The roots of your tree are *just* as important as the above-ground parts, with a lot of responsibilities which aren’t immediately obvious. I’ve summarised the main ones here but there is a lot more detail in separate posts with links provided below. So why are roots so important?
- they absorb water from the soil to meet all the tree’s needs (both for photosynthesis and transpiration)
- they absorb all the nutrients that the tree needs from the soil (using a different process to water, hence a separate point)
- they transport nutrients & water up to the above-ground parts of the plant, and photosynthates (the products of photosynthesis) down to the root tips
- they produce exudates (secretions) which sense and control the rhizosphere (the environment in which the roots are growing)
- they produce plant growth regulators for signalling and enabling growth within the plant
- they store food for later use
- they provide structural strength and stability for the tree by attaching it into the soil
Points 1 and 2 are fundamental to the health and growth of the tree – the roots are the mechanism for the tree to obtain all of the water and nutrients it requires (despite the mythic popularity of foliar feeding, this is only a way of augmenting nutrient absorption and not a primary mechanism). The mechanics of how they do this is described in more detail in how roots absorb water and nutrients – in summary it’s the fine roots and their root hairs which do the majority of the absorption since they have the closest and most expansive contact with the soil. There needs to be enough root surface area to supply the stems, shoots and leaves with the water and nutrients they require.
Point 3 reflects the fact that roots are part of a tree’s vascular system, that is to say, they transport the fluids necessary for growth around the tree. Above the ground the vascular system is present in stems, shoots and leaves, and below the ground it is present in the roots. Water and nutrients are transported up from the roots through the xylem and photosynthates (the products of photosynthesis) are transported down from the leaves and other storage organs in the tree via the phloem, to provide the energy and nutrients for the roots to grow and function. Do roots grow all year round? Find out here: when do roots grow?.
Points 4 & 5 show that roots are very much an active participant in tree growth and not simply a set of supply pipes. They produce both cytokinin and auxin (read more in the post about plant growth regulators), they also produce a huge variety of substances known as exudates which both sense and control the rhizosphere (the environment in which the roots exist). Researchers believe that roots use exudates to “regulate the soil microbial community in their immediate vicinity, withstand herbivory, encourage beneficial symbioses, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species”ref. Read more about exudates and how they are produced in root exudates.
Point 6 reflects the fact that roots are used to store food, in the same way that the trunk and branches do this aboveground (throughout the ‘woody skeleton’ (Ennos)). I was going to tell you that a lignotuber is an example of this and show you a lovely picture of my eucalyptus, but then I read “contrary to common assumptions…the lignotuber in young eucalypt trees did not appear to be a specialized starch storage organ. Rather, the lignotuber resembled an extension of the stem because its starch concentrations and temporal fluctuations mirrored that of the stem.”ref How roots store food and how much of a contribution to the plant’s overall storage capacity they make is debated. More on that in Root Food Storage (or, can I root prune before bud break?)
Finally as per point 7, the roots are responsible for physically holding the tree steady and stable against wind and gravity. They do this in many ingenious ways by adopting different root architectures – combining vertical taproots, lateral roots & sinker roots, creating ‘buttress’ roots, sending roots far from the trunk when needed and managing new root development in ways which stabilise the tree. More about this in root structure and architecture.
What all of this means from a bonsai perspective is that you need to pay just as much attention to the health and care of the roots of your tree as you do to the above-ground parts. Never mind developing a strong nebari for aesthetic purposes, you need to ensure that even though the roots of your bonsai trees are squashed into teeny-tiny pots, they are still able to perform the vital functions outlined above. Neglecting the roots will negatively affect the overall health of your tree.
Practically speaking, this is why you should aim to develop a well-ramified fine root ball, to provide the tree with lots of root surface area for nutrient & water uptake – taking into consideration the amount of biomass above-ground as this will determine how much root mass is needed.
The growing medium plays a huge role as well – this is your tree’s rhizosphere. It should provide the water, nutrients and micro-organisms the tree needs as well as (some) oxygen for root cell respiration, and ideally should not be disrupted so much so that exudates and microbes (fungi or bacteria) are lost. The risk associated with bare-rooting a tree (or excessive repotting) is that it destroys the rhizosphere all at once, leaving the tree vulnerable to pathogens and forcing it to regenerate exudates it has already created (which can use up to 40% of its stored carbon).
Your tree’s roots need to have a regular supply of nutrients, so they require fertiliser of some kind. Even if good compost is added during potting, the small size of bonsai pots will mean the nutrients won’t stay in there for very long. Trees will need added fertiliser – either home-made (for example, regular doses of diluted compost leachate), or purchased. And obviously – watering is critical. Given the role of symbiotic partners (such as fungi & bacteria), you can also add these to the soil – if your tree senses their presence and wants them to stick around – it will probably produce exudates to achieve this.