It is common sense to work on a tree at a time when it is possible to obtain the best result, while interfering as little as possible with the plant’s natural activity. Causing stress to a plant will slow down its process of development instead of speeding it up, so timing your work is important!
Any intervention must have a specific purpose. To obtain noticeable results, it is very important to work on vigorous plants. Weak plants respond badly, and their survival may be put at risk. To boost the strength of a plant that you want to work on, you need to apply fertilizer in the previous year: nearly all species absorb and store up nutrients in order to use them during the following growth season. This article is written by Bruno Mazza, republished from the Esprit Bonsai magazine.
The tree’s yearly cycle
Within a given year, plants go through various phases:
- winter dormancy
- revival and flowering
- producing new growth and fruit
- a short period of summer dormancy
- consolidation of the new growth
- preparation for the winter dormancy.
The start points and lengths of the phases are very much dependent on variations in the photoperiod (the relationship between the length of the daytime and of night) and climate, which can alter the periods of rest or activity. For example, if there are very high temperatures, above 32–35°C, that last for more than a very brief spell, this will slow down or inhibit photosynthesis: the plant will then go into a period of dormancy that will not end until normal conditions have been re-established.
When to intervene
The time to intervene on a bonsai is usually dictated by the plant’s life cycle. Sometimes the right moment is limited to a very short period of just a few days: this is the case for pinching buds on maples. Sometimes, on the other hand, the work can be carried out over an extended period, by adapting the procedures or replacing them with other solutions which, if carefully applied, may even be preferable: this is the case for the repotting of some broadleaves in summer.
In general, the period for intervention more or less coincides with seasonal changes:
- in winter, the tree is dormant because of very cold weather
- in early spring, growth begins again
- in spring and early summer, there is major growth
- in high summer, the tree is dormant due to the heat
- in early autumn, fruit is produced and growth stabilises
- in late autumn, the tree prepares for the winter dormancy.
These are only rough guidelines, however. Geographical position and local microclimate are highly variable factors, so it is necessary to check the plant’s ability to adapt to the chosen location. Paying close attention to the biological cycle of plants to glean useful pointers and to choose the time and method of intervention is much more pertinent, and gives more accurate information than following tables of prescribed data.
Phase 1. During winter dormancy
Broadleaves are inactive during this period. The absence of leaves does not allow photosynthesis, and there is no activity in the organs. The part above ground has no need for either light or fertiliser. Stick only to routine maintenance, to avoid the possibility of infestation by parasites or fungus. The roots, on the other hand, need a certain amount of moisture in the soil to stay alive and not dry out. But take care not to over-water; otherwise you risk asphyxiating them. During this period, because of the low temperatures, water evaporates slowly and is not absorbed at all by the leaves. The soil dries so slowly that it is easy to forget to check it. This is a good period for a number of types of work on bonsai.
Most bonsai can be repotted during the winter, but the best period for this is the weeks immediately prior to the early spring revival, when you judge that there is no longer a danger of intense cold that could damage the roots. If repotting a tree in the middle of winter, you need to remember to put it in a place where is safe from frost – although not in a heated room, because you do not want to stimulate premature growth.
Broadleaves are generally repotted after hosing down the root ball to clean it and leave the roots bare. This provides an opportunity to have a good look at all of the roots every two to three years. You therefore need to take advantage of it to improve them, by removing thicker ones so as to stimulate growth of finer ones which absorb nutrients dissolved in water. The improvements must concern either the buried roots, or those on the surface, and the nebari which should be attended to at each repotting.
The root mass of a vigorous broadleaf can be reduced by 60%, or even more, without causing any suffering to the plant.
Repotting conifers is more complicated and carries a slightly higher level of risk than that of broadleaves. It should never be done with bare roots; instead, part of the root ball must be kept intact. The fine roots of conifers generally have more trouble forming. To absorb food, they need mycorrhizas, which therefore need to be retained.
The time available in late winter for repotting broadleaves is rather limited. Repotting must be suspended as soon as the first buds start to open. A second possibility for repotting arises in June, when the leaves from the first budding are mature. The period for repotting conifers is much longer and can extend right up to the point when the candles open for pines, or to the growth of new foliage for junipers: in practice, it can last from the first half of February until mid-June, depending on the microclimate of the location concerned. Cutting back the roots during repotting stimulates the plant to quickly produce new roots, new growth, and to repair its wounds: healing is very speedy in this period.
As for the above-ground parts of the plant, this late winter period is a good time for structural pruning, replacement pruning and maintenance pruning:
- Structural, or ‘hard’, pruning is intended to give an initial form to virgin material, reducing the length of the trunk and accentuating the taper if this is part of the design conceived for the bonsai, or removing unnecessary branches.
- Replacement pruning is used to reduce the height of a bonsai or the length of its branches. It thus serves to make the bonsai’s form more compact. This and the previous type of pruning use a smaller and shorter secondary branch to replace the one that has been cut off. If necessary, before growth starts again in the spring, this is the moment to carry out a second styling exercise, to go back over the design or perfect it.
- Maintenance pruning. It is also possible to use a very light pruning to get back the shape that has been lost through the new growth from the last season. This type of pruning, which is usually combined with wire training, allows the bonsai’s form to be re-established and the foliage to grow correctly. Repeated every year, this procedure leads the bonsai towards maturity and encourages its growth activity to stabilise.
It is always best to treat the severed ends after pruning with cut paste or adhesive aluminium tape, to protect them from microorganisms and insects, and to retain a minimum level of humidity, which is necessary for the development of healing cells.
The use of adhesive aluminium tape (0.5 mm thickness) for healing has been adapted to bonsai by the master Harumi Miyao, renowned in Japan as the greatest expert on Japanese maple, Acer palmatum. The technique consists of covering the perfectly flat (not concave!) cut with a piece of aluminium tape which goes 2 centimetres beyond the edge of the cut all round. Healing then takes half the time that it does with other methods, and there is no swelling.
Other tasks for the dormant period
The winter dormancy is a good time for:
- working on jin and shari (dead parts of branches or trunks), whether small or large
- major work on deadwood on junipers, pines and Prunus
- bending trunks and branches, small or large
- treating deadwood with jin seal to prevent it from rotting
- using jin seal as a preventative measure (dilute 1 part jin seal with 30 parts water) on trunks and major branches of broadleaves.
- On pines, just before maintenance pruning and wire training, if it has not been done already, it is possible to remove the old needles to allow more light to penetrate, to activate the dormant buds inside the tree and to sort through the terminal buds at the ends of the branches to leave just two of them.
Phase 2. Reawakening in early spring
At the beginning of spring, broadleaves need to be carefully inspected to spot the first signs of budding. Some species, such as elm and hornbeam, should initially be left to bud freely. Only a few weeks later, when the new shoots have developed at least four to six leaves, is intervention necessary by cutting with scissors after the second leaf.
A few weeks after the plant has started to grow again, this hornbeam’s new shoots need to be cut off with scissors after the second leaf.
Others, however – especially in the case of bonsai that have already been styled – need intervention to keep overly vigorous growth under control and avoid compromising the elegance and refinement of the ends of the ramification. This is the case for Japanese maples in the styling phase: if they are left to grow unfettered, during the season they will produce thick, straight shoots a metre long.
On the other hand, on a maple that is in the phase of refinement, intervention on the buds is required every day, from the moment the first ones start to open until the last ones have finished budding. The procedure consists of opening the first two leaves, which will be retained, and removing the leaves inside. This operation, which is called “pinching”, induces the growth of new and smaller buds, which will sprout from the axils of the remaining leaves. The procedure, which needs to be constantly repeated and is often carried out alongside deleafing, generates small leaves and an elegant and slender ramification, which is what characterises Japanese maples. Beech is an unusual case, because it only buds once in the spring. This idiosyncrasy has the result of increasing the time required for the ramification to densify.
Phase 3. New growth in Spring
All varieties of juniper (Juniperus communis, J. chinensis, J. phoenicea L., etc.), if well cultivated (with the necessary sunlight, water and fertiliser), bud continuously from spring to autumn. New shoots should be pinched back with the fingertips every 10 to 15 days, or trimmed with scissors two or three times during the season. If you choose to use scissors, then cut by inserting the scissor blades parallel to the stalk of the shoot.
As for firs, only large buds should be pinched, when they reach 2 to 3 centimetres (about an inch) long. Break them off with your fingers, halfway down. Leave small buds to develop, to balance the energy of the different areas of the tree.
Plants have a natural tendency to grow most in the areas that have the greatest amount of light, so that they can photosynthesise as much as possible. The bonsaist’s task is to spread the growth as evenly as possible, to balance out the energy across all areas of the plant. Pinching is needed for the most vigorous buds, to limit their development and encourage strengthening of the weaker ones. Usually you should only keep the first two buds, and remove the others. However, in particularly thick areas, it is better to leave only one, while leaving three or even four in the thinner areas.
This procedure should be repeated as and when necessary as the new growth comes out, and should continue throughout the growing season.
Flowering and fruiting plants are a separate case, because they should be left free to grow. They should only be pruned at the end of summer, after the differentiation between flower buds and leaf buds has been established. Flower buds develop at the bases of branches that have grown during the year.
If you prune the branches before the buds are differentiated, which generally happens around the end of June, the reduction of the leaf surface area obliges the plant to increase the number of leaf buds and abandon flower buds in order to re-establish a balance in the existing foliage.
The months of May and June are a period of intense activity in pines. If you want make a pine into a bonsai, pinching candles and pruning new shoots are absolute musts. If small branches are not slowed down, they will continue to grow longer in all directions, seeking out as much light as possible. They will then be impossible to make more compact in order to create foliage pads. The energy will then be concentrated around the thickest areas – the branches at the top of the tree and the ends of the other branches – as is the case for almost all plants, which will increase the energy in these spots, to the detriment of the thinner areas which will end up perishing. To reverse this tendency, you need to cut off a greater or lesser amount of the candles. The period to do this is difficult to establish a priori, because it varies according to the specimens concerned and the geographical region. What you need to remember is that, on vigorous specimens, candles should be pinched when they have developed adequately, to somewhere around 3 centimetres (an inch and a quarter). To pinch them back, take the candle between your thumb and index finger and cut it off, while gently twisting it. You should not use scissors, because if you do, the ends of the needles will turn black as they develop. When candles do not open all at once, you need to pay careful attention and pinch them all back bit by bit, as they develop.
Pinching candles on vigorous pines, using fingers and a slight twisting gesture.
When the plant is at the height of its activity, it is the right moment for layering. Sap is flowing abundantly and all the organs are working at a good pace, which encourages roots to sprout quickly. Layering (also called marcotting) can be used to thin down an overly thick trunk, to create a new specimen by using an interesting part of an over-thick plant, to improve nebari that is not particularly attractive, etc. The procedure takes advantage of the capacity many plants have of rapidly producing new roots at a point where a ring of bark has been removed. Layering is an easy method of multiplying numbers of plants. It is used a lot to produce specimens with good proportions and interesting characteristics, and quite quickly at that.
Cuttings are a reproduction technique that allows plants with perfectly identical characteristics to the parent plant to be obtained. Again, this technique takes advantage of the capacity that certain species have – junipers and almost all broadleaves – to put out roots. Pines are not recommended here, because they usually have too slow a metabolism for the cutting to take root before it dries out. If the part destined for the cutting is well chosen, the new plant will have good characteristics, but will need plenty of time in order to reach the appropriate dimensions for creating a bonsai.
Phase 4. Summer
Sometimes, for lack of time or by force of events, you cannot repot during the ideal period, which is generally at the end of winter. Some species can be safely repotted outwith this season, usually in June, by taking certain precautions. This summer repotting is done when broadleaves are mature – that is, when they have finished developing and are completely fulfilling all their functions (first and foremost of which is photosynthesis). They change slightly in colour (growing darker), as well as in texture – they become more resistant, and rubbing them between your fingers makes a sound similar to rustling a sheet of paper. This is the best time for an “off-season” repotting. In summer, roots should only be cut back by a maximum of 40 %, while in spring they can be cut back by up to 60%. In addition, it is likewise advisable to cut back the foliage, by defoliating to a greater or lesser degree, so as to improve the water balance – between absorbed and evaporated water. After repotting, it is important to protect the plant from direct sunlight and wind for a few weeks, and to shelter it in a bright spot. To ensure the substrate has the right level of moisture, which encourages the plant to sprout and develop radicles, on the surface where it dries out most quickly, cover the soil with a layer of small pieces of sphagnum; these can be left permanently in place.
If you work carefully, broadleaves generally withstand being repotted out of season without any problem at all. For conifers (pines and junipers), since their repotting period is longer, the need to repot them in June is eliminated, and all the more so since June is the month when candles develop and new needles and shoots are put out; and it is dangerous and not right to stress plants without good reason.
Pruning new shoots on pines
During June, and sometimes up to mid-July depending on the climate and the specific characteristics of each plant, new shoots on pines should be cut back with scissors. These new shoots are buds that have become candles and have sprouted needles. The aim of pruning new shoots is to keep the lengthening of the branches under control, to encourage dormant buds to be roused and consequently to form dense, compact ramification, as well as reducing the length of needles.
In August, below where new pine shoots have been pruned, buds will grow that will be selectively sorted in autumn.
As high summer temperatures start to arrive – above 32-35°C (90–95°F) – almost all plants stop or considerably slow down their growth rate. Metabolism, photosynthesis, transpiration and gaseous exchange undergo major changes, forcing the leaves to put mechanisms in place that can reduce the absorption of heat and can dissipate it as much as possible through stomata. Plants stop growing and go into a state of rest. When conditions return to “normal” for them, all their mechanisms start to work normally again.
During this period, full attention needs to be paid to watering: often, very high temperatures and wind will dry out the substrate very quickly and literally burn the dehydrated leaves. The leaves begin to go dry (turn brown) on their outer edges, and if the plant is not quickly watered, including being sprayed with a fine mist, they will end up drying out completely. It is easy to understand that in order to function properly, plants need a quantity of water that is at least equal to that used to carry out all their functions. When the amount of water that is lost through foliar transpiration, evaporation from the substrate etc. is greater than the amount being absorbed, the leaves will wither.
If the imbalance is only slight and does not last long, normal conditions can quickly be re-established by watering, and no permanent damage will be recorded. Only photosynthesis and growth will be temporarily inhibited. Without watering, the loss of water will spread from the leaves to the trunk and right down to the roots, and the leaves will fall off within a few days. As with leaf loss in late autumn, the loss of all leaves in advance through drying out is not always a sign that the plant is dead. If the buds for the following spring have been produced, and the dehydration has not affected the branches, trunk or roots, the plant will bud normally in the spring.
If the plant is rehydrated, it may even recover during the season that is already underway, and reconstruct its root system and buds.
Phase 5. In autumn
Once the hottest period is past, in late August or early September (depending on the region), temperatures return to normal, below 30°C (86°F), and plants then readopt their normal growth activity. This is also the moment when attention needs to be focused as much as possible on fertilisation, which needs to supply the plant with the substances it needs to develop, consolidate new growth, reconstitute exhausted reserves and strengthen itself to best cope with the winter season.
In autumn, it is preferable to use fertilisers that are low in nitrogen, which is better for encouraging vegetative growth, especially for young plants and those that are in the construction phase. The feed also needs to be rich in phosphorus – to stimulate root growth and prepare a good flowering for the spring – and in potassium, to strengthen the plant, increase the roots’ assimilation capacity and consolidate the new growth that has been produced during the season.
Chemical fertilisers, which should be used with great care, can be put quickly to use by plants. Conversely, the nutrients of organic fertilisers need to break down by fermentation to be assimilated: this is why they can only be used by the roots 20 to 30 days after they have been applied.
Phase 6. Winter rest
Winter is the time when, after lavishing care and attention on plants, you can finally reap the rewards of your efforts and enjoy them: trees offer up bunches of little apples, orange kakis, a whole host of multicoloured berries and splendidly coloured leaves. But there are still some small tasks that need to be done, to guide plants towards their rest period as best as possible:
remove old pine needles – those that have not grown this season – to allow light to penetrate between the branches and activate the reawakening of dormant buds;
remove dead leaves on broadleaf trees, to prevent stagnation of the moisture from creating problems;
treat branches and trunks of broadleaves with jin seal to guard against disease and fungal infections (1 part jin seal to 30 parts water).
Winter has arrived, and this shohin laden with kakis is a joy for the eyes and the spirit.
Experience and observation
Each plant is a unique specimen as far as its physiology, aesthetic appearance and energy are concerned, all of which can influence its reactions to intervention. A plant’s age can likewise deeply affect the speed and level of its response: the primary objective of young plants is to develop and grow, so that they can attain reproductive functionality as quickly as possible and accomplish the cycle of life. A mature plant is more inclined to maintain its situation and keep itself going by slowing down its organs’ functions. Each situation is different and requires adaptation of the methods and moments of intervention. And each bonsaist’s experience also plays a very important role. - Many thanks to Bruno Mazza and the Esprit Bonsai magazine.