Bonsai & Bleeding Trees.


A little lesson on grammar and the English language with a healthy dose of personal opinion mixed into my experiences of bleeding bonsai trees.

Warning: Contains strong language and personal opinions. If you are easily challenged or offended please f’ off now.

Look up the word bleeding in the British English slang version of its usage and you will find it’s an adverb used as an intensifier. As an example, were I to describe someone as stupid but needed to add additional weight or offence to my opinion I could add the word bleeding as a precursor. Collins dictionary lists seven explanations for the word with the primary definition being the act, fact or process of loosing blood. You’ve just got to love the English language.

Bonsai & Bleeding Trees

Bonsai & Bleeding Trees. Tis but a scratch!

Sadly, in this day of ‘alternative facts’ language is being messed with on an unprecedented scale. Just yesterday (Monday) the almighty and holy BBC reported that oil prices rose sharply on news of the recent conflict that’s kicking off. Apparently that’s going to lead to another pump price increase. On the face of it all sounds reasonable right? However for the previous 11 days the price had been falling quickly from $94.3 to $84 but they picked up on the fact that it jumped $4 a barrel (Brent) to $88. So they were right the price did rise but overall it fell when looked at in the context of a few days. Seems like everyone wants a sizzling headline whilst the actual facts are irrelevant.

Another area where understanding is being lost, or at least confused is in the act of conflation, the merging of two or more sets of information into one. For instance nuclear generation of electricity being labeled as clean green energy. It might produce relatively little carbon pollution at the moment of generation but in all other respects it’s as grubby as a teenage boys under-crackers. 

Our English language has a huge and easily mis-understood vocabulary, by some estimates over a quarter of a million words most of which we will never use (or possibly understand) but, by and large we work on the basis that if something sounds right we generally believe it. After all figuring out the truth takes some effort.

For example climate change, sorry i’v got to get this out there. “They” recon it’s all our fault for driving cars and heating our homes with the fuel ‘they‘ provided for us. That means 1.47 billion cars need to be replaced along with practically every other piece of electrical and industrial equipment on earth and that has to be done without using oil or coal. Are these folk fucking mental? I’m about to be 60 years old so have limited experience and they may well be right, however the postulation that climate is stable appears (to me) flawed even though it’s presented as the bedrock upon which all this BS is founded.

Seeing as the area i am currently sitting in was, supposedly once, both a warm tropical sea and a glacial till plain it strikes me the climate is prone to change from time to time. For sure the exploitation of oil has allowed the human race to burgeon way beyond anything that is either sustainable or sensible. Ultimately the conflation of virus like human multiplication and tiny increases in average temperatures have given rise too, possibly the largest and most lucrative money and power grab in our history.

Ignoring the fact that we cut down all (or most) of the worlds trees over the last couple of millennia. And conveniently ignoring the fact trees lift moisture to cool the atmosphere (as we will see later), create cooling shade, trap moisture and light, collect carbon and produce oxygen, alongside our once healthy seas it’s YOU and I that fucked it all up with our cars and gas boilers. I could rant about this shit show for a month but best I just offer this as an example of how conflation can lead us up a blind alley. Time will tell.

We live in one of the greatest times in human history and have the assembled knowledge of the entire human race at our fingertips. So, what exactly are we doing with that? Looks to me like not a lot, getting at crossed purposes with folk, causing conflict and in many instances becoming more stupid……bleeding stupid by the day. I while away many happy hours with a herf at my local old school motorcycle shop where the conversation revolves primarily around two subjects, everyones medical woes and the stupidity of youngsters and folk not in the room. My friends, ignorance abounds.

It’s so easy to find stuff out today but sadly so many of us just don’t bother, we’re all guilty of taking the easy route much of the time. However real knowledge is a hard won thing, especially where that’s learnt through life experience. Like the BBC’s fuel price shocker, it might be true but reporting that fuel prices spiked without the context is at best deceptive and misleading and will cause considerable angst among some already hard pressed folk trying to make ends meet.

Bonsai & Bleeding Trees

So what’s all this got to do with bonsai? Bleeding bonsai, or more specifically trees that bleed when they are pruned. Over the years I have had a lot of discussions with folk over the subject and thought it might be helpful if I put what I have learned down on paper or to be precise pixels.

Liquids flowing out of our trees generally cause a little panic. This typically happens in spring around the time leaves begin to appear and follows damage of some kind like pruning. Liquid running out of a tree raises the spectre of a slow but imminent death because we have made an unconscious conflation between our knowledge of animals having a limited amount of blood and applied that to our bonsai tree. Cut a good lump off most animals, including ourselves and in pretty short order the heart will pump our veins and arteries dry leading to oxygen starvation with obvious consequences. 

Trees are different, they don’t have hearts, veins, arteries or limited blood supplies. The amount of liquid available to a tree can be large because it’s water from the soil. Trees do not rely upon a single organ (lungs) to absorb oxygen or a liquid like blood to distribute that oxygen to its extremities. Trees are able to absorb air, including oxygen through their leaves and external bark, including the roots so all parts of a tree are independent of other parts for oxygen supply.

So why do trees bleed? Does bleeding affect or weaken a tree? Does bleeding indicate declining health? How can it be prevented or treated? In order to answer those questions we need to understand exactly what’s going on.

A tree, including a bonsai tree, is a naturally closed hydraulic system. Water is absorbed from the soil into the roots via osmosis where it slowly moves through the plant to the leaves where a small amount is combined with carbon dioxide and made into sugars. Up to 99% of the water absorbed by the roots evaporates into the atmosphere by a process known as transpiration.

Movement of water from soil to roots to leaves is driven by two forces or ‘pumps’, one in the roots and the second in the leaves. These are connected by fine tubes, xylem vessels in broadleaves and tracheids in conifers which extend throughout the entire plant.

Water only rises through the tree in the outer edge of the woody material, the xylem. The remainder of the trunk is actually dead and serves as the supporting structure for the living part which is why a tree can be hollow but still live happily for centuries. The water rising up a tree is commonly known as sap, a solution of mineral salts in water all drawn from the soil.

In late winter or early spring this sap is enriched with sugars released from storage in the roots and used as energy for initial leaf and shoot formation. Maple syrup anyone?

Bonsai & Bleeding Trees

Bonsai & Bleeding Trees – Not a recognised bonsai technique.

Osmosis happens when water moves from a weak solution in the soil through a semi-permeable membrane, the root cell wall, into a more concentrated solution within the cell. Water entering the roots in this way creates a tiny pressure which forces water up through the xylem tubes into the tree. Further movement is assisted by capillary action in the very fine xylem vessels that are connected from the finest root to the last leaf.

Evaporation from the foliage forms the second ‘pump‘. As water evaporates from the leaf more water is drawn in to replace what’s lost via the xylem. An unbroken hydraulic column of water is formed right from the fine feeder roots to the very last leaf. The leaves will wilt if water evaporating from them is not replaced as required. A prolonged water imbalance results in leaves withering and ultimately dying off which reduces the plants water needs and hopefully returns everything to a balanced state. Young trees have little defence in this situation but older mature trees are significantly more robust.

Evaporation from leaves increases with warming temperatures and warm air movement. We all know watering is required more often in summer unless you live in an area that is so warm and humid that evaporation ceases in which case water usage is minimal, not a problem in Blighty. Here cooler wet days reduce transpiration and may see our trees almost stop absorbing water from the roots, especially when it’s raining or misty as water can be drawn directly into the leaf thought the stigmata or should that be stomata?

In dormant leafless trees evaporation from the upper part virtually stops but roots remain active. Evergreens need to replace small amounts of moisture lost through their foliage. Whenever the soil temperature rises above freezing roots will be busy absorbing water until the weight of the column of water within the tree is equal to the force exerted by osmotic pressure and balance is achieved.

The resultant pressure of water within the roots ensures that water carrying channels are full and primed to supply water needed to swell buds, leaves and shoots as soon conditions are suitable for growth to begin.

With positive water pressure within the tree any damage, for example pruning, may release pressure and allow sap, normally a colourless watery liquid, to escape. We call this ‘bleeding‘ because it looks a lot like what happens when we suffer injury, we spring a leak. However the conflation of leaking sap and blood are as relevant as pissing and a punctured tyre.

This leaking process, trees that is, can continue for many weeks as long as there is water available to the roots to continually add to the flow. However bleeding usually stops once new leaves have grown and evaporation begins to relieve the water pressure within.

Bleeding, following wounding, is not inevitable, not all tree species bleed. Evergreens tend not to bleed as a small amount of evaporation from their foliage helps relieve sap pressure. On the other hand maples or birches can gush sap for weeks. A trees response to wounding varies with the time of year and when damage occurs. In most broadleaved species the serious risk of bleeding is in winter and early spring just before buds open as the internal pressure is high.

Not all damage will result in bleeding. For instance where the trunk of a conifer is grazed, removing an area of bark but not damaging the underlying wood the tree may ooze a sticky resin in an attempt to re-seal the effected area. Where damage breaks the capillaries, the xylem vessels, there can be a release of watery sap.

Late winter or early spring pruning, particularly of deciduous trees, is the most common cause of bleeding simply because the sap pressure will be highest then and a prune will normally sever the xylem vessels. Very sharp scissors will make the bleeding worse because so many capillaries will be left open ended. In order to prevent the possibly distressing sight of your bonsai tree bleeding profusely it’s best to do your pruning in autumn (which may not always be advisable on all species) or late spring just as the leaves begin to open. Personally i’m not squeamish and far from bothered about gushing sap bleeding out of my charges and in over thirty five years mucking about with all this I am yet to see a single example of a problem directly attributed bleeding.

It’s a natural reaction to try and stop our tree bleeding following pruning. However in my experience, regardless of what’s used to try and seal a cut, the product will be at best ineffective. Conventional putty type wound sealers are easily pushed off by sap pressure. Water based sealers like Kiyonal tend to get diluted and washed away. Oil based paints are simply circumvented. I have seen folk try to cauterize a wound with a flame of some sort, this may be temporarily effective but sap will soon begin to flow again through a wound that will now be larger than it was at the start. My advise is DON’T BURN TREES!

Some bleeding from a cut may be beneficial in keeping the wound free of infection. Bleeding will stop once new leaves appear and sap pressure is reduced. If this whole thing bothers you the best cause of action is to avoid pruning in late winter or early spring. Now I realise that’s not always convenient or desirable and should a spring trim be required I would suggest allowing the trees soil to dry out for a few weeks prior to working. This slight drought should reduce sap pressure a little and reduce the sap flow. I should caution, do this carefully as it’s possible to do more harm than good.

We know that water moves upwards within a tree however there is significant evidence to prove that it can also move downwards too. Experiments on the desiccation of nursery stock have proven that water can be drawn out of roots and back into the soil where the growing media becomes critically dry. That’s not unlike fertiliser burn which causes reverse osmosis. In bonsai controlling soil moisture is something we have to become ‘past masters’ of as I discussed in my recent post on – How To Stop Killing Bonsai Trees – Thoughts On Winter Care.

Very occasionally it’s possible to find the leaves of a tree bleeding sap. Typically this happens in summer when the conditions are nicely warm, very humid and the air is still. Droplets of water can be seen on leaf edges. This is not the result of wounding, it’s an indication of excess water pressure within the tree caused by excessively wet soil conditions favouring rapid absorption from the soil whilst evaporation from the foliage is very limited by conditions. This process is called guttation and is a common phenomenon in temperate areas where conditions are warm and humid. The phenomenon is more commonly seen in species with toothed leaf edged like elm varieties. Guttation is very common in tropical species like ficus.

As mentioned earlier conifers don’t tend to bleed sap in the way deciduous trees do. Where a conifer is damaged it will typically exude a sticky resin. Where wood boring insects enter conifers the tree will flood the area with resin that will seal the wound but will also trap and kill the offending intruders, that’s how we get insects trapped in amber which is just fossilised tree sap. Once the resin solidifies the wound will be permanently sealed and protected. In general conifers don’t bleed resin for no reason, it’s almost always a response to an attack by something undesirable wether that’s an insect or a knucklehead with carving tools. 

Resin leaks from bonsai can cause a problem in as much as they look bad. A slick of sticky resin down a trees beautiful craggy bark can ruin the appearance of a beautiful bonsai so be careful where you go cutting, choose the time of year appropriately and go little by little. In the past I have seen resin run out of conifer for more than a year. That made a hell of a mess and in the end the only way I got it to stop was isolating the exact point of the leak and then using a flame to boil the resin which did stop the flow once it cooled. Cleaning resin from bark or deadwood in bonsai is not easy but it can be dissolved using turpentine. Do not use white spirit, that’s made from oil whilst genuine turpentine is made from distilled tree sap. Methanol (methylated spirit) may also be effective. DO go easy with this as either could prove toxic to you or the tree used inappropriately.

To sum up, sap bleeding from a wound or guttation from leaves is not normally harmful. However do bear in mind the sap will contain a weak solution of sugars and whilst this may not be detrimental to the tree, even over several weeks it could be possible to cause weakness if one continues to cause extensive bleeding year after year. Also bear in mind sugary sap can attract insects or fungal moulds if allowed to dry on the tree over an extended period if rain does not wash the sap away from time to time. There is always a reason why a tree may be bleeding sap and if it’s happening spontaneously a careful investigation into the cause should be made.

I’ll give the last word here to Walter Reeves from Georgia

River birch tree sap is known to cause babies to be born naked and to make your hair turn grey when you’re old. Other than that, the sap that is dripping from the cut branches is harmless. 

Thanks for reading. Please share if this has been helpful 😉

Graham Potter

October 2023