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



A few seeds with papery coats, like desertwillow and catalpa, ailan- 

 thus, and the elms, absorb moisture readily but may be helped by a 

 little soaking before spring sowing. 



METHODS OF SEED STRATIFICATION 



Stratification is accomplished best by holding the seed for the 

 required period in a moist medium at a low temperature. For most 

 species the best temperature is about 40° F., and stratification is best 

 accomplished in cold-storage chambers where the temperature can be 

 held close to that point. 



Sand, peat, or a mixture of the two are commonly used by commer- 

 cial and Government nurseries for stratification of seed. The facilities 

 available and condition of the seed — whether depulped or not de- 

 pulped — will determine the best medium to use. In instances where 

 no temperature control is available, or if the seed retains its pulp, pure 

 washed sand is the safest, although unwashed sand has been used with 

 satisfactory results. Where temperature control is available, and 

 where the seed is free from pulp, pure granulated horticultural peat 

 obtained from acid spruce swamps or a 1 to 1 mixture of sand and 

 peat will give better results than sand. Peat is favored over sand 

 because its light weight, moisture-retaining capacity, and texture 

 facilitate handling, make the addition of water unnecessary during the 

 stratification period, and afford better aeration for the seed. For 

 seed retaining their pulp or where no temperature control is possible, 

 difficulty has been experienced with peat because of heating, and it is 

 necessary to use sand. 



The moisture content of the peat or the mixture of peat and sand 

 used as a stratification medium should be such that no water can be 

 squeezed out by closing the hand firmly on a small quantity. Where 

 the proper moisture content in peat mediums is achieved at the start 

 of the stratification period, it is usually maintained evenly for the 

 duration of the stratification period without further watering. When 

 sand alone is used, it should not be saturated but the moisture content 

 should be such that when a handful is picked up and compressed it will 

 hold its shape without crumbling. It is difficult to maintain a uniform 

 moisture content in sand since the moisture tends to drain to the bottom 

 causing a soggy condition in the lower portions of the mass and 

 leaving the top too dry. 



The correct proportion of seed in the stratification medium will vary 

 according to the size of the seed. A safe rule to follow is that the seed 

 and the medium should be thoroughly mixed and each seed should be 

 practically isolated. The volume of the medium may be somewhat 

 less than that of the seed, but generally more satisfactory results and 

 greater safety will be assured by using a liberal quantity of the medium. 

 If practicable the stratified seed should be shoveled over at 10-day to 

 2-week intervals, assuring more uniform stratification through better 

 aeration. This process is especially important where sand is the 

 medium, since only in this way can the moisture content be evenly 

 maintained throughout. 



Small or moderate-sized containers such as boxes, barrels, or ice- 

 freezing containers (fig. 8) are preferable to large ones in stratifying 

 seed of the smaller sizes. Large boxes or bins have not proved as 

 satisfactory as metal containers because of the difficulty in maintain- 

 ing uniform moisture and temperature throughout the entire volume. 



