CHAPTER XL 



PHENOLOGY OF WEEDS. 



Several species of weedy plants bloom very early in the spring, 

 responding promptly to a comparatively limited amount of light 

 and warmth; others come to bloom after the growing season is 

 more advanced ; while a number blossom throughout the season, or 

 from spring until fall. 



The time of bloom is, in each species, related to its definite 

 physiological constant of warmth, sunshine and moisture. "Warm, 

 sunshiny weather in the early part of the season will hasten the 

 time of bloom, as cold, cloudy weather will retard it. There is a 

 difference of about two weeks between the blooming date of plants 

 of northern and southern parts of Iowa. 



The season of a plant's activity is related in part to the hardiness 

 it has developed, and to the climatic conditions of the geographical 

 region selected by it, as most favorable to its existence. 



It is vigor and tenacity as well as wide seed distribution which 

 contribute to the troublesome character of the plants which we call 

 weeds. 



It is indicated by the accompanying table, as known also by com- 

 mon experience, that the rain, warmth and sunshine received at 

 the arrival of midsummer produce greatest bloom of weedy plants 

 for the season in both number and kinds ; and since weeds quickly 

 mature, it is apparent that the harvest of weed seeds steadily in- 

 creases from that time until fall. 



The crucifers are among our earlier blooming weeds, as the 

 winter annuals, shepherd's purse and peppergrass; the legumes 

 and umbellifers are at their full growth about midsummer; the 

 plantains begin in May, the polygonums in June; whilst com- 

 posites are usually blooming throughout the latter part of the 

 season. 



Grasses distribute their blossoming-time throughout the summer 

 from May until October; vanilla grass and blue grass appear in 

 May, orchard grass, quack grass, needle grass and timothy in June, 

 crab grass, fescue grass, and rye grasses about July, Bouteloua in 

 August, blue-stem and drop-seed grasses in September. 

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