CRIMSON CLOVER AND OTHER ANNUALS 437 



greater cold, more even than red clover. Mixtures with 

 crimson clover are very satisfactory, but probably do not 

 increase the total yield. Under like conditions, yellow 

 trefoil will probably not yield more than three-fourths 

 as much as crimson clover, but with its wider range of 

 adaptation and cheap seed should fill a niche in American 

 agriculture. Difficulty in establishing trefoil may be ex- 

 pected until the ground has become inoculated for it. 



Werner gives the average seed yield as 440 to 700 

 pounds an acre. The commercial supply has been scarce 

 in recent years, perhaps because the practice of using it 

 as an adulterant of alfalfa has greatly diminished. 



BUR CLOVERS (Medicago spp.) 



528. Bur clovers. There are about 40 species of these 

 plants native to the countries about the Mediterranean Sea. 

 Most of these, probably all, are annuals, springing up in 

 the fall, and maturing in early summer. They are all 

 procumbent or prostrate plants when growing isolated, 

 but if planted thickly, make a mass of herbage 8 to 18 

 inches deep. The species are distinguished largely by 

 the burs or pods, which show a wide variation in size and 

 form. 



In America two species have thus far become used in 

 agriculture ; namely, the toothed bur clover (Medicago 

 hispida) and the spotted bur clover (Medicago arabica), 

 the former especially in California, the latter mainly in 

 the Southern States. 



Among the other species that are likely to become of 

 importance are button clover (Medicago orbicularis) and 

 snail clover (Medicago scutellata), both with large smooth 

 pods. 



Toothed bur clover. Toothed bur clover is also known 



