12 Farmers' Bulletin 1096. 



impervious to the waves of outgoing radiant heat from the earth, 

 but allows the incoming heat from the sun to pass through it freely. 

 The expense of covering with glass is too great to allow of its use 

 except for the more expensive plants and flowers. 



Cloth Screens. Experiments have been carried on in California 

 and elsewhere to determine the value of a covering of cloth over 

 orchards and over individual trees. When an acre or more of 

 orchard is thus covered, the minimum temperature may be from 

 2 to 4 higher inside the covered area than outside, if there is little 

 air movement. In experiments carried on in cooperation by the 

 Weather Bureau and the Southern Oregon Experiment Station of the 

 Oregon Agricultural College, it was found that cloth coverings over 

 small areas of orchard or over individual trees do not have an ap- 

 preciable effect on the temperature, even when the coverings are of 

 heavy cloth or canvas. This is due to the fact that the cloth does 

 not prevent an interchange between the air under the covering and 

 that outside. 



Coverings of rather heavy cloth laid directly over garden truck or 

 other low-growing plants are effective in protecting against moderate 

 frosts. In this case radiation from the ground and plants is almost 

 completely cut off and the air movement is so slight near the ground 

 there is little tendency for the cold outside air to be forced under or 

 through the covering. The temperature of the surface of the cloth 

 exposed to the sky is lowered by radiation and may fall to a low point, 

 but as both the cloth itself and the air underneath the cloth are very 

 poor conductors of heat, the temperature of the covered plants falls 

 much more slowly. The heat which has penetrated a few inches into 

 the ground during the day is slowly conducted to the surface during 

 the night and aids in keeping the temperature under the cover above 

 the freezing point. 



It is evident from the above that coverings of this kind should be 

 placed early in the evening when a frost is expected, before much of 

 the heat accumulated in the soil during the day has been lost. Tin 

 cans or other metal coverings should not be used to protect plants 

 from frost damage. Metals are good radiators and conductors of 

 heat and the temperature is likely to fall nearly as low under a cover- 

 ing of this kind as in the outside air. 



Lath Screens. Screens made of laths fastened together with wire 

 (the spaces between them being about the width of the laths) have 

 been used in Florida and California to protect orchards. These 

 screens serve not only to diminish outgoing radiation during the 

 night but also as a shade from the sun. About three- fourths of the 

 sky is screened by a covering of this kind. By placing the laths in 

 north and south directions the direct rays of the morning sun are 



