Lateral roots are ones which branch off from the main root – just like lateral or axillary buds aboveground. Lateral root development is how roots branch and ramify – similarly to stems, which ramify through bud initiation and stem growth. Encouraging strong lateral root development is a goal in bonsai, because we want to create lots of fine root mass to provide maximum exposure to water, nutrients and enabling symbiotic partners (fungi & bacteria); it also helps to stabilise the tree in the pot.

The below image is a nice one illustrating the development of lateral roots. The root grows from the tip – known as the root apical meristem (RAM), where new cells are created and the root tip is constantly extending. Above the apical meristem are pericycle cells which are preparing to initiate lateral roots, and above that these have been initiated and are starting to grow. At the top a lateral root emerges. The pale beige section in the centre labelled the ‘central cylinder’ is the location of the vascular bundle containing xylem and phloem.

I have been astonished while researching this post to discover just how many things can affect lateral root growth. My post on Ramification of Branches and Foliage had just three ways to improve ramification – dividing the apical meristem, pruning (in various ways) and applying cytokinins. In this post there are no less than nineteen different mechanism for encouraging root ramification! I’ll list them (x) as I go.

Before we look at each one of these, let me tell you about the ‘root clock’. The root clock is an oscillating cycle in roots which determines where lateral roots are formed – the spacing between them is dependent on the cycle time of the clock.^ref The way this works is through the oscillating expression of genes in a region close to the tip of the primary root, called the oscillation zone.

As with anything growth-related, our friends the plant growth regulators (along with genes) are involved. Auxin (1) promotes the development of branching lateral roots as well as adventitious roots (such as on air layers) whereas cytokinin (2) opposes these effects.^ref The formation of lateral roots involves both shoot- and root-derived auxin with the root tip responsible for lateral root initiation (the first step of creating a new root), and auxin from the shoot responsible for lateral root emergence (the elongation and growth of the initiated root).^ref The root clock is involved here because the back and forth gene expression causes programmed cell death at the root tip to happen periodically, which releases auxins back up the root and initiates a lateral root.

Ethylene (3) inhibits root growth, and brassinosteroid (4) and abscisic acid (ABA – some species only and in small amounts (5)) stimulate lateral root growth and elongation.^{ref1 ref2} In fact there are complicated interactions between genes and plant growth regulators when it comes to roots with different hormone levels detected in different parts of the root, based on the differing roles they are playing in each stage of root growth. For more check out this article about tap roots which has a good diagram showing plant growth regulators (fig 2).

Other substances produced within plants which promote lateral root development include salicylic acid (6) and melatonin (7).^ref

Aside from plant hormones, the nutrients in the soil also affect the level of lateral root development – for example nitrogen (8): “in low-nitrate soils, patches of high nitrate have a localized stimulatory effect on lateral root development in many species, however where nitrate levels are globally high (i.e. not growth limiting), lateral growth is inhibited”^ref A phosphorus (9) deficit “favours a redistribution of growth from the primary roots to lateral roots”.^ref A sulphur (10) defiency “leads to the development of a prolific root system, usually at the expense of shoot growth…roots elongate faster than those with sufficient sulphate, with lateral roots developing earlier, closer to the root tip and at a greater density.”^ref

Lateral root formation is restricted when water availability is low (11), and somewhat surprisingly, when there is a lot of salt (12) in the soil more lateral roots form.^ref

You might not think that roots need light (13), but in fact light above ground is necessary for maintaining the oscillating signal of the root clock and for the formation of sites where lateral roots can branch off; an absence of light has a strong inhibiting effect on root elongation and branching.^ref This may be related to the point below about sucrose – light drives photosynthesis and the creation of photosynthates like sucrose.^ref

This intriguing study found that drenching roots in sucrose solution (14) “significantly increased lateral root branching and root formation compared with non-sugar supplemented controls.”^ref This may be because of effects on the rhizosphere vs the roots themselves. Just beware what you might attract if doing this in your bonsai garden as a sucrose drench sounds like an insect’s dream come true.

Several studies have determined that root pruning (15) encourages lateral root formation ^ref1, ^ref2 and that this happens most likely due to a surge in auxins after the root is cut. Similarly, synthetic auxins applied to roots of scarlet oak resulted in six times the number of adventitious roots compared to a control, and resulted in longer roots as well.^ref

Another way to increase lateral root formation is via encouraging arbuscular mycorrhizal fungi (16). Interestingly, the mechanism for this is that the plant root detects the presence of chitin in the fungi cell walls.^ref Chitin (C₈H₁₃O₅N) is “the most abundant aminopolysaccharide polymer occurring in nature, and is the building material that gives strength to the exoskeletons of crustaceans, insects, and the cell walls of fungi.”^ref Researchers found that any source of chitin (17) had the same effect – including chitin derived from shrimp shells^ref Chitins can be found in many living creatures, including crustacean shells, insect shells (beetles, grasshoppers, cockroaches, blowflies), bat guano, resting eggs of Daphnia species, spiders, green algae, zooplankton (krill), phytoplankton and fungi.^{ref1 ref2 ref3}

On a similar theme (and maybe a repeat of 14) the presence of Pseudomonads bacteria has been shown to increase adventitious root development in tobacco.^ref

While reading a study about plant growth substances recently I came across (18) – apparently you can physically bisect a root apical meristem and it will become two autonomous RAMs.^ref

And lastly let’s look at two types of pots which affect root ramification: (19) if you use a white pot instead of a black, green or other dark colour, roots will be up to 2.5 times denser^ref, and if you use an air pot (20) they will have fewer circling or malformed roots but also less root mass overall.^ref

So let’s summarise all the different ways you can practically encourage root ramification in your bonsai:

• Root pruning
• Encourage auxin & sucrose production via photosynthesis (leave some leaves and give the tree good light)
• Apply exogenous (from outside the plant) auxins – for example from compost or compost leachate
• Drench roots in sucrose solution
• Add and nurture mycorrhizal fungi and friendly bacteria
• Add a source of chitin (as a vegetarian I can’t recommend any of the animal sources, but some other ideas include scooping the algae from your garden pond (or similar) or adding some mushrooms to your compost and adding that (or its leachate) to your pots
• Bisect the root apical meristems (ie. cut them down the centre with a clean sharp blade)
  • Use a white pot