bonsai tree in hot summer heat

Caring For Bonsai In Extreme Heat

How To Get Your Bonsai Through El Niño

Right now in Australia we are facing a long dry and hot Spring / Summer and possibly into Autumn, This can cause Havoc on a Bonsai Collection if we aren’t carful with our watering and we can also face the risk of water restriction like we did in parts of NSW in 2019. So lets take a look at a few ways we can reduce risk and mitigate any issues that may arise.

Adjust your trees positioning and Watering

With dry heat our trees will transpire at a higher rate with little moisture in the air and high ambient temperatures, This will increase our need to replace the trees water supply if just left in an open and exposed position. Now this if fine if you can keep on top of the trees watering requirements for most of our conifers and evergreens, Deciduous might need a little extra help but more on that later. If you are in a situation where you wont be around to take on the extra watering requirements then you can move your tree into a morning sun / Afternoon shade position. This will allow the tree to still get enough sunlight during the morning while the sun isn’t too intense and allow it to be protected once the intense afternoon sun arrives. This will help slow transpiration lowering your watering requirements. In this case though you still may need to water right before you leave in the morning and again when your return in the evening, but just remember there is no schedule, Gauge your trees and water as necessary.

Ideas To Reduce Water Loss


Soil Dressing

The soil surface is the place where the tree will lose moisture from the pot the fastest in dry hot conditions, once that dries then the body of the soil mix can then quickly begin to evaporate. To keep this under control we can dress the surface of our soil. There are 2 ways I would go about this that work well for large collections and 1 way for people with smaller collections of around 1-3 trees.

For larger collections

Sphagnum Moss : This is a great way to reduce surface evaporation of your soil as Spahgnum Moss holds a great deal of moisture and creates a fairly thick layer on the surface. I generally purchase Sphagnum moss and grate it through my large screen with my soil sieve to create smaller particles, the smaller the particles the larger the surface area for moisture. The only downside I find to the smaller particles is that if you allow them to dry out too much they can very easily blow away in the wind compared to if you just leave it long fibred, but ill leave that up to you. It also helps to have a watering wand with a soft head to prevent blasting your surface dressing away.
I would refrain from using regular moss even though it does actually prevent evaporation and is beneficial in a situation like this but in an extreme heat weather event it will just end up looking dry and horrible the whole time unless you have constant misting. Sphagnum is actually better at holding moisture, it just doesn’t look as good.
Small Particle Akadama : Your 1mm – 2mm akadama that you usually sieve and save for things like creating muck can actually be of great use here, Much like I mentioned with the Sphagnum and creating smaller particles to have a higher surface area for moisture retention the same is true for the akadama. If you put a nice layer of smaller particle size on the surface this will help slow the evaporation of the soil below. This method will be less affective then the Sphagnum but it still works well. If you can doing both is your best shot!

For Smaller Collections 1-3 Trees

Some people find that wrapping wet towels around the pot and covering the surface helps reduce water loss from the pot, As you can see this will only work with small collections unless you have hundreds of towels and plenty of time on your hands.
A Helpful Note
The techniques mentioned above help to reduce water loss at the pot level due to Evaporation so please remember to still check the soil as the top of the tree will still be transpiring at a normal rate even with these techniques applied, they only reduce unnecessary water loss from the pot due to evaporation. You will want as much water available to the tree as possible to slowing water loss from evaporation is certainly helpful.

Misting System

This one isn’t for everybody as this can take some time to set up and can be expensive for larger collections, but a misting system on an automatic timer is a great way to help cool the air and create some humidity around your trees, it is also a great way to help cool the benches and pots as well. Depending on how hot and arid your area is will depend on how often the system will need to spray and for how long. Don’t worry about having moisture on your leaves in the sun as this has been scientifically tested and it was shows that only leaves will small hairs will burn due to magnification of the sun though the water droplets as the hairs raise the water of the leaf, any leave that has a smooth surface means the water sits flat on the leaf and makes it impossible for the water to magnify the sun as there needs to be a space between the water and the surface. I would how ever keep an eye on calcium build up ( white crust ) on your leaves if using town water. This can be rubbed off but you wont want to allow it to get too bad.

Shade Cloth

Another expensive solution that also creates some work for you but none the less an effective solution, Shade cloth can help reduce the intensity on your collection while still allowing your trees to photosynthesis. This will also allow you to keep your trees nice and green throughout the summer period.

Tree Relocation

Like mention earlier picking your trees up and moving them to a position where they receive a good amount of morning sun but are protected from the hot afternoon sun can help slow overall transpiration of the trees. This solution isn’t exactly great for those with larger collections but for those with only a small amount of trees to move this can help a lot.

Water Trays

Placing water trays on each of your benches or even under trees that like it such as Australian Natives can help create some humidity around your trees, This isn’t a silver bullet but it is better then nothing on really hot days. For smaller trees like Shohin or Mame you can also fill a tray with river sand and place the trees on top, this will allow them to have a good amount of humidity while also allowing them to extend their roots out of the pot into the sand for the summer for a little extra water moving ability.

Drip Systems

Much like the misting system a drip system that is placed into each pot can help ensure your trees stay hydrated while you are away or cant tend to your trees. Also much like the misting system the amount of water dispersed and the frequency will depend on your environment and trees. You will need to keep an eye on the drippers with these systems when using town water though as calcium build up can quickly clog the holes, you can clear them by allowing the dripper to sit in a mix of vinegar and bicarb over night. Make sure you visualize the system running every few days to ensure you catch a clogged dripper, you don’t want to be falling back on a system that’s not working when you think it is.

Sprinkler System On A Tap Timer

This is a great way to have the trees get some Rehydration in the middle of the day if you wont be around, if your collection is smaller enough that you can have a sprinkler in the middle that will reach all your trees then you kind of kill all the birds with one stone, when the system goes off you will cool the trees, cool the pots, cool the benches and create a good amount of humidity, you will also replenish water in your pots. This will take some adjusting though as you wont want it going off to frequently causing the tree to stay too wet, but remember we are talking about getting through a weather event and not giving you advise for a permanent set up so do what is needed for your collection.

Other things you can do to help

Cool down your benches periodically : By hosing down your benches especially if they are wood you will not only cool them but you will create some humidity around your trees, but no matter what your benches are made of it’s a good idea to give them a hose when possible and cool them off.
Move trees away from radiant heat sources : Brick Walls, Metal Fencing, Steel Shed Walls, Concrete, Glass etc etc, These are all things that get very hot during summer and having your trees near these radiant heat sources can increase the amount of transpiration or even cook your trees.
Make Sure your tree is fed well with Potassium : Potassium is great for helping a tree deal with stressors such as Heat, Frost & Pests and Disease. Make sure your trees are well fed through out this growing season with something that has a good amount of Potassium.
Identify what trees can handle heat and what trees cant: Japanese maples I’m looking at you! Make sure your thin leaf species get some extra protection, This will be most of your deciduous trees. The thinner the leaf the faster the transpiration and usually most of us don’t catch it in time and end up with fried leaves. Conifers and evergreens tend to have a thicker waxier leaf so they don’t transpire as quick. But remember if your deciduous trees decide to drop their leaves during summer get them out of the sun or you could end up with burn marks on your trunks.
Reduce leaf mass : If you have some trees with a super dense canopy now might be the time to reduce that mass, The more foliage that is on a tree the more surface area it has for water loss through transpiration. Now in saying that I’m not telling you to go around and thin out your entire collection just for the sake of reduced transpiration but if there are trees that need it then now’s a great time to get that work done.
Stay on top of pests with preventative sprays: Pests attacking your tree will weaken it and reduce its tolerance to the extreme conditions, by preventively spraying with something like Congaurd once a month then you can have one foot in front of this issue. Now yes Congaurd ( Which is a direct replacement for confidor ) can be harmful to bee’s, but once again we are trying to get our trees through a tough situation. Your options are to stop the congaurd spray when bees will be present and resume again once they move on or just don’t spray your flowering varieties but do the rest. Congaurd will hang around for about 4 weeks on your trees so keep that in mind if you are trying to create an imidacloprid free window.
Collect Water While You Can And Keep A Reserve : In 2019 we ended up in a situation In some parts of NSW where we had water usage restrictions and one of those restrictions was no watering in the garden, This was at a time when the weather was blistering hot and the trees were drying out fast. After that I made sure I always had my own water storage. In my situation I sourced a 1000l IBC tank which are light and maneuverable and take up little space. You can pick these up on facebook market place for around $60 -$120 depending on the seller. Just make sure you get one in which you can be sure what was originally stored in it as you don’t want to poison your trees with residual chemicals. There are plenty that are just used for water storage though. I have mine hooked up to the guttering on the house and with 1 or 2 good down pours it will be full. I then purchased an 18v transfer pump from bunnings which has enough power to water my trees with my hose like normal and I can even do it if the power it out due to the 18v battery.

Final Words

I’m hoping that the tips above can give you some ideas to give your trees the best fighting chance they have in drought like conditions. I would also try and source out some information on how to keep your property’s protected in these situations as well but I am no where near qualified to give that type of advise, Maybe join your local fire departments facebook group for tips. In this heat and dry weather fire risk is increased greatly and unfortunately a collection can get burnt up in minutes so having some prevention for property damage is also something to be on top of.

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Author : Joshua Hooson

Joshua Hooson is an author and enthusiast of the art of bonsai. He has built his knowledge and understanding of bonsai through a combination of self-experience, lessons learned through hands-on practice, and extensive research. His articles reflect his passion for the subject and offer insights gained through his own personal journey in the world of bonsai. All the information provided in his works is a result of his own experiences and the knowledge he has gained through his studies. He is dedicated to sharing his love of bonsai and helping others grow in their understanding and appreciation of this ancient and beautiful art form.

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