712 



WEED FLORA OF IOWA 



have been introduced into Iowa such weeds as wild carrot {Daucus 

 ca/rota) , Silene noctiflora, S. dichotoma, Gichorium intylms, Conium 

 maculaiwm, Centcmrea solstitialis, Cirsium arvense, C. pratense, 

 Cuscuta arvensis, G. epithymum, Eruca sativa, and others, largely 

 with clover and alfalfa seed. Still another and frequent source 

 of the introduction of weeds is through the cultivation of orna- 

 mental plants which become weedy as : shoo-fly {Hibiscus trionwm) 

 during recent years, the bouncing bet (Saponaria officinalis L.), 

 toad flax (Linaria vulgaris), Euphorbia cypairissias, live forever 

 {Sedum telephium) , snow-on-the-mountain (Euphorbia marginata) 

 in many localities, pickerel weed {Eichornia crassipes) in Florida, 

 water chestnut {Trapa natans) in central New England. The or- 

 ange hawkweed (Hieracium aurantiacum) , and others of this class 

 are excellent illustrations of how ornamental plants become weedy. 

 In some cases plants cultivated for food have become weedy, as 

 in Iowa, the horse radish; in California, the beet (San Francisco 

 Bay region) ; and in Utah and California, the spinach. 



Fio. 546. Snow-on-the-Mountain (Euphorbia marginata). A western weed now 

 common in some places in the east. 

 (After Chesnut, U. S. Dept. Agr.) 



