PLANTS FOR CONGESTED CITY DISTRICTS 119 



providing it receives a certain amount of care. The common arborvitae 

 has generally proved a failure. The exact reason why conifers are so 

 unsuccessful has not so far appeared. Their short life seems to be due 

 to the accumulation of soot which clogs the pores of the leaves and 

 slowly suffocates them. They transpire so much water also during 

 the hot summer droughts that they need an excessive amount of 

 moisture, and they need numerous showers or washings from the 

 hose also to keep them clean and cool. As most evergreens grow- 

 ing under city conditions do not get any care they rarely succeed, and 

 when they do live, they lose their colour and are therefore not recom- 

 mended. 



As a rule, native collected plants seldom or never succeed when 

 taken directly into the congested city districts, and only those plants 

 among the deciduous shrubs and trees which are smooth-leaved are 

 to be recommended for trial. 



LIST OF PLANTS FOR USE IN CONGESTED CITY DISTRICTS 



A. Trees. This group contains trees which may be used with a 

 great deal of certainty that they will thrive under city conditions of 

 congestion and dusty atmosphere. Wherever possible, these trees, 

 especially evergreens, should be thoroughly sprayed at frequent 

 intervals to wash a considerable portion of the dust from the surface 

 of the leaves. 



Abies concolor Pinus montana mughus 



White Fir Dwarf Mountain Pine 



Aesculus hippocastanum Pinus sylvestris 



Common Horse-chestnut Scotch Pine 



Ailanthus glandulosa Platanus orienialis 



Tree of Heaven Oriental Plane 



Celiis occidentalis (North only) Quercus palustris 



Nettle Tree Pin Oak 



Cercis canadensis Robinia pseudacacia 



Red-bud Black Locust 



Crataegus (in variety) Salix (in variety) 



Thorn Willow 



Fraxinus (in variety) Sophora japonic a 



Ash Tree Japanese Pagoda Tree 



Ginkgo biloba Tilia europaea 



Maidenhair Tree European Linden 



Juniper us virginiana Tilia tomentosa 



Red Cedar Silver Linden 



Picea pungens Ulmus campesiris 



Colorado Spruce English Elm 



