332 FORAGE PLANTS AND THEIR CULTURE 



probably provides a somewhat higher soil temperature, 

 and also reduces the percentage of soil moisture. 



Alfalfa is, however, successfully grown in regions where 

 a minimum of 40 Fahrenheit, or even lower, is not 

 uncommon, as in Minnesota, North Dakota and Montana. 

 This is doubtless due in part to protection afforded by 

 snow. 



Young alfalfa is more often winter -killed by cold than 

 older plants, but there are no accurate data as to their 

 relative cold endurance. It sometimes happens that 

 alfalfa especially in low spots becomes covered for 

 a considerable period by a sheeting of ice. This usually 

 kills the plants. 



The degree of dormancy of the plants also affects their 

 ability to resist cold. It is well known that fruit trees 

 are much less likely to be injured by cold when the twigs 

 have become fully hardened and dormant and remain so 

 during the winter. In the irrigated regions, instances 

 have occurred where a portion of the orchard was irri- 

 gated late in the season so that the trees did not become 

 fully dormant. These were winter-killed when adjoining 

 trees of the same variety not irrigated escaped injury. 

 For the same reason, warm weather in late winter which 

 starts growth in the trees is likely to be disastrous if fol- 

 lowed by more cold. The behavior of alfalfa seems exactly 

 comparable to that of fruit trees, in that dormant plants 

 are much more resistant to cold and that high soil moisture 

 tends to retard dormancy. Fortier cites the experience 

 of a farmer at Chateau, Montana, who irrigated late in 

 the fall a portion of a field of alfalfa two years old. This 

 winter-killed, while the unirrigated portion was unharmed. 



Peruvian and Arabian alfalfas are varieties which con- 

 tinue to grow at temperatures lower than that which 



