76 MISC. PUBLICATION 43 4, U. S. DEPARTMENT OF AGRICULTURE 



Throughout the winter, fall-sown seed should have direct contact 

 with the moisture in the soil. Dry, cold ground will have very little 

 beneficial effect on the seed requiring moisture as a part of its pre- 

 germination treatment. Moisture absorption by the seed is negligible 

 in frozen soil, and poor germination often occurs if the soil freezes 

 shortly after the seed is sown. In latitudes where the ground alter- 

 nately freezes and thaws during the winter months and winter pre- 

 cipitation provides adequate moisture, this point is of minor impor- 

 tance; but in the Dakotas fall sowings should be made early enough 

 to permit some moisture absorption by the seed before the ground 

 freezes. 



If the nursery soil is heavy, alternate freezing and thawing, as in 

 mild winters, commonly brings the seed to the surface, exposing it to 

 drying winds, rodents, and birds. This frost heaving can largely 

 be avoided by throwing up a broad ridge of soil to a height of 6 inches 

 over the seed rows. 



Perhaps the most serious problem to be encountered in fall sowing 

 is that a premature warm period in the spring may cause early ger- 

 mination, and that subsequent feeezing weather will destroy the 

 seedlings. If the warm spell is not unduly prolonged, however, this 

 can be guarded against by maintaining a ridge of soil over the seed 

 rows, thus controlling or preventing to some degree the penetration 

 of heat. 



Depth of Sowing 



Small seed should be sown at less depth than large seed. Other 

 important factors that enter into the depth of sowing are the texture 

 of the soil and season of sowing. It is safe to sow at a greater depth 

 in sandy than in fine-textured (heavy) soils, because of the compara- 

 tive ease of seedling emergence. Where ridging is practiced, the 

 season of sowing should not influence sowing depth, but if ridging is 

 not practiced, it is desirable that fall-sown seed be sown at twice the 

 depth of spring-sown seed of the same species. 



Under the practices followed in the plains region, the following 

 rule has been used as a guide in gaging the depth to sow the seed: 

 Sow all seed to a depth oi three times the average diameter of the 

 clean seed kernel but in no case less than Va inch. This rule applies to 

 all spring-sown seed in fine- to medium-textured soils regardless of 

 whether ridging is practiced, and results in 0.5 inch or thereabouts as 

 the sowing depth of a great many of the deciduous species. Very few 

 species are sown to a depth of more than 1 inch below the soil level. 



Sowing Ratios to Determine Density of Sowing 



Data obtained by the Lake States Forest Experiment Station have 

 shown rather conclusively that first-year survival, growth, and vigor 

 of field plantings in the plains region is in direct proportion to the 

 caliper of the nursery stock — larger caliper stock produced better 

 results than smaller stock. It was demonstrated that stock with 

 a caliper of between three-sixteenths and six-sixteenths inch at a point 

 2 inches above the ground line was, for most species, the premium 

 grade from point of percent of field survival and cost per surviving 

 tree. Optimum size varied somewhat by species, a point which is 

 discussed in detail in connection with the grading of nursery stock. 



