42 BULLETIlSr 428, U. S. DEPARTMENT OF AGEICULTURE. 



Among the factors influencing winterkilling may be mentioned low 

 temperature, excessive variation in temperature, protection as 

 afforded by snow and drifted soil, thickness of stand of plants, pre- 

 cipitation, vigor of plants upon entering the winter, and condition of 

 soil with respect to moisture, surface drainage, and existence of ice 

 sheets. 



Medicago falcata is generall}?^ regarded as a hardy species, and it 

 was for this reason that efforts were made to introduce as many of its 

 forms as possible from regions where it is found growing naturally. 

 While it scarcely could be expected that forms of the species from the 

 Mediterranean region would be as hardy as those from the vicinity 

 of Yakutsk, 62° north latitude, where the temperature reaches — 84° 

 F., it is probable that most of the forms can be placed in the class of 

 comparatively hardy alfalfas, regardless of the geographical location 

 in which they were developed. Just why some varieties of alfalfa 

 are more susceptible than others to winter conditions is not clearly 

 understood. It is now commonly believed, however, that an impor- 

 tant factor in the hardiness of any variety is the degree of protection 

 which the plant provides its dormant or resting buds. For example, 

 varieties in which the crowns are produced well above the surface of 

 the ground are uniformly tender, while those in which the crowns are 

 near or beneath the surface are mostly hardy. The Arabian and 

 Peruvian varieties may be cited as illustrations of the former, while 

 the Grimm and Baltic varieties represent the latter group. The 

 dormant buds of the varieties with high crowns are fully exposed to 

 the unfavorable conditions of winter, while those of the varieties 

 having low crowns are protected by the soil, and to some extent by 

 the dead herbage of the preceding summer. The relation of deeply 

 set crowns to hardiness probably was first called to attention in 

 agricultural literature by Thomas Le Blanc in 1791 (^^), but it is 

 only within recent years that the significance of this characteristic of 

 the crown has been duly appreciated {1^5^ p. 4). 



A majority of the forms of Medicago falcata produce at least a 

 part of their new growth from underground members, either rhizomes 

 or true lateral roots, a characteristic which affords material protec- 

 tion during periods of severe conditions. It is largely upon this 

 feature, as well as upon its geographical range, that the estimate of 

 its hardiness has been based. 



While comparative tests of the hardiness of Medicago falcata and 

 strains of Medicago sativa grown in rows and hills have been made 

 since 1910, there are still insufficient critical data from which to draw 

 definite conclusions as to the relative hardiness of the former under 

 actual field conditions. Unfortunately, many of the plantings that 

 were made in 1909 were poorly provided with checks and did not 



