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.


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.



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.


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.



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.


Pests and Diseases


This post is probably not the most exciting of topics to discuss, but we should be mindful of the pests and diseases affecting bonsai especially those, which cannot tolerate cold conditions and are kept inside. All flora are susceptible to attack from pests and diseases whether grown naturally or cultivated. In an attempt to eradicate these unwanted problems most fruiting and flowering specimens are sprayed with a fungicide or repellant. Chemical protection does in many respects produce the results intended, but some will argue that it also destroys those creatures that eradicate the unwanted. Moreover, it can be said that when winter arrives many pests will die due to loss of foliage and the coming cold.

Nonetheless, there are such pests whom are able to survive via hibernation hidden beneath a tree’s bark or in it’s seed pods. For example, the Oak – genus Quercus, a favourite of the Tortricid Moth caterpillar – Family Tortricidae that destroys acorns. Carpenter Ants – Genus Camponotus hibernates under pine tree bark. The spruce beetle, Dendroctonus rufipennis its larvae bores into the phloem of conifers and feeds on the live tissue. And the Asian longhorn beetle Anoplophora glabripennis a native of China and the Korean peninsula excavates 1cm diameter holes in the main trunk causing sap bleeding. Eventually the affected tree dies.

Diseases include; ‘Red band needle blight’ Dothistroma, needle blight affects conifers most commonly pines. It causes needle loss, which eventually kills the tree. Ash die-back Fraxinus excelsior affects ash trees it is caused by the fungus Hymenoscyphus fraxineus, that blocks the tree’s water transport system causing leaf loss and ultimately die-back of the tree’s apex or crown. Horse chestnut canker a bacterium species known as Pseudomonas syringae pv. Aesculi. It causes extensive bleeding areas on tree stems. Phytophthora austrocedri affects junipers causing die-back of foliage, stem and collar lesions and eventually death.

The above mentioned pests and diseases are but a few of the many that exist around the world, attacking many species of tree both deciduous and coniferous. Arguably such devastation is due to climate change and infestation via unwanted importation of packaging material. For example wooden boxes and crates and although authorities do much to enforce regulations, it is difficult to halt the invasion.

With bonsai that must be kept away from cold conditions either pre- (trees in training) or established, they too are susceptible to pests and disease. We like to assume that our indoor environment is free from miniature beast invasion – nothing could be further from the truth. The ‘greenhouse’ or home is riddled with the little monsters and no matter how vigilant we are, eradication is virtually impossible. Arguably, the cause why these mini beasts are able to thrive successfully is due to the temperate conditions abundance of food and water.

In the article ‘New material for bonsai part I’ a paragraph discussing the Red Spider Mite was given including an image depicting its appearance. As stated, evidence of these microscopic arachnids existence was revealed due to their very fine webs, which allow them to roam at will. Getting rid of these unwanted pests is in itself a difficult task. But can be achieved using horticultural sprays and soaps. Other common pests causing disease are described as follows.

Mealybugs – quite visible to the naked eye are related to scale insects and congregate on leaf joints and the undersides of leaves. They damage plants by sap sucking, which causes the leaves to wilt, turn yellow to brown and eventually fall from the plant.Removal of this pest(s) can be achieved either by spraying them with water or via a chemical spray designed for Aphid treatment. The plant then should be isolated from others until the treatment is successful and the plant has recovered.



Aphids – normally a pale green in colour can be found in other shades such as grey and black. Having arrived on a plant their numbers rapidly increase infesting the plant in great hordes with a preference for the underside of leaves. As with the Mealy bug they also suck the sap from the leaves and if not removed quickly, the plant may become infected by disease and viruses. Aphids can be removed relatively easily by the use of warm soapy water directly sprayed on them. Alternatively a chemical application designed for this pest can be used, but the plant should be isolated until the treatment is completed and the plant is free from infection.



Scale Insect – there are more than 25 species of these limpet-like creatures, which makes identification difficult due to their well camouflaged appearance. They devastate a wide variety of plants by sucking the sap and as a result the plant is severely weakened distorting growth. Evidence of their existence can be seen as the growth of black, sooty moulds and or a sticky substance (honeydew) on foliage. Another sign of scale infestation is leaf blemish. Scales have hard shells and removal can be difficult hence the use of chemical application such as an Aphid spray, which softens the shell eventually killing them. The plant has to be isolated until the treatment is successful.


Sawfly Craesus septentrionalis – can be a real nuisance for those including myself whom have Betula species (Birch) as bonsai specimens because, the tender young leaves are prone to be ravaged by the Sawfly larvae running rampant all over the tree. As do the larvae of a large number of species of butterflies, moths and other insects. Female sawflies are so called because of the saw-like appendage at the tip of their bodies, which is used to cut slits into the leaves where the eggs are laid. There are different species of sawfly and the damage cause by their larvae is peculiar to each species for example. Some will leave notches or holes in leaves or devour the leaf leaving just its skeleton, others spin webs, leave galls and some will roll up a leaf completely. Sawfly are commonly found in bonsai for example. Conifer sawflies that feed on needles and bore into buds and shoots. Salix (Willow) the sawfly leaves distinctive red/brown galls, fruit and flowering – Prunus, (Cherry) Pyrus, (Pear) and Malus (Apple) are all affected by the sawfly.

The most common way to eradicate sawfly larvae is either removal manually or by using a horticultural soap as used for Aphids and Mealybugs, but the plant should be isolated so as not to infect others and to allow for the treatment to work.


Sciarid flies often called Dark-winged fungus gnats are commonly found in moist environments including areas where house plants are situated. They thrive on damp soil conditions and can be seen scurrying over the soil, flying around and landing on stems, branches and leaves. Although they are known to be a pest in mushroom horticulture, they present no threat to bonsai plants nonetheless, they can be extremely irritating especially in a home environment. This large Diptera genus is one of the least studied mainly due to its small size 2mm and the difficulty in specific identification. It is said that more than 1,700 species have been described with an estimated 20,000 awaiting further study.



SpringtailsCollembola form the largest of the three lineages of modern hexapods the other two being Protura and Diplura these creatures are not classed as insects, because they are omnivores having internal mouth parts. They are small white or grey in colour and feed off the soil’s dead organic matter. When plants are watered and springtails are present, they are agitated and move rapidly and look unsightly. Springtails pose no threat to bonsai or other house plants, it is their very presence which can be irritating. If the desire is to eradicate these creatures, one can water the plants from the bottom by immersing the container in water and/or reduce the amount of water. But water reduction may not be conducive to some plant species, hence a careful balance should be maintained.



The above mentioned pests and diseases are common to bonsai horticulture, they are but a few of the many thousands that are in existence and to further research them would involve eons of time. Nevertheless, over the past decades horticulturists and scientists have done much of the spade-work hence it is not that difficult to find the answer one is looking for a particular problem. But it pays to be vigilant and inspect your bonsai specimens on a regular basis to ensure they remain in a healthy condition. As the old saying goes – ‘Prevention is better than cure’.

Another factor to consider is the soil composition, because bonsai are confined to a relatively small quantity of soil and this growing medium has to fulfill its needs. It must be able to retain water yet have good drainage and have the ability to allow for air circulation, without this basic criteria the tree will suffer. Soil contains a multitude of living organisms that consume, digest, and cycle nutrients. These living organisms include archaea, bacteria, actinomycetes, fungi, algae, protozoa, and a wide range of insects for example. Mites, nematodes, earthworms and ants all of which are important to the vitality of a soil composition.

Soil in a bonsai pot does not last indefinitely, it decays over a period of time due to the deterioration of living organisms and once expired it is unable to support the tree in order to sustain health and growth. This is when the tree is most vulnerable and attack from pests and disease can quickly take hold. Having said this one may think that a bonsai has to be re-potted every year – not only is this a misconception, it is unnecessary. A tree in a pot or container planted in year one will take at least 2 to 4 years to establish itself although much depends on the species and its growth rate. This can be assessed by teasing the tree out from its container and checking the root ball. If the roots are densely packed with little soil in situ, then it will probably need re-potting alternatively if what is seen is the opposite, then it can be re-placed and left for another season.

For example, one may think that this 13 year old Cotoneaster Lucidus (23cm) planted in a shallow pot (3.5cm x 22cm) would have a substantial root ball and become ‘pot-bound’ after one season. But this specimen is not a prolific grower either above or below the soil level nonetheless, it is inspected bi-annually in the spring. If there is room for the roots to develop it is returned to the pot unmolested and left to grow, but will be fed on a regular basis to ensure the nutrient up-keep. Root pruning and soil change will take place either in year 3 or 4. The next article ‘The need for stratification’ will probably be posted on 29th of January. Until next time BW, N.


The Problems With Bending Or Shaping

Bending branches and trunks – what happens to the cellular structure?

A recent question asked by a student is what happens to the tree or plant when it is wired into position. This may seem a relatively simple question to answer but all trees are different, much depends on the climate zone where they are found. Meaning some species are more malleable and easier to shape, whilst other are more difficult.

For example, Lignum vitae also called ‘guayacan – guaiacum’ or ‘pockholz’, from the genus Guaiacum has a structure so dense that wiring is virtually impossible unless undertaken before lignification has taken place. Alternatively, the Cecropia genus are mainly hollow in their structure and thus are extremely difficult to shape. Some species are prolific in growth (Acer palmatum amoenum) are easily scarred by wiring, whilst others Larix kaempferi (Weeping larch) are slow-growing and more manageable. Arguably, researching a particular species gives rise to a more understanding of its characteristics.

The cellular structure of a tree’s trunk and branches are referred to as the xylem. The inner most part of the trunk consists of the heartwood, which although inactive gives strength to the tree. The sapwood, carries moisture from the tree’s roots to its leaves. The cambium layer is where the tree increases in girth or diameter. The phloem commonly known as the inner bark, carries nutrients manufactured by the leaves down to the roots allowing them to develop. The cortex area gives support and elasticity enabling the tree to withstand bending. The outer bark is the trees protection from injury. The following two images which are NOT to scale, will give some idea of how a tree’s cellular structure and its components work.



As we can see there is much activity within a tree’s structural system and remaining undisturbed will function normally. However, when bending or shaping a trunk or branch the cellular structure of the xylem becomes disrupted. Meaning the outer radius of the bend will be in tension whilst the inner radius is held in compression.

How does the tree cope with this disruption? – More than 90% of a tree’s cells are long thin tubes closely packed together, arranged along the direction of the trunk or branch. Their function is to transport the nutrients and water from the leaves (Phloem) and roots (Sapwood) respectively. They also provide support because, no matter which way a trunk or branch is bent, the internal forces always act parallel to the cells and are able to adapt to tension and compression. In addition, because the cells are hollow, the tree’s trunk and branches can be thicker as opposed to it being a solid mass.

Bending or pre-stressing

A tree does pre-stress its trunk and branches quite naturally as it has to combat the forces of nature. Nonetheless, the long thin-walled tubes in a tree’s cellular structure, have a tendency to collapse when severe bends are applied and this can cause problems for example. Many trees are susceptible to a condition known as ‘brittle-heart’, because as heartwood ages it can be attacked and broken down by fungi. Eventually the tree becomes so weak it dies.

Another problem with bending trunks and branches is that if a bend is too severe, the cortex may splinter or break resulting in damage to the phloem and a damaged phloem is not just open to attack from Lepidoptera and Fungi, it can weaken the movement of moisture and nutrients. It can be agreed that all potential bonsai will at some point in their existence, be subjected to some form of design. In order to produce a miniature copy of what is found in nature and this is achieved via the use of wire and applies to all species both coniferous and deciduous.


Regardless of whether the specimen has been grown from seed or collected from other sources, shaping a tree’s design can often call for some drastic measures. With deciduous species much of the work can be achieved in a shorter space of time than with coniferous varieties. But any severe bending should be done in stages to eradicate the problems mentioned above. Once the initial design has been accomplished, further shaping is usually achieved by pruning, but if additional wiring is needed it should be checked on a regular basis to ensure it is not causing damage. And of course, knowledge of the tree’s anatomy and development in natural circumstances is an advantage in determining this.

Another technique used for deciduous varieties is Uro, (Discussed in a previous article) where a small indentation is left after the removal of a branch. As new wood forms a protective callous around the wound, it forms a small hollow. This is later enlarged going deeper into the trunk and in some cases through to the other side leaving a hole, presumably to create character. But regardless of how deep one bores into the xylem, the tree is susceptible to attack from disease and many varieties are unable to survive. To combat this many bonsai enthusiasts use fire, sealants and pastes to protect the wound, others leave the tree to fend for itself – ‘a natural course of events found in nature?’


Conifers regardless of origin (Seed or other sources) are subjected to the same shaping techniques although Uro is seldom one of them. Conifers have more elasticity when bending for example, long branches can be shaped and reduced in length to add character to a tree. In some cases the branch is cut at strategic points and wired down as shown in the next 2 images.



However, this practice does require some thought prior to undertaking because, the branch can be attacked by disease and restriction of nutrients and moisture to other areas. Once the cuts have been made and the branch is in position secured by guy wires to it’s container, the wound areas should be protected, but some disagree. It is said that a conifer’s natural defence system will heal the wounds via it’s resin or sap.

Other methods for severe bends can be used for example. Wet raffia is tightly wound around the area in question, then wrapped in a rubber tape then the wire is applied and the limb bent into position. In some cases a strand of wire the length of the bend is inserted between the raffia and tape. The ideology behind this approach, is that the inserted wire (Placed on the outer radius of the bend) will protect the bark, cortex and phloem from splitting although much depends of the severity of the bend. (Shown below)

conifer-wiring-cBut it should be known that conifers take some considerable time before a wired branch or trunk will stay in its given shape. Quite often more than one growing season is needed to achieve the required result, but much depends on the species and diameter of the trunk or branch in question.

Other techniques for conifers include Jin and Sharimiki which have been explained in a previous article. Such techniques of removing bark and creating deadwood effects are said to add characteristics to a particular specimen, giving a natural appearance that is highly prized in bonsai. Nonetheless, what ever decisions are taken when designing and shaping trunks and branches; care, patience and the health of the tree is the most important factor. Until next time, BW, N.