The Stratification of seeds

In nature trees have particular ways of dispersing their seeds for example, by the wind and by animals and birds that consume and dispense them through their digestive system. A walk through a city park or tree-lined avenue will produce a variety of seeds depending on the species of tree in situ. Such seeds released from the parent plant are in what is termed as a dormancy stage, that can be associated with the following: climatic conditions, immaturity, light, genetic variation and protection from predation.

scots-pine-seeds

It can be agreed that all tree seeds are dormant, some are considered to have ‘deep’ dormancy attributes requiring long periods of pre-treatment for example, the Ash Fraxinus excelsior 8 to 16 weeks warm at 15°C plus and 16 to 32 weeks cold at 4°C. Whilst Birch (Silver) Betula pendula is considered to have ‘shallow’ dormancy with a treatment of only 3 to 9 weeks of cold at 4°C.

Dormancy is a natural state of being in many plants, its function is to ensure that the seed will germinate at an appropriate time. However, seeds can remain in a dormant state and fail to germinate although conditions, temperature, water and light are in adequate supply. Why this phenomena occurs can be attributed to a seed’s morphological and physiological requirements, because seed dormancy is able to originate in different parts of the seed, for example, within the embryo or its coating – the shell or husk. Thus, dormancy can be deemed not as a constant, but as a variable because it is a common phenomenon encountered in a large variety of trees. To break dormancy and initiate germination, the process of stratification is needed and this method requires different techniques of which, there are various approaches depending on a particular species of seed.

Seeds having two dormancy combinations, a seed coat dormancy and an internal dormancy (Embryo) require the seed coat or shell to be treated first either by soaking in water and/or scarification. The internal dormancy is then subjected to the following treatment. Warm temperatures to initiate root growth then cold in order to break bud dormancy, then warm to encourage the shoot to sprout and complete the germination process.

There are various methods of scarification and stratification – too many to list them all in this discussion, but the most common approaches are mentioned.

Cold stratification – is when a seed/s spends time in or on the ground from the autumn to the spring and during this time it is subjected to the elements, which soften the hard shell, husk or casing allowing the cold to penetrate within. This cold triggers the seed’s embryo to germinate and eventually the seed sprouts pushing its way through its casing searching for light and nutrients. This cold stratification process occurs naturally in the wild, but it can be mimicked in a home environment and the following methods explain how.

Seeds collected in the autumn can be placed in containers with a growing medium for example, a mixture of soil and sand, soil and vermiculite, moss or other potting compost. The growing medium must be damp not wet, because wet soil is apt to cause mould and fungal disease, that can attack the seeds. In most cases the seeds are positioned on top of the growing medium and lightly covered with a sprinkling of the same composition. Then placed in a plastic bag and sealed then stored in the bottom of the refrigerator for 4 months. The temperature must be between 1°C and 5°C (34°F and 41°F) to ensure the stratification process is achieved. After the stratification period has concluded, the containers can be placed in a warmer environment to assist in growth development. However, some species require longer periods of stratification for example 5 to 8 months, whilst others only need shorter times and the easiest way to monitor a seed’s progress, is to check them periodically.

Seeds can also be soaked in water for 6 to12 hours then given the cold stratification process. It is said that this method reduces the amount of time needed for stratification, because the seeds will have absorbed sufficient moisture, which allows the chemical changes to take place. However, the time period for soaking seeds depends on the species and also the hardness and thickness of the husk, shell or coating; excessive water use can cause the seed to rot.

Warm stratification – some seeds including the pomegranate Punica granatum and the lemon genus Citrus, a popular choice among bonsai enthusiasts can be stratified in a warm environment and the sowing process is the same as described above. The container is then sealed in a bag or propagator and placed in a warm environment, temperature between 18-24°C. (65-75°F) The lemon does not require scarification nor removal of any residue, it can be planted immediately once removed from the fruit. However, pomegranate seeds do require the flesh to be removed to avoid pathogens and fungal attack.
Pomegranate seeds if not needed immediately can be placed on a piece of kitchen towel and left to dry. I have pomegranate seeds that are 3 years old, these are stored in an air-tight plastic bag, a few were sown in a small container on a bed of soft paper and covered with tepid water. They have now germinated after 3 weeks and will soon be ready to plant in different soil mediums to determine, which is more advantageous to their growth and well being.

pomegranate-seedlings

3yr-pomegrante-seeds

Warm and Cold stratification – when a seed requires both warm and cold stratification, the warm process comes first followed by the cold process. The warm stratification is required to soften the seeds outer shell or husk, this allows the seed embryo to mature. Warm and cold stratification is relatively easy to achieve, a seed planted in late summer will be warmed by air temperatures, and moistened by watering. The temperatures will gradually reduce as autumn changes to winter, but will rise again in the spring. Seeds using the warm and cold method of stratification need only one treatment then they are ready to germinate, further treatment can result in the seed’s demise.

Hot Water Treatment – seeds with hard shell coats such as peach and plum genus Prunus, hazel ‎Corylus avellana and the brazil Bertholletia require what may be deemed as a more drastic approach prior to stratification. Because their hard shells do not permit water to enter and water prevention stops the seed from beginning its germination. One method of overcoming this problem is by soaking the seeds in boiling water then allowing them to cool down for a day – this is one method of scarification.

peach-plum-and-hazel-seeds

Seeds that have been successfully scarified using the hot water treatment will either swell in size or simply sink to the bottom of the container for example. The black locust Robinia pseudoacacia a flowering deciduous species will swell, whereas Acer palmatum will sink. Seeds not responding to this treatment can be subjected to the process again however, not all will be successful, some may take several years to germinate and some may never do so.

Scarification: preparing seeds for stratification – there are differences of opinion regarding the preparation of seeds, some will argue that all unwanted material such as, pods and remaining fruit pulp be removed leaving a clean seed. In addition, seeds contained in hard shells for example, Prunus varieties, common horse chestnut Aesculus hippocastanum and oak Quercus, should not be removed from their coatings. Because the seed is vulnerable to harm from pathogens and fungi.

Nonetheless, some hard-coated seeds do require some scarification, their hard casings need to be scraped or cut by using a sharp pointed knife. They can also be soaked using the hot water process, both methods permit moisture to enter the casing allowing stratification to take place. But care and attention must be given when using a blade, hot water and indeed such methods using chemicals (sulphuric acid and household bleach) and fire.

Others contend that removing a seed’s hard case or shell prior to stratification, speeds up the germination process providing it is planted in a sterile soil composite. Alternatively just put the seed in a container with a growing medium and let nature do the rest. But allowing nature takes its course does not always produce the desired results for example. A variety of prunus seeds were scarified and planted in the autumn of 2015, thus far there has been no sign of germination. The problem could be due to morphological and physiological attributes or simply that the seeds are sterile nonetheless, perseverance is the watchword, hence they will be stratified again via the cold treatment.

Sowing seeds – seeds are a relatively cheap way to produce new trees and many varieties can be spring sown after cold storage. Others must be freshly sown in the autumn whilst they are still soft and fresh, because they require stratification and nature is arguably the best horticulturist in this respect. However, seeds acquired in the spring cannot be planted in autumn, because they will have lost their freshness and will have dried out. Nonetheless, the above described methods of stratification should help solve the problem.

All tree seeds regardless of their species do require good growing mediums, but there are exceptions to the rule for example. In Finnish quarries wind-blown seeds of the Scots pine Pinus sylvestris can be seen growing quite happily in sand – a growing medium of poor quality with little nutrients, yet they survive quite well. Alternatively a fruit bearing tree requires soil that is deep and fertile and free from high water retention. Good air circulation in the soil sustains health and promotes sturdy growth, it also discourages Botrytis cinerea, a common disease, causing a growth of fuzzy grey mould. Pythium a genus of parasitic oomycotes classed as fungi that can be transferred from the feet of the Sciarid fly or Dark-winged fungus gnat mentioned in the previous article Pest And Diseases.

 

botrytis-disease

It can be argued that seeds are delicate in their form, but in actual fact they are robust and quite hardy able to withstand high and low temperatures and can be stored in the right conditions for long periods of time. What is/are more appropriate to the delicacy issue is/are the seedlings that require care and attention until they mature.

If there are any questions relating to this article or any others posted on this site, please feel free to post them. The next post in about 2 weeks is a topic not often discussed but one of concern ‘Toxic bonsai’. Until next time BW, N.

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