36 



Farmers' Bulletin i208. 



trouble in sewers by filling them with a mass of fibrous roots if 

 access is once gained. Vigorous root growth is encouraged by the 

 moisture from a leak, and the roots ultimately find their way inside. 

 The mature seed is carried by a cottonlike appendage, and in many 

 varieties this is so abundant as to be another serious objection to the 

 planting of these trees. 



The southern cottonwood, Carolina poplar, and the northern cot- 

 tonwood are very similar in their adaptability for street-planting 

 purposes. They are easily propagated, easily transplanted, are quick 

 growing, and where they reach maturity under normal conditions 

 form very large oval-headed handsome trees, but under the artificial 

 conditions existing in cities it is necessary to prime them quite se- 



FIG. 24. A pavement heaved by the roots of poplar trees. 



verely when young to remove the long vigorous growths and make 

 the heads more compact. This pruning stimulates more vigorous 

 growth, which must be removed or they will form long branches 

 with heavy tops that are especially liable to injury by windstorms. 

 The more they are pruned the greater the tendency to an undesirable 

 form of growth. They begin dropping their leaves early in the 

 summer and lose them very early in the autumn. Their root growth 

 is vigorous, especially in the presence of abundant moisture. Ex- 

 cept in regions 6, 7, or 8, or in locations where smoke and fumes in 

 the air prevent the growing of other trees, they should not be 

 planted. 



The Lombardy poplar is a tall columnar tree adapted for use on 

 very narrow streets. (See fig. 1.) It is short lived in many places, 



