224 



WEED FLORA OF IOWA 



Fig. 128-A. Distribution of St. John's-wort. 



each plant after close cutting in hot dry weather — will kill it and 

 may be practicable where the pest is not abundant and the land 

 cannot be brought under cultivation. Prevent it from going to seed. 

 St. John's-wort is easily suppressed on land that can be cultivated 

 under a systematic rotation of crops. Where it is established, it 

 would be well not to seed to grass until it is suppressed. ' ' 



ONAGRACBAE. EVENING PRIMROSE FAMILY. 



A small family, some plants with showy flowers, a few cultivated 

 for ornamental purposes. 



Evening Primrose {Oenothera biennis L.). 



Description. — A stout, erect, pubescent or hirsute perennial, 3-5 

 feet high, sparingly branched; leaves lanceolate, or rarely ovate- 

 lanceolate, denticulate, acute, bracts shorter or as hmg as the cap- 

 sule ; flowers yellow, petals obovate, stigma lobes linear, capsule sub- 

 cylindrical; seeds small, brownish. 



Distribution. — Common everywhere in eastern North America, 

 Rocky mountains and Utah. Occurs in every county in Iowa. 



Extermination. — Spreads by seed. This plant is not difficult to 

 exterminate. Cut off the young plants a few inches below the sur- 

 face of the ground. 



UMBELLIPERAE, CARROT FAMILY. 



Carrot, celery, parsnips and caraway are members of this family. 

 It includes also many poisonous plants. 



