Chapter 19

Plant Propagation from Seed

Environmental Factors Affecting Seed Germination

Seed germination is a complex process that occurs when a viable seed with proper internal conditions is exposed to favorable environmental conditions. Internal conditions that affect seed germination include seed coat properties and dormancy. Environmental factors that influence seed germination include moisture, light, air (oxygen), and temperature. Ideal germination conditions vary by species.


The moisture content of the growing medium can be critical to germination success. Species vary in the amount of moisture needed to prompt germination. For example, the cucurbits (e.g., melons and cucumber) and cole crops (e.g., cabbage and broccoli) will germinate in soil with relatively low moisture content, whereas celery requires soil to be well hydrated before seeds will germinate.


Some seeds require light to germinate while others require darkness. Examples of plants requiring light for seed germina­tion are ageratum, begonia, browallia, impatiens, lettuce, and petunia. Conversely, calendula, centaurea, pansy, annual phlox, verbena, and vinca germinate best in the dark. Other plants do not respond to any light conditions and will germinate in light or darkness. When sowing seed that requires light, these small seeds must be at or near the soil surface to be exposed to light, which enables germination to occur.


Respiration takes place in all viable seed. The respiration in dormant seed is low, but some oxygen is required. The respiration rate increases during germination. Therefore, the medium in which the seeds are placed should be loose and well-aerated. If the oxygen supply during germination is limited or reduced, germination can be severely retarded or inhibited.


Favorable temperature is another important requirement for germination. It not only affects the germination percentage but also the germination rate. Some seeds will germinate over a wide range of temperatures, while others require a narrow range. The temperature range in which seeds germinate extends from a minimum temperature (below which there is no germination), to an optimum temperature (which is the desired level for each species), to a maximum temperature (above which germination cannot occur).

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