Seed Stratification

Most seeds will sprout readily if given moisture and warmth. Some are more exacting with their germination requirements, preferring limited temperature ranges or specific light needs. A few are downright demanding.

Stratification is any method of breaking seed dormancy to initiate germination. One means of stratification is scarification.

Scarification is used for seeds with hard coats. For these tough customers, gardeners must find a way to allow moisture inside the protective shell to initiate growth. Freezing seeds can sometime cause the seed coat to crack. Acid washes or “processing” through an animal’s digestive tract can be the trick for some. Rubbing the seed against an abrasive surface like sandpaper or chipping a tiny hole in the outermost layer are easy and effective. I keep a pair of toenail clippers that are dedicated to this chore. It is important that the wound made by chipping be shallow. Otherwise, there is risk of damage to the underlying endosperm. The endosperm is both food source and protection for the seed embryo. The first sign of germination, a first root called a radicle, emerges from this embryo. Injury to the radicle means deformity or death to the emerging plant.

Other means of stratification seek to mimic nature’s conditions under which the seed sprouts. For most this means refrigeration in damp conditions (think plastic bag with a damp paper towel) but some require storage in warm, damp conditions. A very few require both. For instance, right now I am attempting to germinate seeds of Euscaphis japonica, the Korean Sweetheart tree. The seeds of this plant require a 24-hour water soak followed by sixty days of cold storage, then sixty days of warm storage, then another sixty days of cold storage before being planted in growing media. Whew!

Redbud tree (Cercis Canadensis) requires both scarification and cold stratification. Mother Nature provides the scarification part by making the seeds tasty to birds. Birds crack the seeds as they eat, put them through a stomach-acid wash, and then deposit them onto the ground for the cold, damp days of winter. The success of this process is proved by the number of redbud trees that pop up in woodland areas. Hollies and magnolias have chemical inhibitors in the fleshy pulp surrounding the seed. This pulp must be removed by washing. I have had good success with germinating dogwoods after washing the pulp away with dishwashing detergent, then rinsing with plain water before sowing.

It is important to realize that the germination process cannot be halted once started. If you attempt to stop sprouting after it has begun, the seeds will die. Best success comes from using a seed starting mix rather than a normal potting soil. It is light weight and allows the roots to grow without fighting a denser soil.

Consult a reliable reference (book or website) for the preferred germination conditions of your seeds.