Tree and Shrub Seed Stratification

There are three types of dormancy found in tree and shrub seeds: 

• impermeable seed coats
• embryo dormancy
• combination of the two above

The type of dormancy dictates the method of seed treatment. For seeds with embryo dormancy a rest  period in cool (5°c ) moist conditions is necessary. Fall sowing in most cases will break embryo dormancy.  This takes advantage of cold winter temperatures and seed will germinate the following spring as the soil  warms. Seeds with double dormancy can be sown in early summer for germination the following spring. 

Internal Dormancy

This dormancy is caused by chemical action on the seed’s embryo, preventing its growth. In this case the  following methods can be used. 

Cold Stratification
This involves exposing the seed to sufficient moisture and adequate oxygen at a temperature of 5°c for 30 to 120 days.

Warm/Cold Stratification
The seed is exposed to warm temperatures then cold temperatures followed by germination. This involves a temperature of 20°c to 25°c for followed by cool storage at 5°c.

Seed Stratification  

When seeds are affected by embryo dormancy they require a period of cold or warm, moist stratification after which biochemical processes occur that allow the seed to germinate. The procedure for seed stratification is as follows: 

1. Using clean, sterilized silica sand, moisten to 10 percent moisture content in a plastic bag. Mix sand so that it is uniformly moist. Surface sterilize the seed by immersing in a 10 percent bleach solution for 30 seconds making sure all seed is exposed to the bleach. Using a small sieve makes it easy to immerse the seeds into the bleach. Rinse the seed under cold running water to remove excess bleach solution.
2. Soak seeds for 3 days at room temperature in water, changing water daily.
3. Place the seed in bag and mix so seeds are in good contact with the sand. Do not allow seed to dry (if seed dries reversal of the after-ripening process can occur).
4. Seal the bag and label it including the date that the stratification began. Place the bag in a refrigerator (5°c) or a cupboard at room temperature depending on the stratification requirements of the species you are treating.
5. Check the seed periodically, gently massaging the bag and checking for mold or radicle emergence.
6. After stratification is complete days separate seed from sand by screening. Seed is ready for sowing.
7. Note: These techniques are effective in stimulating seed germination, however germination ranges from 30 to 90 percent depending on species.