(In the early 1990’s, Emeritus Prof. Norman C. Deno, Pennsylvania State University, published a report of a substantial research project “Seed Germination – Theory and Practice” covering the germination requirements of the seeds of over 2500species. It is a monumental work and far beyond the needs of the average domestic gardener. This series attempts to extract some details and information on the germination of seeds that may be of interest to home gardeners – the selection being made on personal experience in consultation with other gardeners. Apologies are offered if your needs are not immediately covered).
1. General Principles
(a) Every species of plant has one or more mechanisms for delaying germination until seed is dispersed.
Without these mechanism seeds would germinate in their capsule or fruit. The challenge in germinating seeds is to overcome these delay mechanisms. Typically these are chemical systems that are destroyed by drying, light or varying temperature cycles. Some seeds have a physical system, usually an impervious seed coat.
(b) Many temperate zone plants us a delaying mechanism that is destroyed by drying.
The common practice of placing seeds in an envelope on the shelf for a while leads to the common misconception that all seeds will germinate when exposed to warmth and moisture. A drying period is necessary before the seeds will germinate. The length of time needed is dependent on the species.
(c) Light is an important variable and is a requirement of some species.
The requirement of light for germination is characteristic of plants growing in swamps or woodlands where the availability of light is more of a problem than the availability of moisture.
(d) Seeds in fruits often have chemical inhibitors in the flesh of the fruit that block germination and must be removed by washing before germination will take place.
Usually these chemical inhibitors are water soluble so soaking in water with a daily rinse for seven days is sufficient for their removal.
(e) Seeds of a significant number of species are sold and distributed by commercial seed companies in a dead on delivery state.
Many seeds do not survive very long in dry storage.
2. Other factors
(a) Temperature. (Working in North America, Prof. Deno operated 4.5 degrees C and 21 degrees C. The temperatures are generally those at which refrigerators are set [4.5] or rooms are set .)
The two temperatures chosen for germination studies also generally equate to Winter or Summer seasonal temperatures. The destruction of delay mechanisms was found to operate most effectively at one or other or these temperatures – not both.
(b) Time is a critical variable
Nearly all the experimental work was conducted over a period of 3 months. This corresponds roughly with the length of Winter and Summer seasons.
(c) Dry Storage is a critical variable.
All seeds are ultimately killed if dry storage is continued long enough. Some species cannot tolerate a week or two of dry storage while others retain viability in dry storage for over a hundred years. There is no point at which all the seeds suddenly die.
(d) Light is an important variable
Light stimulated the germination in many species. Where it stimulated germination, it was often an absolute mechanism.