AusBonsai

Hey good folk of the forum... most would know that I love my group plantings. Young and immature as they may be ...

Well... I am finding something rather interesting with my natives - which generally love water ... and heaps of it...

....the trees are preferring flat pots.

My point is - by their nature, I thought that flat pots dry out quicker! Yet, I have also read (I think) that flat pots don’t drain so well.

It seems counter-intuitive that a flat pot could be wetter but perhaps they hold more water due to the broader surface on the base of the pot ... and thus, water hungry roots relish the flatter tray?

Am I mad? ... any thoughts?

To give you a sense - here’s various plantings in various states of disrepair ... recently cut back, needing a cut back ...
But they are all super healthy... (and of course the last Ian not native... but a ridiculously thin pot of the size of planting.)





Or come to think of it ... is it just that the pots and mounded soil are actually quite voluminous and it’s not about the flat tray at all - there just a lot of soil?!


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I found that the depth of the pot is not the main problem, but if you are using a very fast draining mix, then it can be a problem.
It greatly depends on how long your mix retains water for.

The mix I use drains quite fast, so on a hot and windy day I just find that it would be far too dangerous for shallow pots.

With shallow pots on very hot and extremely windy days, it can be a disaster no matter how thirsty the species is, but this can be compensated for by lining the bottom with a layer of slow draining mix.... but its anyone's guess as to how well this holds up on a 45+ day with gale winds. And I'm not a betting man with very old bonsai material.

By the way Mark, that first group planting is looking lovely. It will be interesting to see how you go with leaf reduction when it reaches the thickness and size you are happy with, but its starting to look very nice.

Yes, excellent point Rory - I didn’t think about the risk of hot windy days and of course, the soil itself. My soil is quite a ‘wet’ mix.

Thanks for the feedback on the gums too. Appreciated. I am enjoying all the [Natural] groups too... and even though they look weird now ... I reckon the Moonah and the Swamp paperbarks have excellent bones and will look much, much better when flushed with growth.

All great fun.


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I agree with Rory in that a less porous mix will hold water much longer than a free draining mix. I started using a slower draining mix during the prolonged drought we experienced a some years back with very good results. This year with a far more normal (read wetter) rainfall some of my trees have suffered from being too wet for too long.

MJL wrote: ↑April 10th, 2021, 7:43 am It seems counter-intuitive that a flat pot could be wetter but perhaps they hold more water due to the broader surface on the base of the pot ... and thus, water hungry roots relish the flatter tray?

Yep, due to surface tension a wide shallow pot will hold onto water more than a standard nursery pot (tall and narrow) will. You can see this in action with a kitchen sponge: soak the sponge and hold it flat on your hand - there's your wide and shallow bonsai pot. Tip the sponge so it rests on one of the longer sides and more water will drain out. Tip it again so it's resting on one of the shorter sides and even more water drains out - there's your tall and narrow nursery pot, holding onto less water.

Using a slower draining mix for natives like river red gums that don't mind having wet feet is a good idea I reckon.

Can't speak to natives but this was one of the issues with my big JM that was so unhealthy. It had been in an incredibly shallow tray for god knows how long and it was just sitting wet in very poor soil that never dried out. It's still in a shallow-ish pot but nothing like as incredibly shallow as originally.

Hmmm interesting - my original thoughts were way too simplistic and clearly would only apply to plants that love wet feet too.... which might explain why some of my natives are flourishing ...

I am getting used to a whole new mini ecosystem/microclimate in my garden courtyard now that I the big elm has been removed removed... from dappled light to full sun ... but fortunately, nearly always protected from the wind.

Thanks for all the thoughts/contributions so far.


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I can’t really see an issue with the depth of pot you have there, just be mindful that they may require less frequent watering than nursery pots or narrower and taller pots you might have kicking around

Rory wrote: ↑April 10th, 2021, 8:15 am It seems counter-intuitive that a flat pot could be wetter but perhaps they hold more water due to the broader surface on the base of the pot ... and thus, water hungry roots relish the flatter tray?

To build on Greg's explanation, the way I understand it is that a given medium will have a (more or less) fixed height of the saturated zone at the bottom of the pot. So a deeper pot will have the same height of water sitting at the bottom as a shallow pot after irrigation, and therefore it will have more soil above that water that is not saturated but also holds air.
In other words, a deeper pot will have a lower percentage of the total potting mix occupied by water and a higher percentage occupied by air.

I think this is also why deeper pots are said to be more forgiving for less than ideal soil types that hold too much water.

Joshua wrote:

To build on Greg's explanation, the way I understand it is that a given medium will have a (more or less) fixed height of the saturated zone at the bottom of the pot. So a deeper pot will have the same height of water sitting at the bottom as a shallow pot after irrigation, and therefore it will have more soil above that water that is not saturated but also holds air.
In other words, a deeper pot will have a lower percentage of the total potting mix occupied by water and a higher percentage occupied by air.

Doesn't that make it 50/50 ? More water at the bottom of the pot, but a greater soil surface and therefore more evaporation.

With the recent spells of heavy rain we have had I got really worried about my trees, but they all came through very well, the shallow-potted ones as well as those in deeper pots, except for one tree that was pot bound and needed an emergency repotting (doing fine now).

Thanks for an interesting thread, MJL. I like your plantings!!

Lisa

When you read about watering and water-retention you will quickly come across these terms: 'perched water tables' and 'pot capacity' (or 'field capacity').

Below is my summary of what I read (and published in the CBS newsletter in 2016).

There many references in the literature to ‘perched water tables in pots’, but the term may be a slight misnomer. In nature, the critical element of the ‘perched water table’, is that there must be an impermeable layer under it. It is called perched because there is another water table below it. This is not the case in a pot.

The following explanation of a perched water table in a pot comes from world-renowned bonsai artist, author, teacher and consultant, Colin Lewis – the bath sponge experiment.

You must have noticed this when you were a kid.... Take a bath sponge and saturate it with water, then lay it flat. The water will drain down but leave a saturated layer about a quarter inch deep. A sponge 5"x4"x1" would drain to five cubic inches of water. Now stand the sponge on edge and it will drain more, leaving a similar quarter inch saturated layer, but only one and a quarter cubic inches of water. This layer of water is called the ‘perched water table’ and is a major consideration in the efficiency of bonsai pot drainage.


Now, a definition of ‘Pot Capacity’: The maximum amount of water a pot of growing media can hold against gravity’.

Growing media in pots act like the sponge in the Colin Lewis experiment. Irrigated potting media that are allowed to drain under gravity will hold water at pot or container capacity and there will be a saturated zone at the base of the pot held by capillary forces against the pull of gravity. Higher in the pot (at pot capacity) there will be less water and more air spaces in the potting media.

The height of the perched water table at pot capacity depends on the proportions of pores of different sizes.

• Many small particles in the mix create many small spaces and so give a high ability to hold water. The saturated zone will be higher with such media and total aeration will be lower.
• Many large particles in the mix create large spaces and so give more air to the mix. The saturated zone will be lower with such media and total aeration will be higher.

Excellent information. Thanks Phil and others who are contributing. Much appreciated.


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This thread by markyscott on bonsai nut explains it all brilliantly, as a bonus he has some/a lot of expertise in the field. From memory he is a geologist or mineralogist or something similar.

https://www.bonsainut.com/threads/intro ... ics.24970/

LLK wrote: ↑April 11th, 2021, 6:30 am Doesn't that make it 50/50 ? More water at the bottom of the pot, but a greater soil surface and therefore more evaporation.

Didn't think of this, that's a good point. I'm not sure how evaporation at the soil surface compares to evaporation from the leaves (or in general the rate that water is absorbed by the roots). Although I imagine a larger surface area, especially on a hot dry day, would be non-negligable.