Physiology of Shaping Bonsai Trees

All of the efforts involved in creating a bonsai rely on a basic understanding of plant physiology. You need to understand how a plant grows and the factors which affect that growth in order to shape your tree as desired. The art of bonsai is creating a pleasing balance of form. The science of bonsai is the physiology of shaping.

The Shoot

The shoot is the basic unit of the plant. It includes a stem and its leaves. A plant is essentially a large shoot with multiple smaller shoots growing off the main one. New shoots are formed by budding. This growth process is aided by watering the plant. As the water is absorbed into the cells of the stem, it enters the cell’s vacuoles. This increases the cell volume, which causes the cell to grow. The growth proceeds rapidly in a longitudinal fashion. This increases the length of the stem and encourages the buds to mature into new shoots.

The ideal trunk of a bonsai is one with a broad foundation and tapered growth. This can be encouraged by promoting the vertical growth of the trunk. This can be done by selective pruning. The stem will also thicken if it has a large number of branches. This is because the trunk has to expand to accommodate the extra nutrient flow to the extra limbs. To encourage the desired taper, limb growth should be limited as you move up the trunk. This principle can also be applied to individual branches if you are seeking to create a long, tapered form.

Because of the essential purpose water plays in the growth of a plant, water is necessary for new and continued growth. When water is not available, the vacuoles collapse and the plant wilts. A consistent source of water ensures a strong, vibrant plant.

The Root

The root is the other basic structure of a plant. In its simplest terms, the shoot is above the ground and the root is below the ground. This is how plants are conventionally understood to be and how they usually are. The soil line is not always an accurate way to distinguish between shoot and root since some plants’ roots grow above ground and some shoots descend into the ground. The bonsai has a root structure that is both below and above the ground. The external roots are called nebari.

Shoot And Root Balance

Shoot and root develop in generally opposite directions from each other, growing in symmetry away from the middle. In the area where shoot and root are closest, cells grow and mature. Cell division is slow as the existing cells get larger and stretch out on a vertical axis. In contrast, the ends of the shoot and root away from the center are areas of rapid, active cell division.

The growth of the roots mirrors that of the shoots. The larger and more developed the shoots become, the more nutrients they need from the root structure. The roots grow to compensate for the increased demand. The roots and shoots will maintain a steady, proportional relationship as the plant grows or is reduced by something like pruning.

Balance between the root and shoot includes balances in water and nutrients. It also stays balanced to accommodate the plant’s increased size and structural requirements. The roots become stout and large as the plant’s size increases to provide extra support for the growing tree. Conversely, if the roots are inhibited by an overly small planter, the shoot will quit growing because of the inability to grow an increased root system to support a larger shoot. This can also happen if the roots are damaged during repotting. In either situation, the plant may show signs of compensation such as losing leaves or wilting. Once the roots are given some time to grow and adequate space to develop, the shoot can regain growth accordingly.

Root Pruning

Root pruning is necessary to create the ideal root structure for a bonsai and make sure that the plant has adequate water and nutrition. Pruning the roots regularly when the plant is young allows the bonsai to develop the desired thin, abundant masses of roots instead of a large tap root and minimum smaller roots.

A tree growing in nature requires a prominent tap root and large root structure to provide physical support for the massive tree as well as supply the large supply of nutrients needed. A bonsai’s needs are different. It does not need the prominent anchor a tap root provides. It needs the fine, thin roots to maintain the reduced size of the tree while still adequately nourishing the tree.

Pruning the tap root and other thickened roots will encourage fine lateral roots to develop. This provides proportionally more nourishment to the tree, which assists in the development of the lateral buds after the branches have been pruned. Healthy shoot growth is proportional to the healthy, fine root system it has. Cultivating your bonsai’s roots will allow the plant to mature as desired.

Secondary Growth

Plant growth progresses from a single shoot to growth of new shoots from that initial shoot. This branching will continue and the plant’s size will grow. The plant will become larger, and the initial stem and supporting stems will all thicken as new stems emerge. Stem growth increases due to increased demand for water and nutrients requiring larger channels for transport and increasing the diameter of the stem. To keep up with that increased demand, the roots also grow.

The thickening of the stem is called secondary growth. This horizontal growth occurs in cell division tissue called the vascular cambium. This increased tissue thickens the cell walls and eventually dies and forms hardened supportive tissue for the stem. Secondary growth happens in roots and shoots. Although secondary growth does not occur in all plants, it occurs with most plants used in bonsai and is an important concept to keep in mind when developing a thickened trunk and an overall tapered form.

The Relationship Between Node Length And Growth

A bud is called a node. It is the emerging growth from a stem. Node growth indicates the general growth of the tree. Nodes emerge as the stem grows, and the space between two of them is called the internode. Balanced growth for the tree is desired, but when growth is excessive, the internode can become overly long. This can be due to many conditions such as excessive fertilizing. Overly long internodes are a problem because they encourage a larger tree size. This is counter to bonsai goals and will need to be monitored and pruned as necessary.

Types Of Buds

Bud growth creates new stems and allows you to craft the bonsai’s shape as needed. Buds appear at different points of the stem. Buds at the end of a shoot are called apical, terminal or end buds. Buds in the middle of the stem are called lateral or axillary buds. Buds that develop from the trunk, roots, or leaves are called adventitious buds. Plants which have flowers form flower or floral buds. Buds which form leaves are called leaf or foliar buds.

Once a lateral bud has produced a shoot, it becomes an apical bud at the end of the new shoot. These shoots that branch off from other stems are called lateral branches. This is the basic growth of a plant, and selectively choosing when and which buds to prune will guide the bonsai’s growth.

Deciduous And Coniferous Trees

Deciduous trees include trees such as the various maples. Deciduous trees grow new leaves every spring. This growth can continue throughout the summer until complete leaf loss in the autumn. This natural cycle of leaf growth and loss aids the bonsai grower, giving a period every year where the branches can be clearly visualized to predict future growth. This aids in wiring and pruning the tree.

Coniferous trees include the many pine varieties. Coniferous trees have leaves on its branches year round. Their leaves are usually the characteristic needles of pines, although some coniferous trees have more traditional leaf shapes. The growth is also often green regardless of the time of year, which is why many coniferous trees are known as evergreens. Leaves on coniferous trees have a 2-3 year lifespan.

When To Prune The Roots Of Deciduous And Coniferous Trees

Root pruning is done on a young, developing plant to create an ideal root set and help regulate the size of the final tree. Desired roots will be dense, clumped balls of thin, hair-like roots. This will be accomplished by pruning the tap root and monitoring root growth through repotting.

Deciduous trees cyclically grow new leaves every spring and lose their entire growth of foliage every autumn. Root pruning for deciduous trees typically takes place in the early spring and late autumn. This bookends the growth season for the trees, before nutrient levels increase for spring and summer bud and leaf growth.

Coniferous trees are evergreen, since their leaves are in a constant, rotating cycle of replenishment. The best time to prune their roots is when they enter their period of dormancy, in late autumn until freezing conditions begin. At this time, the tree’s energy levels are reduced and soil temperature is still warm enough to allow any pruning wounds to heal.

Sometimes you may want or need to prune the roots at other times. Sometimes roots can be damaged or lost during repotting, for instance. In a case like this, you will need to also prune some leaves to compensate for the lost portion of roots. When deciding what part of the plant to prune, keep in mind that the roots tend to serve the shoot structures on the same side of the plant. So, if root loss occurs on the right side of the plant, focus on pruning some of the leaf growth on the right side to compensate.

Achieving Keisho-Sodai By With Balanced Root And Shoot Growth

Stem and bud pruning is necessary for bonsai creation. The emerging plant will naturally want to grow upwards and outwards at a relatively brisk pace. Pruning will contain and control the growth. Focusing on nurturing the lateral buds is of special importance to the bonsai grower, since they allow the bonsai to take its characteristic shape with proper tapering while maintaining a small overall tree size in the spirit of keisho-sodai’s “small size, great similarity”.

Prune the buds which form on the outside of the tree’s frame. Also target the buds on the branches with vigorous growth. These branches tend to have long internodes, and discouraging that growth is important. As you choose what to prune, remember to look for balance in the tree’s form and keep the supply of nutrients steady for the growth it is supplying.

Encouraging Balanced Bonsai Growth Through Trunk Development And Selective Pruning

The ideal bonsai form is a miniaturized version of its full grown counterpart. This will require some special care and work to accomplish. In the beginning, it is important to establish a large, thick trunk that tapers as it rises. Since you are starting with the same seed that typically develops into a large tree, its trunk will not adequately thicken on its own when on the scale of a bonsai. Trunk growth can be encouraged by letting the roots grow adequately. This can be accomplished by giving it a spacious pot to grow in or letting it begin its growth in the garden. You will also need to provide the proper amount of fertilizer and water.

Once the trunk has developed, the bonsai’s final form begins to take shape. Pruning branches, buds and leaves lets you guide the growth to the ideal form. Bonsai growth can be manipulated depending on what you choose to prune. For instance, if you prune away a large amount of growth, the plant’s overall water and nutrient need diminishes. Provided that the root size remains the same, the roots now are able to channel those excess nutrients to dormant lateral buds. This creates new growth from those buds.

If you prune the stems above the lateral buds, vertical growth will cease on the pruned stem. Growth of lateral buds will continue. New shoots will bud and extend at that site. If the lateral bud has begun to develop, the emerging shoot will diverge even more in its growth. This new shoot will be thinner, which can be advantageous when developing the overall tapering form of the bonsai.

Auxins And Cytokinins: The Hormonal Growth Regulators Of Plants

Auxins and cytokinins are two plant hormones that regulate and promote cell division in a plant. They are chemicals which are produced in different parts of the plant. Understanding how they work can help predict growth and make decisions to promote a good bonsai form.

Auxins promote the growth of lateral roots. They are synthesized predominantly in the apical buds on the branches. The auxins are transported to the roots through phloem, which is a channel within the stems and trunk that moves products in one direction, to the roots. Cultivating apical roots encourages auxin synthesis and promotes the growth of the desired thin lateral roots.

Cytokinins work in the opposite direction. They are synthesized in the root tips and transported up the plant through xylem, the plant’s counterpart to phloem, whereas phloem moves one way from the stems to the roots, xylem moves one way from the roots to the stems. Cytokinins are deposited in apical buds and encourage cell division and growth. They also encourage auxin production in the apical buds, which creates a feedback loop. Cytokinins are not deposited in the lateral buds, so growth is suppressed to them. Understanding the cytokinin and auxin influence on bud and root growth will let you make decisions to encourage the type of growth your plant needs.

Xylem, Phloem And The Photosynthesis Cycle

The inside of a plant’s stem is similar in most cases, regardless of species. Vascular cambium forms the outer layer of the stem. It contributes to the growth and thickening of the stem as well as the formation of the protective plant wall. The inner part of the stem is mainly composed of channels of phloem and xylem. Phloem forms the outer channels. It transports carbohydrates from the leaves towards the roots. Xylem is in the innermost part of the stem. It flows in the opposite direction of phloem, transporting nutrients to the stems and leaves.

The carbohydrate products transported by phloem are produced in the leaves through photosynthesis. Xylem’s nutrients include water, nitrogen and inorganic minerals which are provided to the roots by water and fertilizer. When xylem brings water to the leaves, it is broken down to hydrogen and oxygen. The oxygen is released into the air and the hydrogen is used to manufacture carbohydrates which provide energy for the plant. Once the carbohydrate products reach the roots, they combine with nitrogen in the soil to produce amino acids and proteins. This is the basic building block of growth and is necessary for the continued vitality of the plant. This cycle repeats itself throughout the life of the plant.

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