Korean Hornbeam

Korean Hornbeam

There are three main components to tree physiology and they are the roots, the trunk and the crown and here is a brief guide in the context of bonsai trees which are only a scaled down version of a regular tree. Any tree or shrub or plant can be used for bonsai but some work better than others so select your species well and some of the elements of the bonsai tree will work better for this.


Roots or ‘Nebari’

Bonsai tree roots

So always start with the bonsai tree roots, this is the most important aspect for the health of the tree, if the roots are good the tree will grow and develop and remain living, if the roots are bad they won’t. One of the key differences with bonsai is that the structural roots replaced by wiring into pot and this allows the tree to remain secured into the pot and not put energy into developing thicker supporting roots and focus more energy on developing fibrous roots. The whole root area in bonsai is referred to as the nebari and this is can be a key feature aesthetically that often is under developed on bonsai trees if not enough attention is paid to it initially in the early stages of the development.

Fibrous root system

Fibrous roots are the key to a good bonsai, they;

  • Enable good water uptake
  • Enable good nutrient & mineral uptake
  • Allow for good levels of gaseous exchange
  • Enable beneficial symbiotic relationships with beneficial fungi

Root development

Root development should be the first priority on a bonsai tree to develop before focusing on the trunk or the crown, so it is essential to sort out the roots first and look at how to improve the roots through development.

Root development techniques vary depending on;

  • Tree Species
  • Stage of the trees training / development
  • Timing
  • Condition of the roots initially
  • Health of the roots initially
  • Seasons (frost impact / late seasonal changes)
  • Desired effect
  • Style of bonsai – i.e. root over rock, exposed root, formal upright, leaning/ windswept
  • Type of material used to create bonsai
  • Type of pot or conditions the bonsai tree is kept in


Typical bonsai tree root problems are;

  • Crossing roots / circling roots / Pot bound
  • Large structural roots / dense roots
  • Uneven roots / one sided
  • Deep thick roots
  • Lack of fibrous roots
  • Dead roots
  • Root rot
  • Infected roots

Remove bigger to allow smaller roots to develop

All the above should be addressed prior to starting to develop a bonsai further to ensure that longer term these issues have been developed out to achieve the fibrous root pad the bonsai tree will need for longevity.

Fibrous root system

Bonsai root work options include:-

  • Full re-pot of all roots
  • Total root removal
  • Selective pruning/ thinning of roots
  • Under cutting of the roots
  • Pegging or wiring of roots
  • Cutting just round the edge of root plate
  • Cutting out a wedge
  • Adding roots via grafting

Root grafts

Root growth

Hormones are essential for root growth and these are produced by the tree and the two key hormones are;

  • Cytokinins stimulate cell division in root meristem
  • Ethylene stimulates root growth

Nutrients are also involved in forming roots and root growth include;

  • Sulphur (S)
  • Phosphorus (P) (Bone meal)
  • Copper (Cu)

Vine Weevil larvae

 Root pests

It is important to check the roots for pests and diseases as again the roots are fundamental to the trees health. Key times to check would be re-potting but also if the tree starts to show any signs of stress or decline then it is worth examining the soil and the roots to see if any pests or problems are present. The key root pests are;

  • Vine weevil
  • Ants
  • Mealy bug
  • Slugs / Snails

Mr Sekiyama repots an Satsuki Azalea


  • Ensure correct soil type for species and that drainage is good through the soil mix
  • Ensure pot has good drainage holes and wire holes for securing the tree into the pot
  • Take time during re-pot to sort the roots out
  • Wash the roots if necessary (not all trees though for example on pines this is not the case though as you need to retain the mycorrhizae fungi that may be present)
  • Protect after re-potting from extremes of temperature
  • Start feeding around 6-8 weeks after re-potting
  • Season for re-potting Oct – March for most species in the UK
  • Re-pot flowering trees after flowering
  • Timescales dependent on tree & stage of development
  • Good opportunity to check for pests & problem

Trying a pot out for size

Keeping roots healthy

It is important then to keep roots healthy and this can be achieved through good watering and feeding regimes plus a good re-potting frequency and using the right soil with the right aeration in the right pot. There are many types of pots, and select one with good drainage holes and that is suitable for the conditions of where it will be kept and the stage that the roots are, i.e. do not be too quick to put your developing bonsai tree in a small, shallow bonsai pot if you still need to develop it and if need be, place it in a larger pot to allow more growth but prune the roots more frequently and re-pot annually if that is the case.

Options for feeding also vary from;

  • Slow release within the pot
  • Liquid fertilisers – organic and non organic
  • Slow release pellets on the surface
  • Slow release cakes in pots
  • Organic matter  / sphagnum moss as part of soil mix
  • Sugar!!!

Example of large trunk at Lodders Bonsai Nursery

Trunk development

The key to trunk development is the following;

  • Healthy strong growth in crown
  • Good root system to support crown
  • Good soil conditions – aeration
  • High levels of nutrient availability (Nitrogen)
  • No water restrictions
  • No nutrient deficiencies
  • No pests or diseases
  • Lower branches to aid with taper
  • Environmental factors

Trunk cross section

Parts of the trunk

  • Cork & Bark (Protective outer layer and inner bark also called the “periderm”)
  • Phloem rays where nutrients move through
  • Cambium layer (Vascular cambium tissue layer)
  • Sapwood (Xylem rays) water movement
  • Spring and Summer growth = Growth rings (annual rings)
  • Heartwood -at the centre of the trunk
  • Pith

Scots Pine (Pinus sylvestris)

Starting point for trunk

Depending on where you want to start there are many options;

  • Seedling
  • Saplings
  • Collected material – Yamadori
  • Garden centre material / Tree Nursery stock
  • Air layer / Root graft / Sucker
  • Ready made bonsai tree of any size

White pine at T’veerle bonsai show

Trunk styles

  • Wrap around
  • Formal or informal upright
  • Multi trunk, Group, twin trunk
  • Clump
  • Literati
  • Wind swept / Leaning
  • Broom

Wired with rafia as protection

 Trunk shaping

  • Bending
  • Splitting
  • Guying
  • Cutting

Air pot

Trunk thickening techniques

  • Wiring for restriction – works well on Pines
  • Sacrificial lower branches
  • Planting in ground to grow on or in air pots
  • Planting in deep pot
  • Heavy feeding
  • Bark agitation
  • Scoring the bark longitudinally
  • Root placement


Trunk pests and problems

  • Wood boring insects – Saw fly, Wood wasps
  • Scale insect
  • Mealy bug
  • Red Spider mite
  • Aphids
  • Wood rot
Paul Hunt

Adding Lime Sulphur

Keeping trunks healthy

  • Keep them clean
  • Keep them free from lichens / Moss / Algae
  • Check for pests
  • Seal wounds / Avoid making wounds
  • Protect in winter if thin barked
  • Treating deadwood if retained as a feature with lime sulphur
  • Keep them protected from sunlight if thin barked
  • Do Not restrict trunk unless required
  • Do Not cover with non breathable substances

Cryptomeria japonica, Mendip Bonsai Studio at Chelsea Flower show

Crown development

  • Varies with species
  • Limited by seasons / environment
  • Varies with maturity of tree
  • Often limited by root development / health
  • Limited by water / nutrient availability
  • Needs to be in proportion to the scale of the tree
  • Wide range of techniques available

Field Maple (Acer campestre)

Nitrogen (N)

  • Part of all living cells -necessary part of all proteins, enzymes and metabolic processes involved in the synthesis and transfer of energy
  • Nitrogen is a part of chlorophyll, the green pigment of the plant that is responsible for photosynthesis
  • Helps plants with rapid growth, increasing seed and fruit production and improving the quality of leaf
  • Nitrogen often comes from fertilizer application and from the air (legumes get their N from the atmosphere, water or rainfall contributes very little nitrogen)
  • Essential for plant cell division, vital for plant growth
  • Necessary component of vitamins
  • Aids in production and use of carbohydrates
  • Affects energy reactions in the plant
Pyracantha in fruit


Potassium (K)

  • Involved in osmosis and ionic balance and in opening and closing of stomata
  • Potassium is absorbed by plants in larger amounts than any other mineral element except nitrogen and, in some cases, calcium.
  • Potassium is supplied to plants by soil minerals, organic materials, and fertilizer
  • Carbohydrate metabolism and the break down and translocation of starches
  • Increases photosynthesis and water-use efficiency
  • Essential to protein synthesis
  • Important in fruit formation
  • Activates enzymes and controls their reaction rates
  • Improves quality of seeds and fruit
  • Improves winter hardiness and  disease resistance

Satsuki Azalea

Phosphorous (P)

  • Essential part of the process of photosynthesis.
  • Involved in the formation of all oils, sugars, starches, etc.
  • Encourages blooming / flowering
  • Phosphorus often comes from fertilizer, bone meal, and superphosphate.
  • Involved in respiration, energy storage and transfer, cell division, and enlargement
  • Promotes early root formation and growth
  • Improves quality of fruits
  • Vital to seed formation
  • Helps plants survive harsh winter conditions
  • Increases water-use efficiency
  • Hastens maturity

Micro nutrients

  • Chlorine (Cl)
  • Iron (Fe)
  • Boron (B)
  • Zinc (Zn)
  • Copper (Cu)
  • Nickel
  • Molybdenum (Mo)

Macro nutrients

  • Sulphur (S)
  • Magnesium (Mg)
  • Calcium (Ca)

Needle Juniper

Branch development

Various techniques which include;

  • Pruning
  • Wiring
  • Guying
  • Scoring / cutting
  • Bending
  • Growing
  • Grafting
  • Supporting

Red Spider Mite

Crown pests and problems

  • Red spider mite / Leaf gall causing mites
  • Aphids
  • Black & White fly
  • Mildew  / Leaf spots (Fungi)
  • Viruses
  • Rusts
  • Bacterial infections
  • Nutrient deficiencies