A Street Tree System for New York City 57 



way the water from street flushing and storms. Before the 

 advent of the automobile and the use of oil as a dust layer 

 or in road construction this method was of value, but today 

 the injury from the oil and grease which would be thus 

 washed into the soil around the tree roots would more than 

 offset the value of the water received. This has been found 

 to be the case by German tree experts who formerly made 

 use of this practice. 



Where a grating is used over the surface of the tree pit 

 some water is secured from the rains by way of the sidewalk 

 and the rest can be provided by weekly or fortnightly flood- 

 ing with a hydrant hose or sprinkling wagon. 



Where bricks laid in sand are used instead of a tree grat- 

 ing as a traffic surface for the tree pit, a tile or dry well sys- 

 tem or the underground pipe system of irrigation will be 

 needed. These methods are also suitable for trees in un- 

 fenced gravel parkings or continuous strips paved with 

 bricks laid in sand. In the case of trees grown in grass 

 parkings or gravel areas protected from traffic by fences, 

 an occasional heavy surface watering will usually be suffi- 

 cient if the grading is such that a good supply is held in 

 a depression around the tree trunk. 



The amount of water needed by any tree or street of trees 

 will depend upon the species of tree, the physical character- 

 istics of the soil, the drainage below ground, the rainfall, 

 the season of the year, etc., and can only be determined by 

 experience. 



(3) Proper drainage. 



The securing of proper drainage follows, of course, as a 

 corrollary of the above, and if the subsoil does not provide 

 for it adequately, underdrainage in some form must be pro- 

 vided artificially. A layer of loose stones and a tile to the 

 nearest low-lying sewer will provide such drainage in the 

 simplest and most effective way. Sometimes dynamiting the 

 subsoil will provide adequate drainage if the substrata are 

 suitable. 



