Tree roots are in general the art of simplicity, they have three key functions, to anchor the tree, absorb water and nutrients and store excess food for the tree for later. In bonsai they are essential to maintaining a healthy bonsai tree and are often over looked as out of sight, out of mind, but it is important to dedicate time to refining the root system of your bonsai tree to ensure its longevity.
This can be done in one of two ways, firstly through the production of a tap root and large structural roots and then by occupying large volumes of soil with a dense mat of fibrous root system. Both of these types of roots can form independently or simultaneously and taproots are a good initial anchor but degrade over time once the larger fibrous root network has developed. As bonsai trees are usually wired into their pots, the need for root anchorage is removed and roots can concentrate on the other two functions of nutrient and water uptake as opposed to support.
Storage of nutrients and uptake of water
All roots have the ability to absorb water and nutrients, and the taproots have a greater capacity to store more food than smaller roots. Fibrous roots tend to be shallow in depth and this allows for improved gaseous exchange as well as water absorption where the presence of water is also higher closer to the soil surface in most soil types. In bonsai for the majority most taproots will have been removed or should be removed to encourage greater fibrous root development to support the crown and whole tree. The removal of larger structural roots also enables bonsai trees to be kept in shallower pots to improve on the aesthetic impression presented by bonsai trees.
Roots probe soils for water and mineral nutrients and to do this require the ability to extend the length of roots, which is done through the success of the apical meristem and that is where most of the new cells are produced and laid down behind the growing tip. As the roots extend and develop they push through the soil and in order to protect the cells a root cap forms on the root tips and is readily replaced once worn away. Therefore bonsai soils should be kept free from being overly compact to allow for root growth and movement within the pot for most root types. The soil moisture content should be monitored also to ensure that roots have access to good levels of moisture but do not remain sat in waterlogged or stagnant conditions so drainage in the bonsai pot as part of the soil structure is important and this will aid root growth and development if the soil is of a free more defined texture.
Roots have root hairs on then set back from the root tip and these aid the roots ability to absorb water due to providing a larger surface area. However, root hairs are easily damaged but they do re-grow quickly and care should be taken when re-potting bonsai trees to avoid excess root damage.
Roots have the ability to survive in most environments and do this with a variety of amazing adaptations. Some trees will develop aerial roots like Ficus species while other species like Alder have developed beneficial mutualistic relationships in the soil with bacterium that fixes nitrogen and the roots develop nodules storing the nitrogen. Subject to the conditions, some of the root adaptations will be more noticeable than like the pneumatophores or ‘knee roots’ produced by the Swamp cypress or Bald cypress (Taxodium distichum) which allow the roots to emerge from swamp conditions for increased gaseous exchange.
Some additional adaptations of tree roots include:-
- Forming adventitious roots for climbing or clinging to structures ie Ivy, Virginia Creeper.
- Buttress root formation for support of large stems like Ficus trees.
- Prop root formation for additional support in waterlogged soils like the Screw pine.
- Ability to produce runners, suckers or stolons to aid with spreading a plant like Blackberries.
- Adaptations for increased water storage with the formation of bulbs, rhizomes, corns or tubers.
Roots and re-potting bonsai
As we are well into re-potting season this is an excellent opportunity for you all to examine the roots of your bonsai trees and see how they are developing. It is just as important to assess and examine the roots as it is to prune the foliage. The roots are the key to a healthy bonsai tree and therefore need attention and the best time for this is during re-potting.
This is also a chance to look out for pests and diseases, get to see if the fibrous roots are sufficient to support the density of foliage and are healthy and not restricted, dead, in stagnant soil conditions. It is also a chance to re-position any roots to improve the nebari, replace the old soil and if necessary place additional soil release fertilizer in with the soil.
It is a good opportunity to change pots, consider if the existing pot allows for new root growth, is deep enough but also fits the tree and I also find that it is a chance to really examine the roots and notice the subtle differences in types of root shapes, colours and forms that different tree species have.
Here are a few examples of different bonsai root systems on some of the trees I am still developing the roots on.
Notice how fleshy Swamp cypress, Wellingtonia and Ginkgo roots are, these are much fatter and fleshier than for example Hornbeam roots and also the distinct colour differences between the species.