Flat leaves are found across the conifer families including Taxus, Cephalotaxus, and even in Pinus (check out Pinus krempfii) as well as many of the Araucariaceae and Podocarpaceae families. Commonly known trees with this leaf shape include all varieties of yew, swamp cypress, dawn redwood and coast redwood. Here are some examples:

Conifer flat leaves are relatively inefficient from a photosynthesis point of view, since water and photosynthates have to travel further to reach the vascular bundle/s.^ref As a result, they have better photosynthetic efficiency in humid, low light environments^ref and are more common where it is wet or tropical. In these areas their greater leaf surface can help them attract the energy they need without drying out due to having more stomatal openings on their surface.

The primarily southern hemisphere family Podocarpaceae is super interesting and not one that I have worked with much from a bonsai perspective, although buddhist pine (Podocarpus macrophyllus) is a species commonly found as an indoor bonsai in the UK. (It is hardy down to just below -10°C so could be an outdoor bonsai as well). Podocarpaceae are interesting from a foliage perspective, as they have evolved a wide range of different leaf shapes which in some cases have become similar to angiosperms and take the forms of flattened leaves with modifications that allow them to grow larger. Below are some examples from which it’s clear that this family has found a workaround for the venation constraints of others in the conifers.

One study mapped the structure of Podocarpaceae leaves and created a cross-sectional image for each sub-family, their results are shown below (apologies for the poor resolution but the original paper wasn’t great to start with). You can see that although most species only have one vascular bundle, they also have various mechanisms to get substances to and from it – including the orange ‘organised accessory transfusion’ cells and ‘pitted thick-wall mesophyll’ cells. Interestingly leaf (f) from the image above – the largest leaf – is also shown below and it’s the only one to have multiple vascular bundles (N and O) which is similar to angiosperm leaves.

Whilst the majority of Podocarpaceae are flat-leaved, quite a few members of Araucariaceae have flat leaves as well. The famous Wollemi Pine or ‘pinosaurus’ Wollemia nobilis has flat leaves, as does the equally well-known monkey puzzle tree Araucaria araucana and various others in the family. These trees don’t tend to be common bonsai subjects as their leaves are quite large and observing the Wollemi pine in my garden, they don’t have much variation in their form so wouldn’t be that easy to style.

Also unusual among conifers is the Phyllocladus genus. Plants in Phyllocladus don’t have true leaves at all – any leaves they develop are non-synthetic and ephemeral – ie. they quickly drop off. Instead Phyllocladus use their stems, which have developed a leaf-like flattened form, to photosynthesise.^ref These are called phylloclades – see below for some examples from New Zealand:

I know they look identical to leaves – and they contain all the same components including vascular bundles and photosynthesising palisade and mesophyll cells. The nuances of why they are not leaves but flattened branches instead are based on the way they develop and branch, and the relationships between organs – if you want to read the details refer to this excellent paper. But sometimes they may get lumped in with flat leaved conifers.

Which brings me to Taxus or yew. This is a very common bonsai subject, with its own family, Taxaceae. Yew leaves are famous for two things. Firstly, along with every other part of the yew except for its aril (the red ‘berry’), yew leaves are renowned for being extremely toxic. They contain ‘taxine alkaloids’ of which only a small amount is needed to bring on “dizziness, nausea, vomiting, diffuse abdominal pain, cardiac arrest, respiratory paralysis and death” in all animals including humans^ref. Secondly, they are one of the few conifers which are known to do just fine in shady positions, although they are also happy in full sun as their leaves adjust to different light levels.^ref

Taxus have a unique stomatal feature called ‘papillose’ cells^ref which can only be described as ‘pimply’ (see below) – basically the entire surface of the leaf has tiny protrusions as part of the cuticle structure. The presence of these pimples is one of the ways of identifying a Taxus leaf. I couldn’t find any clear explanation as to the role of these papillae, except perhaps to provide some level of water-repellence.^ref Taxus are known to harbour endophytes (microbes) in their leaves which help combat pathogens^ref so it could be that the nooks and crannies created by papillae are a nice home for endophytes.

Anyway, what of all this is relevant for bonsai? It sort of depends on which flat-leaved species that you have. Many will be suited to humid and/or low-light environments, so keep an eye on your watering and don’t let them dry out – they will probably appreciate a misting every now and then. Full sun may not be necessary – or may even be over the top depending on your location – but as always find out what a given species needs as there are lots of quirky members of this club with their own unique requirements.

I’ll be honest, I find conifer needles to be quite weird. As someone raised in the southern hemisphere, true pine trees were something we had at Christmas and that was about it (this is true because the most common Christmas tree found in Australia is Pinus radiata). But actually, needles really are just plain old leaves. They contain all the same components as ‘normal’ leaves, like an epidermis, photosynthesising mesophyll cells, green chlorophyll pigment, xylem/phloem, stomata and so on. They just happen to have pushed the ‘leaf’ form to the extreme, ending up extremely long, thin and tough.

Within conifers, what are *called* needle leaves are found across most of the families, including Araucariaceae (Norfolk Island pine, Cook pine, Hoop pine), Cupressaceae (juniper, Thuja, Cryptomeria japonica), Pinaceae (pines) and Podocarpaceae (Platycladus).

At this point though I think it’s important to point out that the Pinaceae family is the least similar to all the other conifer families, having diverged from them very early in evolutionary history. Look at the ‘family tree’ of conifers below and you can see that Pinaceae including pine, cedars, larch, fir and spruce, has been evolving separately for the longest of any conifer family. This means that ‘needle leaves’ in Pinaceae are not the same thing at all as needle leaves in other families, and they probably need to be treated as two separate sub-categories.

This is why it can be confusing to understand what people are talking about when they refer to needle leaves in conifers. Needle leaves in Pinaceae are obvious – they are long, thin, spiky, tough, 3-dimensional and in mature foliage form in clusters known as fascicles which are actually short shoots (see my post on shoots for more on these). What you and I would call pine/fir/spruce needles – as per these examples:

Needle leaves in other families are a bit more ambiguous. Sometimes they are referred to as ‘awl-shaped’ or ‘sabre-shaped’ and often they have a needly element but also a scale-leafy element. Needle leaves in these families, such as Juniperus and Cupressus, often mature into scale leaves. Here are a few examples of non-Pinaceae needle leaves:

As is obvious, these are different to Pinaceae needles and it’s not just the leaf shape and configuration – one major difference is that these species usually have a photosynthetic stem which pines definitely do not have. You can sort of see how these leaves could change to become scale leaves by the way they are attached to the stem in alternating pairs – if they just shrink and get closer to the stem you could see a mature scale leaf emerge.

For the sake of the rest of this post, I’m going to focus on Pinaceae needle leaves, since these are persistent needles whereas most of the needle leaves on other families are juvenile (although there are some exceptions).

One of the key attributes of Pinaceae needle leaves is that they have a 3-dimensional profile – usually with a quadrangular, triangular or semicircular cross-section, as shown in the images below (1,2,3 & 5 needle pines):

Anatomically, Pinaceae needles are like other conifer leaves in only having one or two vascular bundles for water & sugar transport. Their stomata are arranged more or less evenly around the needle and appear in lines. But needles are unique in having an unusual type of mesophyll cell (a cell used for photosynthesis). Instead of cylindrical palisade cells lined up under the epidermis like other plants, needles have frilly looking ‘arm palisade parenchyma’ (see below). These have very lignified (woody) cell walls which intrude into the cells and it is this feature which is thought to provide needles with their extreme cold resistance.

Pinaceae needles are actually *the* most frost-hardy leaves of all. Mountain pine needles survive temperatures down to −93°C and can still perform gas exchange (oxygen for cell respiration) even when their needles are frozen.^ref They are also highly resistant to herbivory, due to being tough, spiky, full of toxic resins and not very nutritious. And they live anywhere from 2 to 45 years (Bristlecone pine has the record).^ref The needle is one tough leaf!

Pine foliage is also heteroblastic – which means it has one type of foliage during its juvenile phase, which lasts 1-3 years, and a different type during its mature phase. Juvenile needles don’t appear on fascicles, instead developing directly on the stem, and their profile is more flattened – although it’s not super noticeable until you know to look.^ref Mature foliage develops in fascicles (bundles) and is the familiar three dimensional profile – they fall off as a group when the fascicle falls off. See examples below – Pinus cembroides juvenile on the left and adult on the right of both frames.

Juvenile and mature needles have different properties with juveniles having 3x the photosynthetic capability of mature needles. This is believed to be able to provide such species with a ‘fast return’ strategy whilst establishing and a ‘slow return stress-resistant’ strategy when older.^ref

Surprisingly needle leaves actually have better photosynthetic performance than other conifer leaves – despite that fact that Pinaceae have lost key genes required for photosynthesis in other plants.^ref Their improved performance is believed to be because water has to travel further than in non-needle leaves, but there is likely also a genetic factor which hasn’t been discovered yet.^ref Needle leaves take a couple of years to reach their full photosynthetic capacity, but once they do, from then on it reduces with age. One study found that for spruce leaves this declines linearly from the 2 year point, reducing to 30% or less of the maximum photosynthetic capacity by the 6 year point.^ref

When photosynthesising, trees need to deal with transpiration, which is the evaporation of water from the stomata in their leaves and is the main driver of their water requirements. Needle-leaved trees have a massive advantage in this domain, as many species have wax deposits in their stomata which reduce transpiration. It has been found that wax deposits in Sitka spruce stomata reduce transpiration by two thirds but photosynthesis by only one third.^ref Unfortunately these waxes are degraded by pollution, so needle leaved trees can dry out in high pollution areas.^ref

Since needle leaves can generate more energy from the light they have, and from a wider range of sun angles, they can survive in poorer light conditions than flattened-leaved species. Along with their cold resistance and ability to minimise transpiration, this is why the boreal forest comprises mainly Pinus, Abies and Picea species, which all have needle leaves.^ref

Finally a note on buds and how needle leaves develop on a tree. Leaf buds on mature needle-leaved species form as part of a shoot rather than individually, and these are usually determinate, which means that everything is formed inside the bud. So before bud break the leaf primordia (baby needles) are sitting inside the bud. Below are some cross-sections of long shoot pine buds (which you might know as candles) and a spruce bud (on the right). The brackets show where short shoot buds are located, and within these the baby leaves are waiting to emerge with the shoot.

Contrary to some advice, needles do all their extension in their first year of growth. After this they replace or add to their phloem annually, but not their xylem.^ref This means there may be some thickening of needles, but no lengthening after the first year. They may be shorter with increased dryness and poverty of the soil.^ref Eventually needles fall off along with their fascicle and the other needles in their group. However, pines have been shown to retain their needles up to twice as long if they have been defoliated (eg. by insects).^ref

What’s the impact of this all for the bonsai enthusiast? Firstly your needle-leaved species are going to be tough, they will cope with reduced water, poor soil, wind, rain and freezing temperatures. They definitely do not want to be inside.

Secondly as per conifer leaves in general, needle leaves are not as plastic or regenerative as angiosperms – they are part of a shoot and form in the bud, so there aren’t as many styling options as you find with angiosperms or even flat or scale-leaved conifers. And they have a more relaxed time frame than angiosperms do – needles may stick around for a long time – usually this will be 2-4 years in most species and low-medium elevations but can be a lot higher. So your styling decisions can’t be completely redone on an annual basis – a better approach for needle-leaves is a gradual evolution towards a vision. These trees suit bonsai practitioners with patience and a slow, thoughtful approach.

Bonsai folk like small leaves and in the case of needles, short needles. This is achieved in one of two ways. The first way is that long shoots (candles) are completely broken off early in the growing season. This forces the tree to activate dormant foliage buds at the base of the shoot, which don’t have the time or resources to develop full length needles. I’m not sure whether breaking the part of the candle off (as is also advocated) would also reduce needle size since this retains the active short shoots at the base of the candle to still develop. This practice might have a slightly different effect of creating more short shoots with more needles, so giving denser foliage, rather than shorter needles. The second way to reduce needle size is to starve the tree of water and nutrients, but I’d say manipulating candles would be better for the health of the tree. Obviously to add branches to a tree you need to leave the long shoots in place to develop, as these are what create the long-term framework.

To finish off, I just have to share one of the brilliant images created by Gerhard Vicek who does microscope cross-sections of plants – below is a cross-section of a Cedrus atlantica needle. The beautiful staining he does of his samples makes the different cell types really clear. There are epidermal cells (in red) on the outside, below these the unusual frilly arm palisade cells (in green). Then you have the transfusion cells in a brown & white ring, which move water to the outside of the needle and sugars to the centre. In the centre you can see two vascular bundles with the tiny xylem & phloem cells. Truly his images are art for the bonsai science nerd!

Scale leaves are a curious form of conifer leaves which cover up the stem in interlocking patterns. I believe they are called ‘scale’ because they look a bit like fish scales in the way they overlap but I have not found an authoritative source which confirms this. Scale leaves appear primarily in the Cupressaceae family – including junipers, various cypresses, Arborvitae/Thuja, redwoods & Callitris, as well as in the Podocarpaceae family including Dacrydium and Acmopyle. Sometimes the scale leaf form is the mature foliage, while the juvenile foliage takes a needle form (see my post on conifer needle leaves).

Some examples of scale foliage leaflets are below:

You’ll note that I called them ‘leaflets’. The actual leaves are the individual scales that you see in the images, which all combine to create a larger leaflet which is actually a short shoot. The leaves are wrapped around and connected to the stem of the shoot underneath.

Scale leaves are usually in opposite pairs, and depending on the species can have main or ‘facial’ scales and lateral scales with slightly different anatomy. Below is a scanning electron microscope image of Thuja occidentalis leaves which demonstrates these two scale types.

Probably the most distinctive attribute of scale-leaved species is the leaf pattern. Each species has a distinct cross-sectional profile, with different leaf shapes and configurations, these are what ultimately create the three-dimensional shape of the leaflet.

Great work was done on this by some Iranian researchers, who created the following cross-section drawings which I have matched to images of the species in each drawing. These show how the scale leaves attach to each stem, the positions of vascular bundles, resin ducts (large holes) and stomata (which are mostly present in the grooves indicated by ‘S’).

As can be seen in the drawings, scale leaves are very simple, usually with a single vascular bundle (other than Juniperus excelsa above which has none), palisade and spongy mesophyll cells for photosynthesis and the darker transfusion cells which move water to and from the stem.^ref

In terms of their performance, there actually isn’t a lot of information out there comparing scale leaves with needle or flat leaves. One study found that Thuja leaves were about on par with pine needles in terms of photosynthesis^ref and another found that juvenile needle leaves of Juniperus sabina outperformed its scale leaves^ref. Many studies seem to conflate needle and scale conifer leaves and talk about both of them having strong performance in high sun, low humidity situations. There must be some benefit, because quite a few species ‘graduate’ to scale foliage as they age, but I haven’t found any research explaining what that benefit may be.

One weird and wonderful variation of the scale leaf is the unusual ‘axial’ leaf of the coast redwood. Most people associate the coast redwood (Sequoia sempervirens) with a flat-leaved leaflet as shown on the left – and in fact this type of leaf makes up 95% of the leaf surface area of these trees. But 5% is made up of the axial-leaved ‘twiglet’ on the right. These leaves are optimised to absorb water, having much less waxy coating than the flat leaves and contribute up to 30% of the water requirement of the tree. Which can be high, given the size of a coast redwood!

So what are the bonsai implications?

Since scale leaves are associated with older trees, they are preferred for bonsai. If your tree is still in its juvenile phase, you need to let it grow as it’s believed that the trigger for changing phase to mature foliage is the number of meristem cell divisions.^ref Pruning the leader on these trees will keep them permanently in a juvenile state, so let the tree grow until it develops mature foliage, then you will need to use all branches & foliage *after* this point to style your tree.

Also remember that scale leaves usually appear on short shoots, which abscise (fall off) as a unit. But don’t worry because usually there will be a bud waiting at the base of the short shoot to replace the one which fell.

Finally if you look at how scale leaves connect to the stem, I believe that the technique of pinching leaves (and stem) off instead of pruning with secateurs or scissors would leave less dead material on the tree. Cutting straight through a stem is always going to sever one or more scale leaves and cause them to die and go brown. The pinching technique is when you use your fingertips, and pull the stem gently so it breaks at a natural breaking point between leaves.

I’ve been planning a post on this subject for a while because conifers have always been a bit scary to me from a bonsai point of view – they don’t seem as forgiving or obvious in terms of their growth behaviour. This was one of those subjects which ended up being a lot more interesting and complex than I was expecting – once I hit 3000 words for this post I realised I needed to separate things out! So below is a *summary* overview of conifer leaves, and detail on the three different types of conifer leaves are in separate posts: conifer needle leaves, conifer scale leaves and conifer flat leaves.

But let’s start from the start. What are conifers? Strictly speaking they are any of the species in the family Pinopsida also known as Pinales or Pinophyta (for a reminder review the previous post on The kingdom Plantae and where trees fit in), that is to say, the Pinophytes. Pinophytes are cone-bearing plants, hence the name conifers. They include six different families:

• Araucariaceae (including monkey puzzles and the Wollemi pine)
• Cupressaceae (including cypress, juniper, redwood, Cryptomeria japonica)
• Pinaceae (including pines, cedar, spruce, hemlock, larch & fir)
• Podocarpaceae (mainly southern hemisphere evergreens including Buddhist Pine), including Phyllocladaceae (celery pines from New Zealand)
• Sciadopityaceae (Japanese umbrella pine is the only member in this family)
• Taxaceae (yews) including Cephalotaxaceae (Japanese plum yew)

So why do these families have different leaves to those of angiosperms/flowering plants? It’s because gymnosperms (including conifers) and angiosperms diverged in their evolutionary paths 350 million years ago^ref and as a result they have evolved with key genetic differences. These are exposed in leaves in five key areas:

1. Venation – the structure of the vascular system which transports water through the leaf and products of photosynthesis back into the tree (ie. its ‘veins’) (and thus determines the possible leaf shapes)
2. Stomata – the distribution, density and effectiveness of the pores on the leaf which allow air in and water vapour/oxygen out
3. The photosynthetic apparatus – how the cells in the leaf are arranged to perform photosynthesis and which reactions are used
4. Heteroblasty – the phenomenon of ‘extreme variation in leaf morphology during plant development’ or in other words, leaves being completely different on young plants versus old plants of the same species (trees which have different juvenile and mature foliage) – although this also exists in angiosperms the versions in conifers are unique genetically
5. Resin canals – the ducts in conifer leaves & stems containing secondary metabolites

Starting with venation, the vascular system of conifers (which performs water & sugar transport) has only one single vein or two parallel veins per leaf, running up its centre. This is shown in the examples of conifer leaf cross-sections below – purple shows the xylem (water transport) and the blue shows the phloem (sugar sap transport). (2), (10) and (14) have two parallel sets of veins and (5) & (12) have a single, larger vein.

By contrast the vascular system in the leaf of a flowering plant is much more sophisticated with many different vein patterns across species^ref and the average vein length per area in an angiosperm leaf is 2 to 5 times higher than in conifer leaves.^ref Some examples of angiosperm leaf venation are below – you can see veins branching and extending to every part of the leaf and this is one of the advantages that allow angiosperms to create larger leaves (hence the name ‘broadleaf’).

The vascular structure of conifer leaves limits how much water can be delivered to their outer edges. From the vascular bundle/s, ‘transfusion tissues’ or specialised cells conduct water and photosynthates to and from the margins.^ref Their conducting capacity is limited, which in turn limits how wide a leaf can become. In layman’s terms, because conifer leaves have basic water piping, they can’t grow too wide – which affects the size and shape that conifer leaves can take.

Conifer leaf shape categorisation is inconsistent across the literature, and you may see different descriptions such as awl-shaped, sabre-shaped or even intermediate (a catch-all for anything which doesn’t fit). A reasonable set of descriptions has been created by Paul Fantz at the North Carolina State University. But at the end of the day most conifer leaves fit into one of three types – flat, scale or needle leaves. A nice study was done in Iran which produced line drawings of the main three types of conifer leaf, which you can see below (and here). On the top is a flat leaf of Taxus baccata (yew), on the bottom left is a scale leaf (and stem) of Cupressus sempervirens (italian yew) and on the bottom right is a needle leaf of Juniperus communis (common juniper). Due to their shapes, each type of leaf is a little bit different in terms of how they perform in a given environment, and you can learn more about this in my posts about each type: flat, needle, scale. The fact that the same tree produces foliage of more than one type is covered below in the section about heteroblasty.

Now let’s consider the stomata on conifer leaves (to learn or remind yourself about stomata you can read my stomata post). Whilst conifers have the same basic structure for their stomata, with one guard cell on either side, they differ from angiosperms in their arrangement and effectiveness.

Conifer stomata develop at the base of each leaf, meaning that they grow out in longitudinal bands as the leaf emerges, whereas angiosperm stomata develop at multiple points on a leaf, resulting in more variation in their patterns^ref. In needle species they are arranged around all sides of the leaf (with a few exceptions), in scale leaf species they appear in the grooves between scales and stem, and in flattened leaf species they appear mainly on the bottom of the leaf. Below is an image of the stomata from a Picea species, showing them arranged in lines:

Stomata in conifers have a couple of other characteristics – often they are ‘sunken’ or set into the layers of the leaf, as well as filled with wax plugs.^ref This massively reduces the gas exchange capacity of the leaves – one study found that gas exchange was only 35% compared to species without wax plugs. Their conclusion was although this blocks the stomata and reduces photosynthesis, it may have been an advantage during wetter periods of earth’s history by keeping the pores free of water. The wax plugs also prevent fungal intrusion – which is more of a risk for conifers with long-lived leaves. Finally a less open stomata also reduces water loss. This allows conifers to survive in drier areas and to stay alive for longer with minimal water – hence they are now found in more extreme environments where angiosperms can’t survive. Below is a sunken stomata from a Tsuga canadiensis on the left and a Cryptomeria japonica stomata full of wax on the right.

Next we need to look at one of the most important attributes of a leaf – its photosynthetic apparatus and performance.

Whilst conifer leaves photosynthesise about 30% less effectively than angiosperm leaves^ref, they live and photosynthesise on average 50% longer when compared to angiosperm evergreens – and obviously much longer (around 300%) when compared to deciduous angiosperm leaves.^ref So overall conifers need to invest less resources to generate their energy, since each leaf works for longer periods. Where angiosperm leaves have a ‘live fast, die young’ lifestyle, conifer leaves are more ‘slow and steady wins the race’.

One surprising fact I came across while researching this post was that conifer seedlings can actually grow in the dark. They are able to synthesise chlorophyll and create the photosynthetic apparatus without light, and these are ready to work as soon as the plant is illuminated- although the amount of chlorophyll produced is lower than if the seedling has been illuminated.^ref This makes sense since seedlings may often germinate in low light conditions on a forest floor.

Like angiosperms, conifers can have different shade and sun leaves (this is known as ‘heterophylly’). In Abies alba (silver fir) sun leaves are on average longer, have thicker cuticles, more photosynthesising palisade mesophyll cells, fewer spongy mesophyll cells and more stomata than shade leaves, as well as significantly higher photosynthetic performance.^ref By contrast shade leaves contain 3 times more chlorophyll content and 2.5 times more carotenoids than sun leaves. Even the arrangement of sun and shade leaves look quite different – see the image below showing sun leaves on the left and the shade leaves on the right.^ref

Source: https://onlinelibrary.wiley.com/doi/full/10.1111/pce.13213

Another factor which determines the photosynthetic performance of a leaf is its age. Except for the few deciduous conifers, conifer leaves can last anywhere from one to 45 years, although the latter is unusually long. The data is scattered across many papers but to provide some examples, the majority of pine needles live for 2-8 years^ref , the scale leaves on Thuja plicata live on average 8 years^ref, and flat yew leaves also live up to 8 years.^ref Needle leaves live longer at higher elevations and with poorer conditions in general (such as lack of water).^ref

Which brings me to the topic of heteroblasty, or trees which have obviously different juvenile and mature leaves. It’s a well noted phenomenon in bonsai circles that certain junipers have needle leaves when young and scale leaves when older. It turns out that heteroblasty is observed in Cupressaceae^ref, Pinaceae^ref and Podocarpaceae^ref and results from what is called a ‘phase change’ in the shoot apical meristem. This is when the growing tips change to produce different organs – so instead of producing buds that become juvenile leaves, they produce buds which become mature leaves – and eventually buds which become reproductive organs as well. This phase change is relatively stable, so once a meristem produces mature foliage, it will continue to do so. It is also position specific – so the lower branches may retain juvenile foliage even when the rest of the tree has mature foliage.^ref

One explanation for heteroblasty is that it’s a useful way for plants to deter herbivores or other environmental hazards that exist for smaller, younger plants. New Zealand has a high number of heteroblastic plants (200 species), and academics have proposed that the unusual branching form in juvenile trees which is specific to the area has specifically developed to deter large ratite birds like emus and moa.^ref 10 such species were found which changed their leaves and branches once they surpassed 3m in height (the maximum bite-height of the ratites). However since there are no more moas, it’s hard to prove the theory, which is apparently hotly debated.^ref

Phase changes are controlled by genes and plant growth regulators, which change their expression when a meristem has undergone a certain number of cell divisions.^ref This was demonstrated by showing that mature flowering meristems, when rooted as cuttings, also flowered and so retained their mature state. This is why position matters when it comes to heteroblasty and only meristems which have reached the mature phase will produce mature foliage.

Since phase change to a juvenile state is desirable for plant cloning, there are studies which have considered how to maintain juvenility or reverse it in mature plants. One method for delaying phase change is to ‘hedge’ – what you and I would call pruning – presumably because this removes the apical meristem programmed for the new phase and reverts to meristems lower down the plant which haven’t changed phase. Another is to apply stress to a plant by starving it, dehydrating it or exposing it to heavy metals.^ref

The final and fifth familiar attribute of conifer leaves that differs from angiosperms is that they are almost all resinous. Conifer resins are mostly terpenes made up of linked isoprene elements (C₅H₈) and are conducted through leaves (as well as some cones and wood) through resin canals. 30,000+ different terpene structures produced by conifers have been identified – some of which are used to produce various products including turpentine, printing inks, soap, plastic, fireworks, and tar. The effect of resinous leaves is to deter insects (Farjon, 2008) and microbes.^ref Resin doesn’t feature too much in bonsai (other than when you’re cleaning your branch cutters), but the resin does provide a defensive benefit to your trees which is probably better than many of the chemicals that are sold for the purpose.

Anyway what does it all mean for bonsai? (Thank god I hear you say – it only took her 2000 words!!)

Well let’s start by acknowledging that conifer leaves are quite different from those of angiosperms. Their vascular system dictates that the leaves take one of the three forms – needle, scale or flattened, and aside from the few deciduous conifers, in general their leaves are designed to stay on the tree for much longer than most angiosperms. This means you’re not going to get the same level of leaf turnover on your coniferous bonsai as you would with your angiosperms, and your styling decisions need to be more carefully made and executed. It is going to take longer to fix a mistake on a conifer.

Similarly, their photosynthetic rate is not as high as an angiosperm, so in many cases a conifer is not going to be able to achieve the same growth rates as an angiosperm unless they have a lot of light, although there are some more fast-growing species. As per the previous point, conifers are less forgiving of poor styling decision.

Depending on its leaf type, your different conifers will prefer different conditions (full sun for needle, humid and less sunny for flattened), but you should also be thinking about how to cultivate the types of leaves you want to see on your tree. Sun needles appear denser and better for bonsai, so shading a fir or a pine is probably not a great idea. Similarly making use of short shoots with their increased leaf numbers is important (see my post on shoots).

Species which display needle-scale leaf heteroblasty are a special case as usually you want them to take on mature scale foliage which is preferred in bonsai. To do this, lower, older branches (with the juvenile form) will eventually need to be removed, and you shouldn’t prune the apical stem of these species until they have reached the mature foliage phase. Or sidestep the juvenile phase altogether by taking cuttings of mature foliage which should stay mature unless they are seriously stressed.

A final point would be to say that although conifers all fall under Pinopsida (etc) they have a much longer evolutionary path than angiosperms and more divergence between them, so lumping them all together into one post is not really comparing apples with apples (hehe). So have a look at the other posts which spawned from this one to dive into a bit more detail: conifer needle leaves, conifer scale leaves and conifer flat leaves.