Is it ok to water with hot rain water?

[Rodrigo]

I have a few bucket fulls of rain water that I'm using on my Maples to try to fix some issues I've been having. But having them in buckets, in 100+ degree heat obviously heats up the rainwater significantly.

Is this an issue? I understand using cold water when the roots are hot may cause shock but is there a limit to how hot the water can be?

 

[TrevorLarsen]

I personally would just wait until a morning to use it when the water will be coolest, but I have no experience if it’s bad to use hotter water.

 

[WNC Bonsai]

Imalways drin my hose before watering just in case the sun heated it up. Tree roots don’t like it too hot.

 

[roberthu]

Imalways drin my hose before watering just in case the sun heated it up. Tree roots don’t like it too hot.

Same. Drain the hot water out of the hose first.

 

[Glaucus]

People always warn of hot water sitting in a hose. So during an entire summer, I would spray the first bit of hot water on the same patch of grass on the lawn. It didn't get damanaged.
Would you put 60+C water on your prized bonsai? Probably not. It definitely gets hot in such a small diameter hose, sitting in direct sunlight all day.

During a heat wave, I had to water with quite warm well water that was sitting in a 1000 liter container. But during a heat wave, warm water is better than no water. I'd rather have used chill water. Didn't measure the temp but it was definitely noticbly warm. Having sat in the sun for a few days during 35+C temps.

Not sure about your buckets of rain water. Are they really already that hot? Surely, the water isn't 100F?

Eventually, even the warm water will evaporate, cooling the plant.

 

[Deep Sea Diver]

Believe we are taking about water from rain buckets that is 100F

Really nice question! At 100F the trees are likely shut down, or close to shutting down all metabolization.

Yours seem to me to be a two part question.

What is the temperature of the media?

One year we did a little experiment measuring the difference in media temperatures before and after watering one hot summer day. Actually a closely related question to yours.

- The preliminary data we gathered was on that day was:
The media of black nursery pots 1” in from the edge was 102F / 88.9C
The media of the deeper 3” ceramic bonsai pots 1” in from the edge was 89F / 31.6C
Once watered the temperature in both pots returned to just below air temperature in 10 minutes.

This in no way was an exact study, just some backyard science. Hopefully this summer we have enough time to do a more comprehensive run and include shallow pots and inner media temperature.

What are the functions of water in a bonsai pot?

Besides the basic function of water (as applied to this question)?
- Exchange of O2 and CO2 and transportation of nutrients
- Hydration of the roots and media
- If cooler than the media, cooling of the roots and media.
- If warmer then the media would heat the roots. Be interesting to check inner media temperatures in your pots?

Water of the same temperature as the media, 100F would perform the first two functions. As @Glaucus mentioned.

It wouldn’t hurt the roots if they were already at that temperature and were unharmed (Another good question - dependent upon the tree species)…. and once the water began to evaporate out of the pot, likely might have a slight cooling effect.

Watering in the early morning would work then, but the rest of the day the trees would need water too. When it was 114F here we needed to water some trees three times along with periods of misting.

Cheers
DSD sends

 

[Glaucus]

Even if the soil is 95F then 100F water will cool becuase of evaporation, As long as the soil is dry. 95F air can't cool. 100F water can.
Water is cooling fuel for the plant. No water means no cooling. As long as it is liquid, it can evaporate, thus cool. The energy of the phase transition is key here. Doesn't mean you should pour near-boiling water on a plant, though.
But if there's still 70F water in the soil, maybe don't poor that 100F water on the roots.
Very likely the water is cooler than the air. But maybe not the soil? Though it may depend on time of day and if the buckets had direct sunlight.

If you put 100F water on your skin, it may feel warm at first. But if you put a fan on your wet face, the water will start to feel quite cool.

 

[WNC Bonsai]

Even if the soil is 95Fm then 100F water will cool becuase of evaporation, 95F air can't cool. 100F water can.
Water is cooling fuel for the plant. No water means no cooling. As long as it is liquid, it can evaporate, thus cool. The energy of the phase transition is key here. Doesn't mean you should pour near-boiling water on a plant, though.
Very likely the water is cooler than the air. But maybe not the soil? Though it may depend on time of day and if the buckets had direct sunlight.

Yep, I have a dark green watering can I use for spot watering and the outside can feel quite warm, however the water inside is usually much cooler which may be the case with the water buckets.

 

[Glaucus]

Using bigger buckets, using white buckets, covering the buckets with shade cloth or a parasol, etc would all help as well.