TREE SURGERY 355 



sheet metals to use. Galvanized iron, tin plated sheet-iron and 

 uncoated sheet-iron may be used if kept painted. The tensions 

 developed when a tree twists and bends under wind pressure 

 must be taken into account in fitting the sheet metal. 



Two things are attempted in filling cavities : (1) to prevent 

 the entrance of fungi and insects ; and (2) to strengthen the tree. 

 As a matter of fact they accomplish neither and there is no sound 

 argument in most cases for the additional cost which filling en- 

 tails. If the cavity is to be filled, it must be cut back on all sides 

 so that the cavity enlarges abruptly just inside the opening. A 

 shell of about two inches should be left if possible as a shoulder 

 to hold in the filling. If the opening is so wide that a shoulder 

 cannot be shaped to hold the filling, reinforcing of some kind 

 should be used. If the shell is thin or the cavity is a long one, 

 over two or three feet, it should be bolted. Bolts should be used 

 at intervals of two feet. The manner of placing and size of bolt 

 will depend on circumstances. After the holes are bored, areas 

 of bark are cleaned away so that the washers and nuts can be 

 countersunk. The holes should then be tarred and the bolts 

 fastened in place after which tar should be applied again. If 

 the shell is thick, nails may be driven into the wood to serve as 

 reinforcing to hold the filling. Also wires fastened to the nails 

 and bolts may be used. If the cavity is very large, ordinary 

 reinforcing steel rods wired to the bolts may be utilized. 



The two types of fillings generally used are concrete and 

 asphaltum. Concrete may be put in dry or wet. Dry con- 

 crete is built up by tamping and troweling. Account must be 

 taken of the size of the tree and amount of swaying in the wind, 

 and joints used or the concrete will crack and fall out. Joints 

 should slope downward and drain easily. Newspaper or heavy 

 tarred paper and felt are used to separate the blocks. Bricks 

 and stones may be tamped in where large fillings are made. If 

 wet concrete is used, a facing of canvas and oilcloth is employed 

 over the opening to shape the surface. In all cases the surface 



