k2bonsai wrote:by the way, do not put larger particles at the bottom of your pot and finer mix on top UNLESS you want your finer soil layer to hold extra water as you are basically creating a perched water table.
I looked up what a perched water table is to try and understand, but I'm still not getting it. Is there a 'simple' way to explain why having larger particles at the bottom don't help? Is it that the smaller particles block up the larger ones? Am just trying to wrap my head around it
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So really, its not a bad thing to have larger particle size at the bottom of your mix (Or different levels), in fact, it's a good thing.... you just have to remember that it does not mean the mix will dry out quicker, but actually the opposite and will retain moisture for longer.
I'm going to need to read through that again to make sure I have it right, watering really is a lot deeper then I ever would have thought
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Jason wrote:I'm going to need to read through that again to make sure I have it right, watering really is a lot deeper then I ever would have thought
Yeah, that's for sure. I've been taking a one size fits all approach with my mix, and while the results haven't been bad, surely I can do better. Time to buy some sieves and get serious. Could really help with the hot summer weather around here.
Jason wrote:I'm going to need to read through that again to make sure I have it right, watering really is a lot deeper then I ever would have thought
Yeah, that's for sure. I've been taking a one size fits all approach with my mix, and while the results haven't been bad, surely I can do better. Time to buy some sieves and get serious. Could really help with the hot summer weather around here.
Exactly the same... and I've actually got some seives coming in the mail already, should be here today/tomorrow
I use larger particles over the drainage holes at the moment, but my reasoning for doing it was totally wrong as it turns out!
Last edited by Jason on May 29th, 2014, 1:40 pm, edited 1 time in total.
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Here is a simple way to sum up the benefit of a perched water table that is created deliberately to server a purpose.
Most plants that go into pots use only about 30cm of the soil. So if i was to have a very large pot that was long and wide but only 30mm tall then i could just fill it wih potting mix and put the plant in and water regularly and everything would be good.
Now lets say i want to look really trendy so i buy one of those really narrow long tall pots and stick the same plant in (lets say the pot is 600mm deep). If i was to now water this on the same basis the water would end up further down into the pot and not be available to the plants roots.... Most people who use pots like this end up planting things like yakas and agave etc... why? well because these are drought tolerant plants and can handle times of no water. Lets say i wanted to plant somethign that was not drought tolerant but i wanted to keep it in the same pot then i need to raise the water level up into say a 30cm soil layer. So to do this i fill the bottom 30cm of the pot with gravel and then place 30mm of soil on top. The water that would normally reside in the bottom of the 30cm high pot now resides above the gravel layer.
Any easy way to demonstrate this is to take a rectangular spong and fill it with water so it is saturated. Place this on a bench width wise and then watch as the water drains out. There will be a layer on the bottom of the sponge which retains water just as your pot does. Now do the same but place the sponge vertical and you will see there is much more dry space before you get to this bottom layer of water (aka my example above with the plant in a long narrow pot).
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Last edited by k2bonsai on May 30th, 2014, 12:52 am, edited 2 times in total.
"The problem with socialism is that eventually you run out of other people's money."
Thanks k2, an outstanding illustration. So using larger particles in the bottom and smaller at the top is kind of like cutting the sponge down and setting it on top of something, effectively taking the water holding capacity to the top of the pot. A perched water table. As you say, seems particularly important for cascade style pots.
Thanks for the interesting replies, and pictures from Peter Warren's blog are interesting. And that leads to another long term argument on bonsai forums about the necessity of drainage layers. I prefer the term aeration layer, as this is what these layers of courser material do. The perched water table effect diminishes quickly as roots grow through the layers. Interestingly, I have found that sometimes roots won't grow into the bottom layer, they are effectively air pruned, but a similar plant right next to the air pruned pot will have roots growing through the same course bottom layer.
Additionally, as roots colonize a pot the pore spaces become smaller and so a pot retains more water, less air. This is important for plants like tridents that produce tremendous amounts of roots and colonize the bottom of pots, causing a large saturated zone to form. This is also why Japanese maples that haven't been repotted sometimes suddenly die in spring. The saturated zone isn't a problem during plant growth, as water is removed by transpiration, in winter the roots are dormant and have a low oxygen requirement. In spring, the roots become active and have a high oxygen demand, much higher then in summer or autumn, and it cannot be met in a pot that has been colonized by roots. Most soil pathogens need warm temperatures and reduced soil oxygen content (not complete lack of oxygen, they won't become active in zero oxygen levels) to become pathogenic to plants. This combination of oxygen levels and temperature is why sudden maple death is a hit and miss affair. And why they should be repotted yearly.
I also have plants that shows signs of water stress before others and often base the timing of watering on these plants.
Someone wanted to know about whether to water before forcasted rain. Do it, water away knowing that it won't hurt. The weather people often get it wrong, also, one day of wet soil isn't going to be a problem where as one day without water can be a disaster.
My initial question was rhetorical, you can't effectively water a tree and keep the soil moist at the same time. A plant with soil that doesn't drain properly has to be repotted. If it's out of season the best technique is to slip pot the tree into the ground and then repot at the correct time.
I know there are always exceptions. Plants never cease to amaze me with what they can tolerate.
Here is a Japanese maple that self seeded in an icecream container that was supposed to be growing Marsellea (a swamp plant so no drainage holes in the container). It has been totally waterlogged for its entire life and has grown quite well and appears healthy.
palmatum waterlogged.JPG
However - don't try this with your precious trees. Just because 1 has managed to survive against the odds does not mean it is good for the majority. I use and promote the use of very open, well drained mix for bonsai.
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