786 WEED FLORA OF IOWA 



Many plants, either annual or perennial, which have a recog- 

 nized weedy character, have a prolonged period of bloom and seed- 

 ing, thus maintaining a vigorous hold. 



Common cosmopolitan weeds, belonging to various orders, have 

 often remarkable perennation, as in case of the dandelion, shep- 

 herd's purse and chickweed; these weeds begin bloom in earliest 

 spring and are also more resistant to frost than are other weeds. 



The period of time required for maturing fruit after blooming 

 varies in different species; a comparatively short time is needed 

 in case of most weeds, especially the late blooming ones. 



The dormant period required for a seed before germination will 

 take place differs with varieties, and with individual seeds of the 

 same variety. In seeds requiring a period of rest, this delay may 

 be a matter of days, weeks, or years; therefore there is a distribu- 

 tion for seeds through time as well as through space. Many seeds 

 germinate immediately if conditions are favorable. Many young 

 plants must be lost by this autumnal vegetating, in case of seed- 

 lings overtaken by frost before they can bear seeds or establish 

 roots; but the hardy habit of the winter annual, the biennial and 

 perennial, protects for the most part such young plants after fall 

 germination. 



Quack grass has a crop of seedlings of the same season as the 

 ripening of seed ; this is true also of wild carrot, burdock, thistles, 

 horse nettle and ribgrass, all of which being biennial or perennial 

 persist throughout the winter. 



The freezing and thawing of winter facilitates germination of 

 weed seeds in the spring ; and very early one may note, in the loca- 

 tion of the parent of last season, as soon as the soil grows warm 

 enough, many flourishing young colonies of seedlings, such as 

 spurge, oxalis, pigweed and smartweed crowding each other for 

 foothold. The culturist is greatly influenced by considerations of 

 blooming-time, seed-time, and time of seed-germination in his ef- 

 forts to control and to exterminate weeds. 



