WINTER INJURY TO THE ROOTS 317 



ground freezes and often dug rather early must have very tender roots, 

 so tender in fact that exposure to a sUght frost after digging in this stage 

 is hkely to have very serious consequences. Extreme care in protecting 

 tree roots against any freezing from the time they are dug until they 

 are planted is amply justified. 



Summary. — Root killing is particularly common in sections with 

 low winter temperatures and little snowfall. Minimum soil temperatures 

 of 24° to 25°F. at a depth of 6 inches are very common in deciduous fruit 

 sections and soil temperatures of 7°F. have been recorded in Nebraska. 

 Freezing temperatures are frequently registered to a depth of 2, and occa- 

 sionally to a depth of 3 feet. The critical temperature for the roots 

 of most hardy species during their dormant season ranges from about 

 14° to 5°F. During the growing season it is much higher. Minimum 

 soil temperature is influenced greatly by soil covering, being distinctly 

 higher under snow or a mulch formed by some cover crop than under 

 bare ground. Fertilizers may indirectly protect roots against severe 

 freezing by promoting the growth of weeds or of cover crops. Frost 

 penetrates more deeply in light than in heavy soils. Roots are killed 

 more readily in dry than in moist soils. Considerable differences exist 

 in the relative resistance of the roots of different species and varieties. 

 Preventive measures include moderately deep planting, the use of cion- 

 rooted trees or trees on hardy stocks, the choice of locations not unduly 

 exposed to the wind, the use of cover crops to hold the snow and thus 

 both directly and indirectly blanket the soil and in some cases artificial 

 mulching. Remedial treatment consists chiefly in judicious pruning. 

 Care should be taken in handling nursery stock that the roots are not 

 exposed to freezing temperatures in packing, unpacking or heeling in 

 and they should be protected from freezing while in storage or transit. 



