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TOPIC: About roots, heat, soil and pots

About roots, heat, soil and pots 8 months 3 weeks ago #33337

Hello, all;

I have some of my bonsai in ceramic or porcelain pots, some in clay terracotta pots.
Concerning the trees that are in the afternoon sun (we are having a very warm Autumn in Brazil), by the end of the day the pots get really hot on their outside walls.
We know the roots usually don't like light (well, my Ficus seem to grow roots anywhere, be it dark or light, but that's another topic), so my questions are:
1-) Is there a problem with the roots if the soil gets warm?
2-) Is the soil we usually use for bonsai worse than organic soil to maintain the temperature inside the pot cool? (I get this impression; my pomegranate is planted in organic and feels cooler than the inorganic to the touch).

Thanks in advance, any info will help.
  • Clicio
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if you have never killed a tree, you are not doing bonsai

About roots, heat, soil and pots 8 months 3 weeks ago #33345

Yes. Heat is unnatural and potentially damaging. I would imagine organic substsrate to heat up less. I have thought about a stad that shields the roots from summer sun myself..
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About roots, heat, soil and pots 8 months 3 weeks ago #33346

Clicio wrote: Hello, all;

I have some of my bonsai in ceramic or porcelain pots, some in clay terracotta pots.
Concerning the trees that are in the afternoon sun (we are having a very warm Autumn in Brazil), by the end of the day the pots get really hot on their outside walls.
We know the roots usually don't like light (well, my Ficus seem to grow roots anywhere, be it dark or light, but that's another topic), so my questions are:
1-) Is there a problem with the roots if the soil gets warm?
2-) Is the soil we usually use for bonsai worse than organic soil to maintain the temperature inside the pot cool? (I get this impression; my pomegranate is planted in organic and feels cooler than the inorganic to the touch).

Thanks in advance, any info will help.


These are very interesting questions. I am going to take an educated guess at them, but not an "experienced" guess, mainly because I am interested in the discussion.
1) I don't think so. As long as they don't dry out, and you keep watering, you are fine. I wonder if there is a temperature where they pot would become like an oven and literally "cook" them.
2) This is a good question, and I think there are two ways to look at it. For the organic soil, when you touch the pot, it feels cold, because there is heat transfer from the surface of the pot to the soil, since the soil is cooler (heat flows from hot temp to cold temp), there is distributed heat, but that doesn't necessarily mean the roots are cool, or burning. If you touch the inorganic soil's pot, and it is hot, it means that the pot is hotter than outside, and heat is flowing from the pot to outside (more accurately to your body, so the pot is hotter than your surface temp). This would suggest that the clay is heating up, but the inorganic soil insulates the roots and heat doesn't reach the roots, therefore, inorganic soil is actually keeping the roots cooler, as the clay will absorb the heat and find an insulating barrier (the inorganic soil), which kinda makes sense since inorganic soil is mostly small rocks, right? heating up rocks is a lot harder than heating up wet organic soil!.

Look at it this way, pour a hot drink in an insulated cup. The cup surface stays cold, but your drink stays hot. The cup's surface doesn't allow heat transfer. Now, instead, pour the hot drink into a non insulated cup, like regular glass. The glass heats up to touch, which means heat is flowing from the cup to the atmosphere; your drink is getting colder. Makes sense?

More similar to your case. Put ice and a few beers inside a cooler and go to the beach. The surface of the cooler will get pretty hot, but the ice will stay cold.

To play it safe, you could think about burying the pot in an insulating material. Same principle as keeping the roots warm during winter in cold regions; it is all about heat transfer.
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About roots, heat, soil and pots 8 months 3 weeks ago #33348

Also, you should run a quick and simple experiment to make sure what's going on. Put a cheap, common thermometer in each pot (half way to the bottom of the pot or so). There are a lot of things going on here that I am ignoring such as water retention of the substrate, material of the pot, size of the pot, paint, radiation due to location of the pots, etc...
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About roots, heat, soil and pots 8 months 3 weeks ago #33350

eangola wrote: Also, you should run a quick and simple experiment to make sure what's going on. Put a cheap, common thermometer in each pot (half way to the bottom of the pot or so).


Good idea, thanks, I will do it.
Maybe insulating the pots when the weather is very hot (direct sun) will work; I will try that too.
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if you have never killed a tree, you are not doing bonsai

About roots, heat, soil and pots 8 months 3 weeks ago #33376

Living in the tropic like yourself, the sun comes up straight away 5:30am and by 10am-2:30pm it has its highest peaks. I got plants on rock slabs 2-5cm soil Bonsai pots (ceramic, concrete, stone, terra cotta) and plastic, I water in the dry season twice a day, around 7-8am and then 4-5pm. 6:30 daylight is gone, this al year around.

This watering gives the plant enough moisture to survive, I haven experienced any problem yet with heating any plants to death, however more then three days without water then the killing starts (by experience). if you look around especially in the dry season some plants get hardly any water only moisture fluctuation at night, but they still manage to survive (depending on the specimen). I took ficus of roof tops that haven't seen rain for more then six month. I could believe in Europe and areas where the sun has a direct long term exposure on the pots from the side that this may have a different impact, Spain, Italy, Turkey and so on that a generally hotter and have long day sun exposure. I am interested to learn what other experiences are.
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