Cement rock planting?

Ok, I’ve been working on a project recently, a (sort of) rock planting.
My original idea was:
Which is a very poor drawing of what I was intending.

Anyway, i decided to use the technique of carving polystryene blocks with a soldering iron:
Like so… then coating with cement (three thinnish layers):
Then building together with wire and cementing in place:
I’ve embedded wires into the cement for tie-downs, and plan to add moss to grow on some of the cement. I’m planning on using the technique mention on this forum for moss graffiti (which is using buttermilk, water and moss in a blender i think). (Sorry, i cant find the link at the moment.)

A couple of questions:

  1. Am I a lunatic for doing this?
  2. Is it cheating? (yes, yes, it is…but if its any consolation, it took a fair while to do)
  3. Have any forum members been using this technique?
  4. If so, are there any tips, shortcuts, points of note?
  5. Am I about to be shunned by the entire bonsai community?

Let me know your thoughts.


Awesome job. It’s not cheating. Check out the penjing books from Zhao Q.
I’m thinking of using pal tiya and maybe mix resin with it.


Hadn’t heard of pal tiya. Just had a look online, very interesting. I’ll have a look at some books too.


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Beautiful work! My only criticism is that there is usually a plant or two in a rock planting…
ha ha got you!!! I would love to see how this progresses. :heart_eyes:

Very nice work…I can see the time involved and you’ve done an excellent job!

I wonder if you have considered staining the rock to make it look more natural. Even with a tree and some moss, a monochrome stone could be difficult to pull off well??

What are your plans to deal with the potential pH issues? Are you going to seal?


This is coming along great!

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Thanks for the responses.

Yes, plants are a traditional addition haha.

Hmm, yes staining might be a good call. In a bizarre way, I was thinking that while the rock itself needed to look like it had been cracked and eroded (mostly wind, the directionality of the crevices will sort of match with the cascade / windswept plants I’ve planned for it), I wanted it to visually represent the elements (to be almost cloud-like and pale). However, I’m reconsidering and definitely open to suggestion.

pH issues! Yes, this is the type of thing I need to be thinking off. Thank you. I might have to consider some kind of sealant. I’ve have a little look at the effect cement will have on run-off water. Maybe the ion-exchange capacity of an akadama substrate will have a reasonable buffering effect on what the trees roots actually see?


OK…further geek info about the possible pH issues…

We have this that suggests that the cement may raise the pH…
Then we have this:

About the buffering effect of akadama (in shrimp tanks)… it does have a quote “it doesn’t really buffer too much but rather keeps the PH from rising” which made me chuckle a bit, because that’s what a buffer does (sorry. chemist.)
So it may work that the akadama would maintain a stable pH…however, is it worth the risk to NOT seal it?
I’m thinking probably not. Any ideas?

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What plants are you planning to use? Different species tolerate higher pH better than others. Junipers tolerate a higher pH than pines. I have attached a link to tree pH ranges
This is a list of tree pH ranges compiled by Marty Klajnowski of San Antonio and originally published in an ABS Newsletter a few years ago; it is fairly comprehensive and a very good indicator of the pH level for individual species.


Hope it works out well. You are doing an amazing job so far.
Credit for table to ABS and Marty Klajnowski of San Antonio. Not sure how accurate they are but give you an idea of which species tolerate a wider range.


Thats great.
Yes, its two little juniper nanas…so according to that table pH 5.5 to 7.5 should be ok.
You know, I’ve got access to a calibrated pH meter, akadama and a spare piece of formed cement…
Think I’m going to do an experiment:
Deionised water (as blank)- record pH
100 mL of deionised water + ~ 20 g akadama - record pH (at initial and maybe 24 hours?)
100 mL of deionised water +formed cement - record pH (at initial and maybe 24 hours.
100 mL of deionised water + ~ 20 g akadama + formed cement (at initial and maybe 24 hour)

This will show:
The effect of cement on pH (with reference to blank), and if this increases over time.
The effect of akadama on pH (with reference to blank), and if this increases over time.
The ‘buffering’ effect of akadama (assuming that cement signifcantly affects pH).

I’ll try run this next week. I’ll give you all an update, you deserve it for getting through this post :grin:


Good. I’m super stoaked.
Use newly formed / aged / washed cement. Note cure age and wether it has been well WASHED (soaked) after curing. Also rough total surface area. 20 gm of cement.?
(APOLOGIES… I AM an R & D Chemist.)
My best guess is the cement water goes >pH 8. And the akadama has little effect. The ion capacity vs the leached alkaline… :thinking:

Just drop it in a bucket of water and let is sit for a few weeks(20-30 days) The acids will leach out and it will be close to ph neutral after. I have made pots with cement, or shapecrete in the past and it works for me. Added bonus if the water gets funky it will stain your “rock” and naturalize it too.
I am not a chemist, or have any science past college in my brain. I have just made stuff and tossed it in a bucket, and maybe put some mosquito killer in it while it soaked and then tossed a tree in there and not killed it in a year or two. public\service warning complete.


SPOT on. End run… just soak em.
Sometimes it’s about the journey and not the destination.
The info on paper would be nice.
I’m trying to get similar info on replacements for akadama… pH , water retention, ion activity… long term geekyness to improve bonsai.

Yep, the chances are I will give it a good soaking and possibly apply some kind of transparent polymer coat to the tree-contact parts…I think i’ll still run the experiment, just because.

I’d be interesting to do a loss on drying experiment for moisture retention too… not sure I could swing the use of the lab oven and LOD vessels though. I’ll get some funny looks just on the pH meter…

It’d be fairly straight forward to use kitchen scales and a fan oven though, I guess…hmm

Whats the consensus about the staining, by the way? Is it that it’d be better with a less uniform tone? (even if its just grubby water staining :grinning:)

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Its good to see another chemist on the board too.
By the way, Moon, in the Pharma industry throwing stuff in a bucket with water qualifies you as a formulation scientist. haha

I think a less uniform tone makes for a more natural look. Darker areas, drips, seepage lines, etc.
-Newly minted Formulation Scientist

Many concrete colour additives are available. I had to mix n match house stucco. Brown’s, reds, grays.
Dang it…now I want to do rock planting. Maybe use hydraulic concrete with fine pumice for weight and strength.

I liked your idea about the moss. Any staining from the soil and watering would be more natural looking than a deliberate color. IMHO

Yeah, I like the bucket of water idea Moon suggested, or maybe a clear rubble bag full of water. Get some natural algae growth and staining.

Maybe try some smoke staining too? Ill have to try some things out on left over pieces.

Thanks, people!

Ok, a quick update for the science geeks:
All solutions were mixed and allowed to stand for ~ 20 min.

So the deionised water had pH 6.01

A 6 cm x 6 cm square of set cement (no washing or exposure and only 2 weeks old) in 100 mL of deionised water had a pH 10.95… fairly disturbing for tender roots…

~ 20 g of akadama in 100 mL of deionised water had a pH of 5.48 (slightly lower than the water… not too significant

A second 6 cm x 6 cm square of set cement in 100 mL of deionised water with ~ 20 g of akadama had a pH of 9.90.

So the akadama has a slight buffering effect but not enough to stop plants curling up and dying. I think this was pretty much what Kurt had predicted.
When the original water was drained and replaced by a second 100 mL portion of water in the cement only beaker, the pH did reduce to 10.7…so it may be after a fair bit of flushing the alkaline content will reduce enough.

Which brings me to staining… I tried insoluble suspended oxides (black, red and yellow primarily) and applied them as washes (after a few test pieces) and got the following result:

I think this looks more blended as a unit and a bit more rock like…but getting right close…i’m not sure I could convince myself that it was a genuine,as the surface of the cement seemed too smooth.

This led me to consider killing two birds with one stone (puntastic) with an acid wash! This should help neutralise the alkaline we have just seen demonstrated and also provide a more natural pitted surface to the cement. I’ll have to redo the oxide washes, but i’ve learnt a bit more now anyway.

I’ll give a bit more of an update if this works…and if it doesnt so you will all know what not to do.