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Overhead Watering Bonsai, Good or Bad?

Updated: Jan 7, 2023

We have all heard the story of " don't water the foliage on your tree as you will get leaf burn ". But is this really true or better yet is this the real issue? Lets take a look.

Watering bonsai with a watering system

In a perfect world we would just have a watering system set up to run at different intervals each day to keep our trees happy and hydrated but the more you get into bonsai the more you realise this is not a reality and overhead watering is best left to the large nursery industry. In short i will tell you that you should always hand water your trees where absolutely possible, not only is this better for your trees but you also get to closely inspect each tree on a regular basis so you can catch other issues early such as pests and disease and things like wire bite, But what are the issues with overhead watering?

Leaf Burn : Now let me just start by saying this is an old wives tale and not something to worry about, as the story goes if you water your foliage on a hot sunny day the water droplets will concentrate the suns energy on a pin point location and cause some leaf burn. Think about this for a moment, if you are trying to burn a leaf with a magnifying glass will it work if you sit the glass directly on the surface of the leaf? No, you have to move the magnifying glass a certain distance away from the leaf until the sunlight is all directed to a single point. Now the same goes for a water droplet, if it is sitting directly on the leaf's surface it wont be able to concentrate the suns energy into a single point, you would have to suspend the water droplet above the leaf's surface to be able to get the light concentrated enough to cause damage.

So if leaf burn isn't the problem then what actually is the problem?

Calcium Build Up : Ok so for most people using town water there is a chemists list of chemicals that go into treating the water that comes out of our tap to ensure there are no nasty bacteria that will harm us when we consume the water. The first big problem we have with town water is usually a PH that is higher then optimal for the tree to be able to uptake nutrients, but we have a separate article for that. The main issue is in hot weather if you water the foliage on your tree as the water evaporates it leaves behind those chemicals on the leaf's surface, the main one you will see is calcium which you can normally see as a white residue on the lips of your bonsai pots or down the sides of your bonsai pots. When this calcium builds up on your leaf's surface it creates a barrier causing a reduction it the photosynthetic capabilities of the leaf. This is much the same as if you have a Solar system on your house roof, if you don't clean the panels every year you will notice a reduction in the amount of energy produced by the system due to the build up of dust and grime on the solar panels surface. Obviously if you are watering with collected rain water this is not really an issue for you and giving the leaf's a good douse during hot days will actually be beneficial as it will cool the outside of the tree and reduce transpiration. You can see how this single technique can be both good and bad at the same time depending on your specific circumstances.

Fungal Infections : Ok so what about cooler weather and overhead watering? When the weather is cooler we don't see so much evaporation of the water on the trees foliage but what we see is almost worse, we see standing water. When water just sits in our trees inner branches and exteriors in cool weather we are basically inviting fungal infections over for a dinner party. in cool weather there should be no reason at all logically that you would need to actually water the canopy of the tree so it should be avoided at all costs.

Insufficient Water Levels : When watering by hand we can be absolutely sure the tree got enough water by continuing to give the pot a good drench until we see the water rushing out the bottom of the pot like a tap. When overhead watering with an irrigation system for example or even worse a misting system it might take a good few hours for the pot to actually get enough water in it to be sufficient. When the water is delivered at such small rates this can be confusing as the soils surface will appear to be soaked but how much of the core is really properly watered? You may recall an article i wrote a while ago about the need to still water on some days when it rains due to the rain actually not having enough volume to properly water the soil mass, the same applies here. This provides me with another good point which is if you have a full canopy on your tree is the water penetrating the canopy and hitting the soil surface and is the trees dripline over the pot or does it land outside of the pot? This could mean that even though you run your watering system for 2 hrs lets say in that 2 hrs your trees pot may have only taken on a very minimal amount of water that is not sufficient for the tree, especially in hot weather. You will see it all too much if you pay attention to Facebook pages where people state that they went away on holidays for 2 weeks over summer and they had a watering system that came on every 4 hrs during the day for 30mins but when they came back all their trees were suffering or dead. they cant figure out what happened.... well now you know.

I hope this has given you something to think about when watering your trees moving forward, i know most people water by hand but there are some who try to leave the watering to watering systems and usually pay the price for it without understanding why. I know the common argument is usually " but what about large scale nursery's, trees in nature or large scale agriculture " and my counter argument would be to think a little harder about the differences of each of those and you will find the answers and it will also teach you how to think more critically about your bonsai practise which will be the best technique you have ever learnt.

Until Next Time, Enjoy Your Bonsai Journey And Enjoy Your New Year.

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3 commentaires

03 janv. 2022

Hi Josh, Although I fully agree that watering pot plants, bonsai including, it's much better done by hand than by rain or mist, I have a very different experience in regard of "burn" marks left by water droplets. Those are much more real than "old wives" tales. I regularly observe such marks after accidentally leaving a plant with wet foliage in full sun so I try avoiding it by any means.

Your analogy with a magnifying glass is incorrect for two reasons. You seem to assume that the marks are real "burns" resulting from heating leaves to a few hundred C. But they are not. It sufficient to heat up a leaf tissue to 60-70 C for it to die.…

04 janv. 2022
En réponse à

Thanks for the interesting and useful report Josh. It's a relief to know that I can be less cautious when watering some of my plants. However, some of paper conclusions are consistent with my experience. I have observed "burn" marks on my hoyas, jades, and other succulents with hydrophobic leaf surfaces. Although their leaves aren't hairy but very waxy so supporting more spherical droplets and making "burns" possible according to the paper. But bonsai are safe :).

It's surprising that the authors assume the horizontal leaf surface and then are concerned with the sun inclination angle. Real leaves are at all possible angles so there are always such that are positioned for more efficient sunlight focusing at any time of…

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