130 MISC. PUBLICATION 4 3 4, U. S. DEPARTMENT OF AGRICULTURE 



Heat 



Heat, unless associated with drought, will not have any serious 

 effect on well-established seedlings, except to retard growth. It is 

 common knowledge that young seedlings that are in the process of 

 germinating or that still have succulent stems, are particularly sus- 

 ceptible to high temperatures even though adequate subsoil moisture 

 is present. Such injury may appear either as heat lesions on the 

 stem at the ground line, or burning of the leaves and succulent tops. 

 The logical solution of the problem of heat injury is to use nursery 

 technique that will produce sturdy woody stems as early in the 

 season as possible. 



Shading and irrigation by overhead sprinklers are helpful in reduc- 

 ing damage from heat, but shading is not a practical measure in large 

 deciduous tree nurseries unless the seedlings are grown in beds. 

 Sprinkling may have to be done twice a day on soils of low water- 

 holding capacity. 



For a dark-colored, loamy sand nursery soil in North Dakota, it 

 was found that IK hours of overhead watering, starting at 12:10 p. m., 

 reduced soil-surface temperatures from 125° to 96° F. on a day when 

 the air temperature in the shade was between 100° and 106°. The 

 hourly record was 103°, 117°, 119°, and 107° respectively at 2, 3, 4, 

 and 5 p. m. On the un watered areas the temperature at the surface 

 remained around 125° until 4 p. m., but dropped to 110° by 5 p. m. 

 Six-week-old seedlings of ponderosa pine in this nursery showed 

 definite heat lesions on the southwest side of their stems when soil- 

 surface temperatures exceeded 120°. It is believed that the tem- 

 perature at which plants will suffer injury depends not only on the 

 length of exposure, age, and development of the species but also on 

 the nature and size of the soil particles and color of the soil. 



Heat injury to nursery stock is always more prevalent in coarse, 

 sandy soils than in fine-textured soils, even in cases where soil tempera- 

 tures are not essentially different. This appears to be due to the 

 greater reflection of heat from the sandy surface, and higher conduc- 

 tivity of the sandy material. The lower water-holding capacity of 

 the very sandy soils makes it necessary to watch surface temperatures 

 more closely; heat injury may occur as soon as the surface quarter 

 inch of soil has dried out, even though there is abundant moisture 

 below the surface level. 



During the prevalence of high temperatures in 1936 in the plains 

 area, heat injury caused considerable loss in the nurseries, especially 

 where no overhead irrigation was available. Furrow irrigation failed 

 to distribute moisture on the surface of the soil either rapidly or 

 evenly. Fortunately, such extremes of temperature are the exception 

 rather than the rule, and it is not considered justifiable to install 

 overhead systems to cut down severe heat losses occurring only at 

 such wide intervals. 



Observations indicate that stand density in seedlings growing in 

 narrow bands has little effect in preventing heat losses during the 

 critical period of emergence and establishment. The main advantage 

 of denser stands is that there is less mechanical injury from wind and 

 beating rains. After passing through the initial critical period, how- 

 ever, stands of average density (8 to 12 seedlings per lineal foot), 



