94 MISC. PUBLICATION 4 3 4, U. S. DEPARTMENT OF AGRICULTURE 



natural method of hardening off nursery stock. All cultivation and 

 irrigation should generally be tapered off after midsummer and 

 usually should cease entirely from a month to 6 weeks in advance of 

 the average killing frost, the time depending somewhat on the water- 

 retaining capacity of the soil and on the size of the stock. A gradual 

 depletion of the soil moisture will bring about the hardening of the 

 seedling tissues and cause cessation of growth. When growth has 

 ceased and tissues have hardened seedlings can withstand fairly 

 severe frosts without having the tops killed back. 



Such natural ripening processes are much to be preferred, but 

 when they are rendered impossible by heavy fall rains, the only re- 

 course is to such wholly artificial methods as spraying the seedlings 

 with a copper sulfate solution to bring about defoliation, or digging 

 and burying the seedlings in pits for several weeks to sweat off the 

 leaves. Another method is to undercut or root-prune the seedlings 

 with the ordinary U -blade digger with the lifter removed. Artificial 

 ripening should be used, however, only as an emergency measure, 

 when natural ripening has been delayed beyond the safety point be- 

 cause of abnormal weather conditions. 



Spraying with copper sulfate solution causes a drying up of the 

 leaves at a rate in proportion to the concentration of the solution. 

 Parker (28) states that a solution containing 1 pound of copper 

 sulfate to 50 gallons of water affects the leaves like a light frost, 

 2 pounds per 50 gallons like a heavy frost, and 3 pounds will cause 

 ripening of the foliage within a week. This method of hardening 

 stock has been used to some extent in commercial nurseries. 



In the frequently used practice of digging the seedlings while in 

 full leaf and burying them loosely in pits under several feet of soil, 

 the seedlings are left for 2 to 3 weeks until the leaves loosen and can 

 be detached easily by shaking or stripping. This method, while 

 apparently safe, involves considerable extra handling and requires 

 care that the seedlings are not left buried too long, to avoid heating 

 or molding of the stock. 



Undercutting the seedlings to a depth of approximately 12 inches 

 while in full leaf reduces the moisture-absorbing capacity of the root 

 system and brings about a wilting and gradual drying of the foliage. 

 Undercutting has been used by some commercial nurseries with con- 

 siderable success, and in recent years by some Lake States nurseries 

 in hardening-off jack pine. 



Top Pruning to Increase Survival 



Top pruning has not proved to be of any marked benefit in in- 

 creasing field survival of deciduous stock which is reasonably well 

 balanced but has in some instances increased survival of seedlings 

 weakened by improper handling or having poor root systems, and, so 

 far as known at this time, can be recommended for such stock. It 

 has been observed that top pruning tends to induce lower branching 

 and more compact growth during the first few years after planting, 

 which is in many respects desirable, especially for the shrub species. 

 It may result in certain tree species having a somewhat distorted 

 form, but for windbreak planting the form of the tree is not regarded 

 as being of primary importance. As a practical nursery measure it 

 is desirable, since it reduces the bulk and thereby the cost of handling 

 and planting the seedlings. 



