4 FARMERS' BULLETIN 788. 



impenetrable it is. The ordinary windbreak does not provide an 

 absolute barrier to the wind ; a certain amount of air forces its way 

 between the branches and foliage of the trees, so that the movement 

 of the air on the leeward side is not completely stopped but only 

 greatly reduced. When windbreaks composed of such trees as cot- 

 tonwood become old, wide openings are left between the bare trunks 

 and more wind gets through near the ground than higher up. 1 Such 

 windbreaks can be made efficient only by underplanting the cotton- 

 wood with other trees or shrubs. 



An ideal windbreak for checking wind currents would have the 

 contour of an earth dam. In the central rows would be planted 



FIG. 1. On irrigated lands in western Colorado (Mesa County), the Lombardy poplar, a 

 close relative of the cottonwood, grows thriftily, and is used for the protection of 

 orchards. 



the tallest trees, such as cottonwood; on either side, rows of shorter 

 trees, such as ash and locust; and outside of these, low bushes or 

 cedars. Such a windbreak would not be easily penetrated, and its 

 inclined surface would divert the air currents upward and relieve 

 the horizontal wind pressure. 



Breaking the mechanical force of the wind benefits the farmer 

 most directly by protecting his grain crops and his orchard. The 

 value of the windbreak in giving this protection is, of course, difficult 

 to measure in dollars and cents, but where winds are at all frequent 

 such protection alone may be equal to the rental of the ground occu- 

 pied by the trees. In one case in southern Minnesota a windbreak, 



1 A dense mulberry hedge of a single row may offer quite as much resistance to the wind 

 near the ground as several rows of open-growing locust or cottonwood. 



