312 FUNDAMENTALS OF FRUIT PRODUCTION 



which retained its leaves but matted down too close upon the ground. 

 The stalks left standing after a crop of corn grown in the ordinary way has 

 been harvested make a very efficient snow holder but furnish very little 

 protection to the ground at times of intense cold unaccompanied by 

 snow." 



The superior snow-retaining qualities mentioned, particularly in the 

 case of corn, are operative mainly when the snow fall is accompanied by 

 wind. 



Summarizing the requirements for a cover crop under Nebraska 

 winter conditions, Emerson says: "It should start growth promptly in 

 order to insure an even stand and to choke out weeds. It should grow 

 vigorously to insure a heavy winter cover and to dry the ground in case 

 of late-growing trees so as to hasten their maturity. It should be killed 

 by the early frosts so that it will stop drying the ground after danger of 

 late tree growth is passed and help to conserve our light rains so much 

 needed by the trees in winter. ... A cover crop should be heavy 

 enough to furnish as good direct protection as possible against freezing 

 and thawing and it should stand sufficiently erect to hold snow against 

 the power of strong winds." 



Of the crops tried, that which appeared to come nearest meeting these 

 requirements in Nebraska was German millet. 



Root Killing in Different Fruits. — There is less latitude in the root 

 hardiness of the various species than in the hardiness of their tops. 

 Nevertheless there are enough differences in many cases to make the 

 choice of root stocks very important. 



The Apple. — Carrick^^ found that the majority of dormant apple roots 

 were seriously injured at a temperature of — 12°C., with considerable 

 injury at — 7°C. He reports the cambium as the most tender tissue, 

 followed closely by the phloem, with the cortex less tender. Under 

 extreme conditions xylem and pith are said to be killed. French-grown 

 stocks were found substantially as hardy as the native-grown seedlings. 

 In all cases there was a considerable variation, as would be expected 

 among seedling plants. This difference, it may be remarked, is likely 

 to assume considerable importance under field conditions. 



The Pear. — Studies on pear roots by the same investigator indicated 

 that Kieffer roots were more resistant than the French stock. A tem- 

 perature of — 11°C. during the dormant period produced extensive injury 

 in both. In April Kieffer showed only slight injury at — 9°C. while 2-year 

 French roots were killed. Pear roots seemed to acquire hardiness later 

 than those of the apple and never become quite so hardy. 



The Peach. — The peach root is relatively hardier in the zone of dis- 

 tribution of this species than is the apple root along the northern border 

 of apple growing. Occasionally, however, root killing in peaches occurs. 

 Goff^® records that in the freeze of 1899 peach tops suffei;ed more than the 



