared Diamond's uncle Fester is on a roll.
Crosby explains how weeds from the old world prepared the ground of the new world for European colonisers and were critical to the success of European expansion.
"Weeds" is not a scientific word. It refers not to plants of any specific species or genus or any category recognized by scientific taxonomy, but to whatever plants spring up where humans do not want them. More often than not they are plants that evolved originally to fill the minor role of colonizing bare ground after fires, landslides, floods, and such and that found themselves wonderfully preadapted to spread across the expanses stripped clean by the Neolithic farmer's plow or sickle. Already tolerant of direct sunlight and disturbed soil, they added tolerance of sandal, boot, and hoof. Always ready to spring up fast in the wake of disasters, they easily evolved to survive and sprout again in the wake of the tug, tear, and chomp of grazing livestock. The farmer calls them the bane of his life, and they are, but they also provide livestock with feed and help combat erosion.
The Neolithic farmer simplified his ecosystem to produce quantities of plants that would grow rapidly on bare ground and would survive grazing animals, and he got exactly what he tried for, but some of them he cursed: tufted vetch, ryegrass, cleavers, thistles, coriander, and others.
Those plants with a talent for finding and settling on disturbed grounds rose to the challenge when agriculture first emerged. Evolving with agriculture these plants found better and better ways to use the opportunities provided by agriculture's annual cycle of soil disturbance (the decimation following the plough and the harvest). When seafaring began these plants had become especially well adapted to conquer other lands, easily outcompeting local plants that lacked previous experience with agricultures impact on the land.
In other words: agriculture not just created the possibility to think of a plant as a weed, it also created conditions for plants to acquire the persistent characteristics of weeds. Crosby goes on to cite from the Proverbs in the Old Testament:
I went by the field of the slothful, and by the vineyard of the man void of understanding;
And, lo, it was all grown over with thorns, and weeds had covered its face, and its stone wall was broken down.So shall your poverty come like a robber; and your want like an armed man.
There you got the agricultural obsession with strive, purity and ecocide in the language of King James: the damnation of weed as a moral wrong.