Best substrate for tropical bonsai

[SharonP]

Substrate question with a LOT of background:

I started in bonsai 16 months ago (after decades in other horticultural hobbies) and have over a dozen species of Shohin-sized tropicals. I live in Wisconsin, USA (zone 5). Some trees live indoors under good SpiderFarmer lights year-round, others go outside in summer. They all get RO water, liquid and organic/pelleted fertilizer and occasional worm tea.

Unlike all my other plants (garden, indoor houseplants, etc.), my bonsai trees have EXTREMELY slow growth, whether indoors or out. They no obvious signs of poor health (no yellow or brown leaves, root rot, insect damage, etc.), but many can go months without putting out a pair of new leaves. For instance, other local bonsai club members’ Portulacaria trees grow >10 cm in height a week when outside. My 2 year old Portulacaria (also outside summers) is the same small height as when I got it a year ago. I don’t think it’s grown more than a couple of new leaves in the year I’ve had it.

Other club members are mystified why my trees don’t grow, whether indoors or out. I water with RO water and use liquid bonsai fertilizer with nearly every watering (daily outdoors, every 2 days indoors). I also use a microbiome supplement similar to worm tea. When outside in summer, the trees also get high-quality organic pelleted bonsai fertilizer. The only thing I could think of was the Bonide systemic insecticide I use every 2 months is stunting them. However, I use this on my tropical houseplants, too, and those all show very vigorous and luxurious growth.

Then I started looking at the substrate. The difference between my happy tropical houseplants and the unhappy tropical bonsai is that the bonsai are in 100% akadama/pumice/lava rock.

When I buy my (usually 3-5 year old) trees from growers in the southern U.S., they arrive in bonsai pots in 100% potting soil. They are healthy and happy. But per instructions from the 25+ bonsai books I've read and a class on repotting bonsai, I immediately remove ALL the soil and repot the tree in akadama/pumice/lava rock. (To avoid the dreaded "root rot"!) Fellow local bonsai club members will drive hours to buy this mix when they run out. So I just assumed every bonsai tree in the world wants to go in that mix, but that commercial growers simply can’t afford the stuff.

Now I’m wondering if my trees are in a sort of stasis; desperate to return to potting soil where they can thrive once again. Maybe a gravely akadama mix that's great for a 50 year old pine bonsai is not ideal for a 3 year old tropical. And especially a Shohin that’s in like 2 cups of substrate to start with.

To fellow tropical bonsai enthusiasts: I'm planning to repot all my trees ASAP (I think it's okay to repot tropicals year-round). What substrate do you recommend? 100% potting soil? Mixes involving coco coco coir, sphagnum, or akadama? I don’t want another summer to go to waste with my trees in this state where they’re alive but not growing. Thanks!

(Photo taken just a minute ago shows three of my very slow-growing trees: Serissa, Portulacaria, Chinese elm)

 

[dbonsaiw]

Hmmmm, I'm no tropical expert and can only share my experiences. I have a number of ports, vachelia and some other tropicals. They are indoors about half the year under lights. Everything is planted in bonsai jack universal soil (I literally use that for everything, for better or worse). And everything grows a lot, except the ports I have in smaller containers. They grow, but not as much. I keep my Chinese elms outside year round. Even smaller containers can't contain the wild growth. So I have some questions:

- can you describe how often you water? Overwatering can actually slow growth down.
- Do you prune the trees? If so, when?
- when were the last repots?

I do have a few hornbeams and an American Elm that simply did not extend for maybe 2 years. I have no clue why and no idea why it started again.

 

[Michael P]

Some will regard this as heresy, but try adding 25% to 33% organic matter to the APL. It should be something that is the same particle size as the APL and does not quickly break down. Fine pine bark is what I use. My inorganic component is usually expanded shale (haydite) or low fired clay based spill absorbents like Safe-T-Sorb.

 

[SharonP]

Thanks for the replies. To answer dbonsaiw's questions:

1) I water daily when trees are outdoors, and every other day when indoors. I water the ports once a week.

2) I've been following Morten Albek's writings on shohin bonsai to never let the tree branches grow wild, and to keep the tree constantly pruned to shohin size. My ficus are the only ones I'd say behave "normally," where I can pinch out unwanted new growth and they just keep sending out leaves 52 weeks a year. My species like Jaboticaba, Chinese elm, Vietnamese bluebell, or pomegranate will have a small flush of growth maybe once or twice a year. They send out a bunch of long shoots with HUGE internodes. I pinch them back to keep the tree shohin size. Then they literally don't produce another new leaf for months, or even until the next year. Other trees--like serissa, ports, natal plum--never get pruned and they still don't grow, not even half an inch a year. They are effectively like Tillandsia air plants...

3) I acquired all my current trees between 4-14 months ago. I repot every tree into akadama mix literally the day it arrives in the mail. Later on, I then repotted about half the trees from plastic pots to ceramic bonsai pots because it was required for our club's exhibition, and because I had bought a ton of nice bonsai pots on auction sites and wanted to use them. So some of the trees have been repotted twice in the past year.

 

[Colorado]

I would guess that it is a water and sun exposure related issue.

It also sounds like you are fertilizing way too much. There is absolutely no need to fertilize with every watering. That could be the culprit too, in my opinion.

Repotting also slows them down.

 

[SharonP]

@Colorado thanks for writing. Everything I read on fertilizing is all over the map. Some sources say to fertilize just a few times a year, and others to fertilize with every watering, even multiple waterings a day in summer. A successful local club member (whose trees grow like wildfire) has seen my trees and guessed I may be under-fertilizing and recommended adding fish emulsion.

What would you suggest for a fertilizer regime for ports (succulents) and tropical deciduous trees? I have available: Osmocote (just ordered from Amazon, arriving tomorrow), organic bonsai pellets from Stone Lantern, a liquid bonsai fertilizer, and a worm-tea-like nutritional water additive.

 

[Colorado]

@Colorado thanks for writing. Everything I read on fertilizing is all over the map. Some sources say to fertilize just a few times a year, and others to fertilize with every watering, even multiple waterings a day in summer. A successful local club member (whose trees grow like wildfire) has seen my trees and guessed I may be under-fertilizing and recommended adding fish emulsion.

What would you suggest for a fertilizer regime for ports (succulents) and tropical deciduous trees? I have available: Osmocote (just ordered from Amazon, arriving tomorrow), organic bonsai pellets from Stone Lantern, a liquid bonsai fertilizer, and a worm-tea-like nutritional water additive.

Personally, I use Biogold pellets on the surface of the soil, and then supplement with liquid fish emulsion once every week or two. I replace the Biogold pellets when they disintegrate, usually after about 4-6 weeks.

My buttonwood seems to like this schedule

 

[SharonP]

Personally, I use Biogold pellets on the surface of the soil, and then supplement with liquid fish emulsion once every week or two. I replace the Biogold pellets when they disintegrate, usually after about 4-6 weeks.

My buttonwood seems to like this schedule

View attachment 548180

Thanks! Love the buttonwood with the great deadwood. That's a species I've never seen before.

 

[TimIAm]

Watering can be an issue and it's best to get your watering and soil mix set up to fit in with your local climate. Where I am, I need some organic to retain water or my trees just don't get enough water. I use fine pine bark as suggested by @Colorado. I fertilise much the same as Colorado - slow release fertiliser (Osmocote Plus Organics) and fortnightly fish emulsion.

Although you are keeping these trees outside, how many hours of sun are they getting and is it direct sunlight or part shade?

keep the tree constantly pruned to shohin size

I'd be interested to read the full context of this (if you have a link). I'm not familiar with Morten Albek, so not a comment on his work. I though constant pruning was considered bad practice?

With ficus:

If I want to build growth into the trunk, I let it grow and only cut if something is either important or going to drastically ruin the design, otherwise it's grow, grow, grow.

If I want to build branches, I let the branch grow long and then cut back and repeat.

If I want to refine or build in taper / ramification, I let the tips grow healthy and then when there are buds below the grow tips (I usually look for a good pair), I then cut back to those buds and then wait for the tips to grow out healthy again. In this way, I am letting the tree let me know when to prune as there are new buds available. Buds will appear if the tips and the tree are growing healthy.

In all of the examples above, I don't see constant pruning as working in favor of what I'm trying to do.

We have wet / warmish weather year round here in Sydney, we can (mostly) get year-round growth. However, even if I wait for a ficus to put out new buds, if I try and year round grow / cut every time new buds appear the tree can stall. I had a couple of ficus in the past where I wanted to push for really dense ramification, but I still need to let the tree just grow for a couple of periods per year or the point above of 'wait for the tips to grow health again' won't happen. The tree just slows down from being constantly cut back.

 

[TimIAm]

I acquired all my current trees between 4-14 months ago. So some of the trees have been repotted twice in the past year.

Also, maybe this your trees are less than a year old and have been repotted twice. Possibly, some of them into small bonsai pots when they are in development.

 

[amcoffeegirl]

A couple of things that have held me back-
I pruned too often never really allowing for growth.
Starting in bonsai pots too soon.
Being afraid of full sun exposure while outdoors
Root pruning in the wrong season
Using a coarse soil mix
Repotting when it’s not needed.

I have in the past used wigerts mix. Currently using a similar mix - lava, turface, pine bark, with added akadama- sometimes some cork, chick grit, perlite. The smaller particle size has helped with finer roots.

 

[Gaitano]

I would second on the repotting so often. When you trim the roots they need to heal in order for them to function properly as the water nutrient highway. Gerberally I water the tree in just after repot and then hold off until the soil is almost dry before I water again. Signaling the tree is ready for water uptake. This is why we typically don’t repot deciduous when they are in leaf. If the roots have been trimmed and need to heal, they can’t immediately run water up to the leaves the way they are meant to. Repotting has been referred to as open heart surgery. A serious undertaking. That would set the trees back a bit before they were able to kick the system back into gear. I usually repot my tropicals in the summer, mid June in Missouri. Finding a soil that works for your specific environment winter/ summer can be challenging. Mine go outside when the overnight temps are consistently above 50 and get as much sun as possible. Back inside in the fall when the overnight temps drop below 50.

 

[dbonsaiw]

1) I water daily when trees are outdoors, and every other day when indoors. I water the ports once a week.

Watering isn’t really on a schedule. It’s more about giving the tree water when it needs water. I try to check the soil daily and see if the tree needs water. Learning when to water is an integral part of bonsai. I find that using a bonsai soil mix gives me more control over the water issue as the soil doesn’t stay waterlogged and I can just water again when needed (thereby also oxygenating the soil).

I've been following Morten Albek's writings

I’m not familiar with him, but pruning limits growth. If you’ve developed a shohin and want to limit the growth to preserve the form of the tree you worked so many years to develop, you prune. If you are developing the shohin and want growth, you grow.

So some of the trees have been repotted twice in the past year.

Yeah, don’t make a habit of that. Definitely something that can slow growth (or worse).

 

[SharonP]

Here's an update 2.5 weeks after my post. The day after my post, I added Osmocote (which had just arrived from Amazon) to both indoor and outdoor trees. What a difference! The trees are suddenly exploding with growth, including ones that had barely put out a new leaf in the past year. I now have visible new leaves on every plant, and some have 4-8" long new shoots.

I thought I might have been overfertilizing by using liquid ferts almost daily. I was so wrong. That stuff was useless--but the Osmocote did the trick. Another factor for the outdoor trees is that I starting shading them more of the day than before. I think they might have been experiencing some heat stress that was halting their growth. But that wouldn't account for the growth of the trees that are indoors under lights. The only thing that changed for those was the fertilizer. I know Osmocote can be a contentious topic, but it is sure working in this case.

Pictured: Natal plum (photo May 18th) had virtually no growth for the previous year. As of June 7th, the leaf mass has doubled. That's a full-time indoor tree under lights. The Ixoria hadn't put out a new leaf in months, whether indoors or out (in summer). The photo shows all the growth in the 2.5 weeks after Osmocote (new leaves are brownish-red in this species).