THE DORMANCY OF PLANTS AND SEEDS.
Dormancy in plants and seeds is a survival strategy and it may be a simple or complex dormancy.
Plant dormancy is usually simple and the best example is with deciduous plants that lose their leaves in autumn preparing themselves for the unfavourable conditions of winter. However, plants do not only go dormant in winter but can go dormant during other periods of unfavourable conditions. The Australian species of Drosera (Sundews – insectivorous plants) grow and reproduce during the cool season and go dormant in summer and succulents usually have a dormancy during the ‘dry’ season and grow during the wet season. With them it depends on their natural habitat – those with winter rainfall go dormant in summer and vice versa and they resent moisture during the dormant period.
Complex seed dormancy is more varied but it is still a survival strategy. Simple dormancy, where conditions of soil warmth plus water break the dormancy and trigger germination is most often seen in annual plants – snapdragons, zinnias etc but it also occurs with many trees. Complex dormancy is when multiple conditions must be met and often in the correct sequence. Examples are:
Scarifying. The seed coat (testa) on all members of the Pea family is impervious to water because it does not have a hilum which allows water entry. The seed coat must be broached with hot water treatment (scarifying).
Stratifying. Some seeds need a ‘winter’ before they will germinate, this can be simply achieved by storing them in the refrigerator for a period, while others need a given number of hours before they will germinate. The seed totals the number of hours below a certain temperature and once the required total is reached, it will germinate when spring comes. If it is short one hour of the required chilling time, then sorry, try again next year.
Smoke Treatment. Some seeds native to Australia and South Africa live in a bushfire-prone habitat. These seeds will remain dormant during a bushfire but will germinate after rain, if the bushfire and rain occur sequentially. If either smoke or rain only occurs, then the seed remains dormant. Smoke from the bushfire coats everything in chemicals and the rain washes those chemicals into the soil where the seed detects them and knows bushfire season has passed and the water triggers germination.
Seasonal Dormancy. This mainly occurs in annual, cactus and succulents. Annuals are dealt with above and need no further explanation. Cactus and Succulents are semi-arid or arid-living plants and dormancy is dictated by their natural habitat. Plants with a winter rainfall regime go dormant in summer to conserve water while plants with a summer rainfall usually go dormant in the winter. This is a survival strategy evolved to cope with weather adversity. The gardener needs to understand this in order to not attempt germination during the respective dormant period except where climate-controlled greenhouses are used.
A full list of seeds, seed treatments and dormancy would easily fill a large volume and does not belong in a simple blog. However, below I have included a short list of succulent dormancy to whet your appetite.
