Soil Mix for Hot & Dry climate?

I live in Los Angeles, San Fernando Valley. The back patio where the my trees are has a lot of southern exposure direct sun and seems to run about 5º hotter than what any weather station says my temperature is.

I’ve got pomegranates, junipers, olives, bald cypress, crape myrtles, bougainvilleas–all stuff that loves heat.

I recently took Mirai advice and went to a non-organic soil mix of lava, pumice, diatomaceous earth, akadama. Before switching, I had some Ocean Forest potting soil mixed in on the advice of a local guy.

Since dropping the organics, my pots are drying out at an alarming rate and need to be watered twice or more a day. While this is ok if I’m working from home and can babysit them, I’m looking for solutions that will take my waterings down to once a day for most of the year (temps under 100º).

I have some ideas, but can people in hot dry places comment with their thoughts?

  1. Ditch the largest 1/8’-1/4" particle size, so I’ve only got particles from 1/16"-1/8" (I have all three size screens). That should increase water retention.

  2. Add finely chopped redwood bark to increase water retention, or even screened potting soil. (seems common here in SoCal)

  3. Put up some shade cloth. I hesitate on this because I don’t want to rob the trees that like the full sun of what they want. I already have shaded areas for my recovering trees, camellia, azalea, etc.

  4. Come up with a creative solution to shade just the pots. (I have tried reflective bubble wrap circles with a cutout so you can put them over the pot and remove to water.)

  5. I have my shohin trees on a tray with water and large pumice stones - this doesn’t seem to help all that much, but it is something I can use in conjunction with other methods.

Since trees don’t do much growing when temps are over 86º, maybe on the hottest days (100º+) I just move everything into the shade. But it is in the 80s and 90s for a large portion of the year, I’d like to find a way to keep them in full sun most of the time.

I am grateful for any thoughts from people in similar climates.


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Hey Keegan,

I live in Livermore, CA 40 miles east of San Francisco. We have a dry climate that sounds similar to yours. I struggle with the same issues. I tried more organics in my soil and found it impossible to strike a balance of H2O and O2. I since have been strictly 1:1:1 pumice, lava, Akadama. If you can find Clay King mix, it has slightly higher Akadama % which will keep your trees moist longer, but still maintain particle size conducive to balance of H2O and O2. In the heat of summer I put 30% shade cloth over my deciduaous, and with a heavy Akadama mix my junipers and pines do fine until I get home from work. I have also added misters tied to my sprinklers timer which I control my phone. Hot days and I open the app and the misters come on not to water but cool the benches/containers, and the foliage loves the moisture on hot days. I am not an expert, but my experience has been any organics in the soil ends up with root gnats, larva and root rot. The only time I use Any organics now is when added to pumice with the trees developing in colanders as the massive airflow keeps the pine bark from staying too wet.


I’m in upstate ny so take what I say with a grain of salt but there are a couple things I can offer that I know could help. First is to double up on top dressing. I’ve heard that people in your area have a hard time with getting collected moss to grow on the soil suraface, I don’t know if that’s an issue for you but it doesn’t matter. Even if you can’t get green moss to grow, just use sphagnum. And put a top dressing layer of sphagnum on that is double thick as to what Ryan might use. This will hold moisture in your soil longer, you will be surprised. I’m not sure if organics is a great idea, and a smaller particle size will retain more water but I’m not sure that’s ideal for every tree you have. Maybe consider altering your soil ratio to incorporate more of the water retaining components. Something like 1:1:2 pumice:lava:akadama/DE?


This is a phenomenal idea! Do you have a brand of wifi timer you recommend? I have a wifi socket for my espresso machine–I assume it’s the same idea.

@Mike - yeah, I had thought about doubling up on top dressing too. I’ll try it.

I just set up an experiment of sorts, with three equal trays by volume of soil:

  1. standard soil mix, 1/16th-1/4.
  2. fine soil mix, same as above but sifted 1/16th-1/8th
  3. Same as #1 except with some organics mixed in.

I’ve laid out the trays and we’ll see if there is a difference in who dries out first.

I did water retention testing (by weight) on them, and #1 with the finer particles was the clear winner–but we’ll see how they evaporate in the heat of the day…


This is the timer I have… little pricey, but worth it.


Do you mist foliage? I’m in a hot area but relative humidity here is 50-70% so water loss isn’t as much. The reason I ask is because I’m preparing to setup a misting system for the hot summer days when I’m at work (12 hr shifts so it’s not like I get home at 6). Lmk if it’s safe to mist foliage or if I should focus on pots.

With that type humidity I’m not sure, but my thinking is you should be fine as long as you maintain balance of H2O and O2 in the container. Misting inhibits the transpiration process, so your vascular tissue uses less of the available moisture in the container when you mist the foliage.

Just keep in mind, it is not the air flow that dries out the soil, but rather the root activity and transpiration at the foliage tips. You can put two pots out in the sun with the same soil, one has an active tree the other just soil, and the pot with the tree will dry out rapidly compared to the pot with just soil.

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I’m near Yuma, AZ, and this year I’m trying out a 1:1 of DE, lava, pumice, and bark. I’ve added small bark nuggets at a 1:1 ratio with the rest of the soil. I found seedling sized bark for orchids that was ideal. It was the G&B brand, but i couldn’t find it anymore on a quick search of their website. I used the size suitable for shohin on mine.

I’m also going with a thick layer of top dressing to keep the soil from drying out too soon. So far, I’ve needed to water daily, but I’ve also moved a lot of my stuff into afternoon shade. Even the bougies seem grateful for it.

After growing bonsai in Southern California High Desert (Palmdale) for 21 plus years I have quite a bit of knowledge in this area. I have tried everything. If you are going to leave your trees in full sun all day you should invest in shade cloth. The temperatures/UV’s are too intense without it. It is not going to weaken your trees. I also invested in reliable automatic watering system with misters and blubblers for my trees. I set them up to water several times daily during the 100º+ weather. I used a water filter system to prevent water deposits from clogging the misters/drippers and bubblers. Replace them annually or as needed. You can determine how much time you need to water. If you have any number of trees and a day job this is invaluable. Soil mix is another thing. I have tried various types to include Akadama, pumice, lava, sphagnum, peat, redwood fines, etc. You have to find what works for you. For the majority of the time I used a mix of peat and lava. It wasn’t a perfect mix but did well. I added akadama toward the end of my stay in the desert as it was the only way for my Japanese Black Pines to survive along with the shade cloth. I now use Akadama/Lava/Pumice mix for all of my trees.

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I guess I should also add that I’ve learned that just because a site or person says a species does well in full sun, it’s not necessarily true for everyone.

I haven’t really wanted to do an automatic watering system, because I don’t really trust it to do a good enough job. I’ve noticed that some of my trees use more water than others, and I’d rather do it myself. I do have a system that I’ve hooked up while on vacation, but even then, I have someone ensure it’s doing it job.

Thanks everyone for chiming in! I think a misting system is a great idea, and moving small pots and all but the most sun-loving trees into afternoon shade seems smart too. Still working on the best soil mix.

My experiment showed that the standard soil mix + bark held the most water the longest, followed by the shohin sized soil, with the standard soil being the driest.


I had to learn to consider the source when being told that a plant is “sun loving”. I started to notice that most places suggesting full sun weren’t as warm as my area, and were also not considering that the plant was living in a pot 2 inches deep.

This last year, as soon as summer started, I moved my bougies from full sun all day, to a spot where they start getting shade a little past noon. I was getting less wilting, less leaf scorch, and the trees were way healthier at the end of the summer than previous years. When I saw how well they were doing, I did the same with my 3 junipers. The junipers also loved getting a little shade in the hot summer afternoons.

So while my advice is anecdotal at best, try giving some of your trees a break from the summer sun. I think Ryan once said it, when he was talking about pines and water, but just because a plant can thrive in a certain environment, doesn’t mean it wants to.

As for your search for the optimal soil composition, I recommend the following video:

Guy does a water retention experiment with all sorts of soil components. Mind the corrections listed in the description.

I would also suggest this write up by Adam Lavigne regarding soil components:

Adam’s Soil Post

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The above video and Adam’s soil post ARE really good info.
Knowledge is important. This is a mind numbing conversation at best. Once you get inside the MATRIX…
I’m seeing a real wide variation in what people USE for bonsai soil. I’ve killed trees by following blindly.
Educate yourselves for where you are (soil types availability, heat, humidity, cold, water purity, fertilizer.)
Eastern Wa state. 90 deg summer, 15 deg winter, dry. What I’m using works for me here. 1:1:1 lava:akadama:pumice. I DO use some organic humus in my small decidius trees-- keeps them from drying out. Survival trumps all else. Moss! Hotter days, half day sunlight for most smaller rees.
Above 90 deg, everything goes under 30% shade cloth.
Yay Marai! WATCH the videos. They are priceless for good bonsai practices ind culture.


Hey Keegan, I talked to Lindsay Shiba that lives in lytle creek and he said he mixes some succulent potting soil in his mixes to help retain moisture. He had an amazing trident maple that he said he leaves in full sun without shade cloth, but he mentioned that he is currently watering twice a day and has organics in there.

I might experiment with adding some organics as well with my deciduous varieties.

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Hi Keegan, You might get some great advice from the members of Sansui Kai a bonsai club that meets at the Sepulveda Garden Center on the second Wed. of the month 6:30 to 9:30 PM. We have all been where you are and have adapted in a number of ways. Jack