144 



it generally tends to reduce the size of the tree, though in very thin, 

 dry soil, heavy pruning has also increased the size of the tree. 



Whitewashing Twigs to Retard Bud Growth. Another means 

 of holding the buds dormant in winter is to keep all twigs and buds 

 covered with whitewash. This keeps down the temperature of the 

 buds during warm days by reflecting heat that would be absorbed 

 by the dark colored pigments in the twigs. The effect of color on 

 the temperature of the twigs has been worked out by Whitten" who 

 found as great as eight degrees centigrade difference between air 

 temperature and that of purple twigs in winter. He also found by 

 carefully sectioning buds that when the twigs are kept whitewashed 

 during the winter they do not push as rapidly during warm periods 

 as do buds on twigs not whitewashed, and in some springs, though 

 not in all springs, he was able to cause the blooming time to come 

 somewhat later. However, it would not be expected that such a 

 process would affect the blooming time as much as it affects the 

 amount of starting during warm days in early winter, since at bloom- 

 ing time the buds are for a good share of the time exposed to an opti- 

 mum temperature for growth or nearly so, and during warm days 

 in winter the air temperature is below the optimum temperature 

 for growth and often even below the minimum temperature. The 

 temperature of these winter twigs may be raised by absorbing sun- 

 light to at least above the minimum temperature for growth and in 

 some cases to the optimum temperature. Thus those who have 

 expected whitewashing to cause an appreciably later blooming in 

 spring would naturally, in many cases, be disappointed. In sec- 

 tions, however, where much killing occurs in winter following the 

 starting of buds by warm periods, it should be expected to give good 

 results. 



While prolonging the rest period by causing growth late in win- 

 ter by the above method applies only to sections far enough south 

 that there would be a considerable number of warm days before the 

 rest period is entirely ended (sections like the southern half of Mis- 

 souri and farther south), the benefits from whitewashing would be 

 expected to be as great say in New York or Canada in March before 

 the optimum temperature for growth is reached as in Missouri, un- 

 less the sunlight should be less direct and there should be less in- 

 crease in twig temperature caused by the absorption of light by the 

 dark color. However, the cost of whitewashing an acre of trees, 



'Mo. Agr. Exp. Sta. Bui 38, 1897. (Bibl. No. 114.) Das Verhaltnis der Farbe, etc., 

 1902 (Bibl. No. 116). 



