960 



COMPOSITAE 



Ambrosia 



o 5o 



Map 2101 



Iva 



ala Willd. 



9146. AMBROSIA [Tourn.] L. Ragweed 



Staminate heads sessile; leaves not divided, mostly less than 1 cm wide; fruit 4-angled, 



each angle ending in a short prickle 1. A. bidentata. 



Staminate heads on short pedicels; leaves 1-2-pinnatifid, 3-5-lobed or, if undivided, more 

 than 1 cm wide. 



Leaves 3-5-lobed or undivided 2. A. trifida. 



Leaves pinnatifid or bipinnatifid. 



Stem leaves petiolate; fruit with 5-7 short prickles about 0.5 mm long, the beak 



generally 1-1.5 mm long; annual, branches widely spreading 



3. A. elatior. 



Stem leaves sessile or with short petioles; fruit without prickles or with 1-4 very 

 short ones less than 0.5 mm long, the beak generally about 0.5 mm long; 

 perennial, branches ascending, more compact 4. A. coronopifolia. 



1. Ambrosia bidentata Michx. Lanceleaf Ragweed. Map 2103. Gen- 

 erally found in hard, white clay soil in low land in fallow fields, in open ■ 

 woodland, and along roadsides. Pioneers have told me that they did not 

 note this species until the past ten years. Blatchley reports that it was 

 first noted in 1895. Schneck in 1876 reports it as "common in prairies." 

 This western species is slowly migrating eastward. Where it is found, it 

 usually forms dense stands. I was told by a farmer that stock will not 

 eat it, although they will eat other species of ragweed. It is restricted to 

 the southwestern part of the state. There are records of its occurrence 

 in Clay and Vigo Counties. 



Ind. to Nebr., south w. to La. and Tex. 



2. Ambrosia trifida L. Great Ragweed. Map 2104. This ragweed, 

 without doubt, occurs in every county of the state. It is usually abundant 

 in the alluvial bottoms of streams. Found in low, open places in cultivated 

 and fallow fields and woodland. It grows to giant size and J. M. Coulter 

 records measuring a specimen 18 feet high. A form with entire leaves is 

 often found growing with the species. I believe that it is a depauperate 

 form of the species and it has no taxonomic significance. 



Que. to Man., southw. to Fla. and N. Mex. 



