Tree and shrub seeds commonly have an embryo dormancy that prevents them from germinating even if growing conditions are ideal. The embryo requires a period of cold moist conditions to overcome the dormancy. The cold period varies and depending on species can range from one to five months. Where a double dormancy exists (impermeable seed coat and embryo dormancy) a period of warm followed by cold stratification is required. The warm period degrades the seed coat and slows water to enter the seed. The warm period to break down the seed coat can be replaced by acid scarification; this shortens the pre-treatment period significantly but requires extreme caution. With some species the radicle will emerge from the seed during the stratification period which is a good signal the seeds re ready to be sown. Once in stratification it is important that seeds are not allowed to dry. The procedure may seem complex but in reality is relatively straight forward just requiring patience.
An effective alternative to seed pre-treatment is to adopt nature’s approach; that is sow seeds and wait. Woody species that have embryo dormancy are sown in the fall whereas those with double dormancy are sown in early summer; both germinate the following spring.
- Using clean, sterilized silica sand, moisten to 10 percent 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.
- Soak seeds for 3 days at room temperature in water, changing water daily.
- Place the seed in bag and mix so seeds are in good contact with the sand.
- Seal the bag and label it including the date that the stratification began. Place the bag in a refrigerator (5oC) for species with embryo dormancy or at room temperature when requiring warm stratification.
- Check the seed weekly, gently massaging the bag and checking for mold or radicle emergence.
- After the specified time period separate seed from sand by screening. Seed is ready for sowing.