peratures, a large number of his 

 plants were so badly damaged that 

 they died — a serious loss to him. 

 Damage consisted of splitting and 

 cracking of the bark, exposing the 

 wood beneath; in some instances, 

 the bark hung in shreds from the 

 branches and twigs. Attempts to 

 explain what predisposed the plants 

 to injury were mere guesses and un- 

 satisfactory. The root systems 

 were well-developed and in good 

 condition; soil tests indicated no 

 poisonous materials or excessive 

 amounts of plant nutrients; the 

 plants had not suffered for lack of 

 water, nor did it seem they had 

 received too much. One possible 

 explanation was that fertilizer heav- 

 ily applied in the spring to prepare 

 the soil for planting caused the 

 plants to produce soft growth, 

 which did not harden sufficiently 

 before the frost. 



many nights the temperatures 

 dropped below freezing. 



Another application of fertilizer 

 was made in the Spring of 1951, 

 and by fall some of the plants were 

 two and a half to three feet high. 

 On November 7, after several nights 

 of below-freezing temperatures, 

 there was serious frost injury to 

 such hardy azaleas as Torch Azalea, 

 Korean Azalea, Korean Rhodo- 

 dendron and Carolina Rhododen- 

 dron, but not to Ghent Azalea, in 

 all the blocks regardless of the kind 

 or amount of fertilizer used. 



Apparently, fertilizer is not a 

 factor in predisposing azaleas and 

 rhododendrons to frost injury. This 

 conclusion is further supported by 

 the frost killing of 114 named var- 

 ieties of hybrid azaleas in another 

 field planted in the Spring of 1951 

 in soil prepared with only a moder- 

 ate amount of fertilizer. 



Fertilizers Do Not Cause 

 Frost Injury 



To obtain more information on 

 predisposing causes, young plants 

 of several kinds of azaleas and 

 rhododendrons susceptible to frost 

 injury were planted in the Spring 

 of 1949 in blocks, some of which 

 were fertilized with organic mater- 

 ials, such as manure, castor pomace, 

 and commercial dried hen manure; 

 others, with commercial chemical 

 fertilizers. The rates of application 

 were made on the basis of 500 to 

 2,000 pounds per acre. That fall 

 there was no injury, although 

 temperatures as low as 26 degrees 

 prevailed. 



In the Spring of 1950, fertilizers 

 were applied again, and although 

 some blocks showed considerably 

 better growth than others because 

 of the beneficial effects of fertilizer, 

 no frost injury occurred even though 



Rainfall Is One Cause 



An analysis of the records of 

 rainfall for each month of the grow- 

 ing season for 1948 through 1951 

 indicated that rainfall in October is 

 related to frost injury. In 1948 and 

 1951, when injury occurred, Octo- 

 ber rainfall was 4.3 and 4.7 inches, 

 respectively; whereas, in 1949 and 

 1950, when there was no injury, 

 although fall temperatures were 

 often as low or lower than in the 

 injury years, the rainfall was 1.2 

 and 1.5 inches, respectively. 



This relationship indicates that 

 frost injury can be reduced, and 

 even prevented, by (1) withholding 

 water from azaleas and rhodo- 

 dendrons in October and (2) root- 

 pruning in rainy seasons, at least 

 two weeks before expected frosts. 

 Both operations will slow up growth 

 and harden the plants against frost 

 damage. 



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