It will soon be winter and many gardeners enjoy starting plants from seed that require special treatments. Here’s Halton Master Gardener, Cathy Kavassalis’ explanation on the process required to break seed dormancy which can help you to create a beautiful native plant paradise.
The terms cold stratification, moist stratification and or scarification are often loosely used but are all methods for breaking seed dormancy. They describe a range of techniques.
- Stratification is a general term that refers to a range of processes used to simulate natural conditions (typically temperature and moisture) that seeds require before germination can occur.
- Scarification refers to various techniques used to break physical dormancy (explained below). I will begin with a couple resources I like and then provide more details.
Cold-moist treatment is effective for many northern, native species. You simply mix seeds in moist media like peat moss, vermiculite, perlite, or potting media to simulate the natural conditions they would experience overwinter. Then store the mixture in a container placed in a refrigerator for a period of time, (typically around six weeks). See ‘Further reading’ below for step by step stratification.
Certain species have very particular requirements. To understand those requirements, one must better understand dormancy. Dormancy is an evolutionary adaptation that prevents seeds from germinating during a time when environmental conditions do not favour survival. Seeds display a range of morphological, physical and or physiological dormancies.
Some seeds are underdeveloped when shed from the mother plant and require time to mature. This is termed ‘morphological dormancy.’ But, time alone may not be sufficient for germination to occur. Seeds may also require mechanical action or environmental triggers to initiate germination.
Seeds are protected by a seed coat (testa). In some species this physical barrier requires scarification (scratching, etching, or burning) or passage through an animal gut where acids or digestive enzymes can dissolve the barrier. These seeds have what is called ‘physical dormancy.’ Soaking (in water or acids), nicking, burning or abrading such seeds can aid in germination. Different techniques are required for different species.
Still other seeds experience ‘physiological dormancy.’ Germination-inhibiting hormones prevent germination until environmental conditions are appropriate for growth. Light, moisture and temperature are among the factors that signal seeds to break dormancy. Some species have a simple temperature requirements: A period of warm or cool temperature stratification. Others have complex requirements for alternate daily temperature fluctuations, or prolonged incubations at cool and warm temperatures to promote germination. A good example is the European ash (Fraxinus excelsior), which requires a period of cold to remove physiological dormancy and a period of warm to overcome morphological dormancy. Thus, there is a period of the year that precedes seedling emergence in which the seeds lose dormancy and can germinate if given light,” (Penfield 2017 https://www.sciencedirect.com/…/pii/S0960982217306164). Each species has its own combination of requirements to break dormancy. It can be complicated and different for each species.
Tom Clothier has compiled a useful database for germination requirements: https://tomclothier.hort.net/. Bottom line… There is no single stratification or scarification method appropriate for all seeds. Check requirements for your particular species.
Halton Master Gardener
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