WEED MIGRATION 



691 



in a few places where tillage was not good; but in nearly every 

 case these perennial weeds disappeared from the cultivated fields 

 which thereupon became occupied by a large number of native 

 annual weeds like the greater ragweed (Ambrosia trifida), Spanish 

 needle (Bidens frondosa, B. discoidea) and smartweed (Polygonum 

 sp.), a few hardy perennial weeds like milkweed (Asclepias 

 syriaca), morning-glory (Convolvulus septum), artichoke (Helian- 

 thus tuberosus) and meadow sunflower (Helianthus grosseserratus) . 

 Such annual or winter annual weeds as squirrel-tail grass (Hordeum 

 jubatum), peppergrass (Lepidium apetalum) began to compete 

 with a host of European and other exotic weeds like lamb's quarters 

 (Chenopodium album), persicaria (Polygonum persicaria) , purslane 

 (Portidaca oleracea), foxtail (Setaria glauca, 8. viridis) and, later, 

 crab grass (Digitaria sanguinalis and D. hwmifusa). In grain 



Fig. 533. Greater Ragweed {Ambrosia trifida). This weed rapidly occupies 



cultivated ground. 

 (Vasey, U. S. Dept. Agr.) 



