This is a rewrite of my original post on shoots, now I know a *lot* more…
So what are shoots? They are the vegetative growth which comes from buds, extending to create new stems. Since stems create the architecture of a tree, shoots are really important when it comes to bonsai.
There are three key concepts to know about when it comes to shoots. The first is the existence of long and short shoots, the second is the way in which different shoots are formed and the third is the concept of the internode.
I had never heard of long and short shoots before researching this site, and I have since found that many articles and books don’t really talk about the fact that many species of tree possess two types of shoots. Shoot differentiation (as it’s known) is present on the vast majority of deciduous angiosperms (flowering trees), all deciduous gymnosperms, and quite a few (around 25%) of evergreen gymnosperms as well, particularly conifers.ref
In these trees, two different types of shoot develop – long shoots and short shoots. Long shoots are exactly as described – they have a terminal bud which continues to build up the length of the shoot over time so it becomes (relatively) long. Short shoots meanwhile don’t persist beyond a limited number of years, they are much shorter than long shoots and have many fewer nodes. Both types of shoots can have leaves, flowers, cones and fruit, but only long shoots can create the long-term architecture of the tree. Importantly, aside from their structural trunk and branches older trees mostly grow short shoots, which is why they look more ramified.ref
In some species (such as pines), short shoots – otherwise known as fascicles – are a feature of the mature vegetative phase of the tree, and don’t appear in the juvenile phase nor with juvenile foliage. An interesting side note is that fascicles can be used to propagate trees with needle leaves, the fascicle is treated like a cutting and placed in rooting hormone and well drained medium – the reason this works is because the fascicle is actually a short shoot and not a leaf.ref
Below is an example of Cedrus libani where the clusters of needles (N1) are on the short shoot (S), and occasionally along the long shoot (L) there are individual needles (N2).
A fascinating – and useful for bonsai – attribute of short shoots is that they almost always have more leaves than the equivalent long shoot. In angiosperms, short shoots have multiple smaller leaves with an almost identical leaf area to a single leaf grown from a long shoot (see example A below).ref And in gymnosperms short shoots have many more leaves and leaf area than long shoots – examples D and E below show the leaves on a short shoot (right hand side) compared to the individual long shoot leaves (left hand side) on larch and dawn redwood.
So hopefully you can see that short shoots are fantastic for ramification! But not so fantastic for building the structure of the tree, since they don’t persist. So how can you tell which is which? Very simply short shoots are smaller, have a lot more leaves, and fall off when their time is up. Often in gymnosperms they will have cones at the end of their leaves.
You may not have realised that whilst the ‘leaflets’ on Cupressaceae species such as dawn redwood, cypress and juniper may appear to be compound leaves, instead they are actually short shoots. When their life comes to an end, the entire short shoot abscises (falls off) along with its leaves. Similarly for pines, what you might know as ‘fascicles’ are actually the short shoots, and on pines only the short shoots bear photosynthesising leaves (needles). Eventually they will fall off.
In angiosperms, a short shoot usually develops from the bud in the leaf axil of the long-shoot leaf, arriving the next season. In gymnosperms, it depends on the species. In Cupressaceae a bud will be sitting at the base of the short shoot so another one should grow once it falls off. In Pinus short shoot buds sit in the long shoot leaves towards the base of the long shoot, and they are positioned at the base of the long shoot bud.ref In Ginkgo both short shoots and long shoots can come from any bud on any type of shoot.
Below is a picture of some interesting behaviour I’d never seen before – this Japanese larch belonging to a member of my bonsai club produced buds and new stems right through the middle of its cones. Pollen and seed cones on larch are terminal organs growing only on short shootsref – which means they aren’t supposed to extend. But Larix is known to be able to change the type of shoot from short to long if damaged (which may have been triggered by the hard pruning it received).ref So in this case what had been a short shoot destined to eventually fall off, instead turned into a long shoot.
So what does it all mean? From a bonsai point of view, the first thing is to work out if a tree has shoot differentiation. If it is deciduous, it will, and if it is a gymnosperm, it still may even if evergreen – gymnosperms which have shoot differentiation include Pseudolarix, Taxodium, Sequoia, Cedrus, Larix, Ginkgo, Pinus & Metasequoia. Understanding the difference between short and long shoots will allow you to understand where foliage will ramify, and where the long-term structure of the tree can come from. On trees which don’t have shoot differentiation, any stem which has a vegetative bud can be used to develop the shape of the tree.
So now we know that long and short shoots exist in many trees, let’s turn to how those shoots form. According to Thomas (2018) , there are three options.
Option 1 is ‘fixed’ or ‘determinate’ growth. These trees preform every part of the shoot in the bud, so they extend very quickly (a few weeks) and then stop. If they are young (less than 10-15 years old) and have the right conditions, they may do this a second time around the start of August (in the Northern hemisphere), this is known as Lammas growth. The shoots from these trees developed based on the conditions at the end of *last year’s* growing season.
Option 2 is ‘free’ or ‘indeterminate’ growth. These trees have only some preformed leaves. Once extended the shoot will continue to produce other leaves from scratch in a continuous fashion. Often these are found in the tropics or warmer climes (my potted Eucalyptus never seem to stop producing leaves even during winter).
Option 3 is ‘rhythmic’ growth. These trees extend in recurrent flushes, with multiple cycles of growth and bud formation during the season.
Outside of the tropics, towards the end of the growing season all trees will stop shoot and leaf growth according to their phenology, in order to complete the formation of buds for next year. If conditions are not good, these buds will be fewer and contain fewer leaves. To see a list of which trees have which types of growth see the Growth Types Table. The relevance to bonsai is that trees with determinate growth are only going to give you one or at most two cracks of the whip in a given season. Those with indeterminate growth might be easier to develop since they will keep extending as long as the conditions are suitable.
Interestingly one study on lammas growth (second flushing) found that 73% of this occurred from lateral buds. We’d obviously love to have this in bonsai as it helps ramification within the same growth season.ref This articleref summarising lammas growth factors says that it can be encouraged by warmer temperatures (Pinus densiflora), extra watering (Pinus sylvestris), nitrogen fertiliser (Pinus sylvestris, Pseudotsuga menziesii) and applying a blackout treatment for less than 2 weeks early in the summer (Picea abies). So from a bonsai perspective see if you can encourage second flushing to generate those lateral buds.
And finally we come to internodes – these are the length of the shoot between each successive leaf. In general bonsai afficionados are looking for short internodes so they can achieve compact, dense foliage. The factors which affect internode length when a tree grows are the same as for any other type of growth – genetics, plant growth regulators and availability of nutrients. Shorter internodes can be achieved by (1) shoot pruning, (2) thigmomorphogenesis and (3) starvation.
If you allow a shoot to extend naturally (and it has no competing stressors), it will prioritise resources into growing as long as it can and the growing tip will suppress the growth of any lateral shoots below it – because the driving force for a tree is to grow large and establish the biggest exposure it can to sunlight. An angiosperm will grow a series of internodes with leaves at each point. What I have observed from looking in my garden is that the internode length on an angiosperm tends to start small (or in some cases leaves are grown directly at the node as well), then increase in size, then reduce again.
To get the smallest internodes, you should prune off the growing tip once the first pair of leaves and the first internode has grown. If leaves have grown at the node, you could remove the shoot altogether (there will be no internode in this case). New shoots will grow from buds in the leaf axils, and if you keep doing this, you will always retain the short first internode and increased ramification.
You could also make use of thigmomorphogenesis which is “the response of plants to mechanically induced flexing, including the brushing or movement of animals against plants, or the flexing of above ground portions of a plant by wind, ice, or snow loading”ref According to this articleref, “the most consistent thigmomorphogenetic effects are a reduction in shoot elongation and an increase in radial growth in response to a flexing stimulus resulting in a plant of shorter stature and thicker, stiffer stem.” i.e shorter internodes and thicker stems.
Thigmomorphogensis is thought to be triggered by plant growth regulators or other substances within the plant signalling when it has been touchedref. To trigger thigmomorphogenesis in your tree, you could expose it to wind while the buds are developing, rub the internodes for 10s daily (seriously, this is what they did in the original studyref which identified the phenomenon), touch the leaves regularly or manhandle the growing shoots.
Another way that bonsai enthusiasts encourage small internodes is by starving the tree. Fertiliser helps the tree grow and this will lead to longer internodes and larger leaves. Holding back fertiliser may result in the desired effect – but also could impact the tree’s health negatively – so it is a balancing act.
So there you have it – shoots turn out to be surprisingly interesting. For your bonsai, try to work out if your tree is shoot differentiated, and if it is, aim to use long shoots for structure and short shoots for foliage ramification. If it has determinate shoot growth, you need to work with the one or two shoot extensions that you get per year, and to get that second flush with lots of lateral buds try using one of the techniques above (warmer temperatures, extra watering, nitrogen fertiliser). Finally keep internodes small with judicious pruning, foliage fondling and holding back fertiliser.