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Self-watering pots


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It seems to me a special version of self-watering pots should be able to solve three problems, especially for big palms: To have to water very frequently, to get even moisture (neither wet nor dry spots), and to provide easy means for leaching. And maybe wouldn't have to pot up as much.

Ordinary self-watering pots (aka "sub-irrigated planters") don't have these benefits though. This is how they seem to work (free pic from Wikipedia):


Basically water tank placed under soil, with some kind of wick to suck water up to soil by capillary action. Some of the most commonly available commerical ones are GrowBox, EarthBox and Ollie Plants Sippers, which all seem to create the wick by letting soil dip down into the water through a small compartment:

GrowBox.jpg.759838c20286828df73a0d809e1e  Earthbox.thumb.jpg.02716c18c3e3ffa847714  Ollie12.png.0944e8b6815880fc963c822ed3bf

Octopot has a different wick in the form of a cylindrical "bag" that pulls water up the sides:


If you make your own, then the wick could be synthetic yarn or part of a mop or cloth - anything that doesn't rot and sucks water.

But all these versions of self-watering pots share some drawbacks: Bottom of soil gets wet, salt builds up, you can't leach.

I have some ideas how to improve this so that it would suit even (or maybe especially) big palms:

  1. Make sure there is a distance between water and soil, because only the bottom part of soil+wick gets wet, so this way one should be able to avoid wet bottom part of soil. I'm thinking maybe 5 cm (2 inches)?
  2. Choose water container maybe 50% the size of soil volume. When you want to leach you could add water to soil and empty water from container using siphon or pump. Repeat 5 or 6 times. Easier than carrying it to bathtub or putting bucket under and carry back & forth.
  3. I suspect an improved wick is needed especially when it is getting root bound: Maybe 5 or 10 wicks, each 5 mm wide, across the pot's bottom area, dug into the soil and hanging into the water
  4. I suspect roots will pinch the wichs so maybe surround them by protective tube.
  5. Manufacturers and users of self-watering pots usually recommend peat based soil mix, something capable of capillary sucking.
  6. Maybe a big palm can keep a relatively small pot for very long, and that way slow down growth a bit for a palm that is about to outgrow apartment?
  7. You can use quite fast draining soil and still not risk that it runs dry even if you don't attend it several times per week, and the palm will probably love the good drainage and non-wet non-dry soil. Actually, a plain pot mimics real ground rather badly, this system gets much closer to natural conditions (with ideal watering as well).

Then there are a number of loose ends I haven't got tied up completely:

  1. Some seem to allow roots to grow into water. Should one allow that, or keep a fine meshed net at bottom? I'm even told that water will get enough oxygen from tube for roots to breathe (like hyprophonics) but is that believable?
  2. Should one water the whole soil thoroughly from top once then never again? Some sellers say self watering will not work for the first few weeks, sounds like they assume roots will grow into water? But peat should suck from day 1 I would think.
  3. Maybe there's a risk container becomes disgusting? For that reason maybe let it dry out - but will not self watering cease then and require watering from top? Perhaps one could let it dry out and water from top once per month or so to kill off fungus?

For people who keep palms indoors and want to be able to leave the apartment for up to a week or so, and for even longer durations be able to give simple maintenance instructions - I can't see that there is any other way to go... But will it work as well as I hope?


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Okay, I'll weigh in on this.  My container ranch has roughly 50% "self watering" pots.  These are mostly the pots that have a depressed circle or X that goes to the bottom of the saucer with holes/vents to pull in water by soil capillary action.  Most match the design below, but a few are tall with the wick and closed reservoir.  So far I have found that they all work Great!  


However, I use them slightly different for different palms.  For most of my palms I still water down from the top and keep adding water a bit at a time until I see there is drainage out the bottom into the saucer.  This gets left in the saucer and gives the palm something to sip through the next couple days to a week.  The top down water ensures that the whole of the root ball stays moist and promotes healthy infiltration of all my coffee grounds, rock flour, fert, etc.  

I also have some that, in addition to occasionally getting a top down watering, also get their reservoirs filled at least weekly, and some every few days.  These are the high transpiration Chamaedoreas and my Dypsis lutey.  Depending on the temp, pressure, and humidity my 1-gall D.lutescens has sucked up over a gallon in a week.  I'm told that larger specimens will push a liter or more per day out their tops.  When I check water levels on most of my palms, these guys are almost always racing towards bone dry.  Filling the reservoir every week makes me less worried that I'm going to lose them if I miss a top-down watering.

I will say that I have overwatered at times, but that the self-watering pots I posted above seem to overflow before they drown the plant or get root rot, but I am wary of any pot that I couldn't see how full the reservoir was.  When repotting I have noticed that the majority of the rootball is mid-level to high in the pot with a few "tap roots" going down to the water level.  Almost all of them punch through the vents and put roots directly into the water, they seem to try to do this as quickly as possible and make their own "wick" as it were.  Sometimes this makes it very difficult to extract the palm when repotting as there is several feet of white rope coiled up in the saucer that is too thick to pull through the mesh/slats.  Usually I just clip these roots off with no damage to the palm, and they grow back fast in the next pot anyway.

I also appreciate that I can fill reservoirs before I leave for a trip and everything will be fine for upwards of a week, and sometimes longer.  Granted, this is in my climate controlled apartment, so your mileage may vary.  If it's summer I will still have a friend check everything out on the balcony after 4 or 5 days of no rain, but everything inside is usually fine.  As for build up of salts and whatnot, so far none of these are too big to manhandle into the shower.  I'll put everything in the tub at least once or twice a year and let them get a good soaking.  I'll even fill the tub to let them saturate all the way and anything soluable will drain out with the water when I pull the plug.  Most of these have removeable saucers, so I can just let them sit in the shower and drain for a few hours before putting them back out by the windows.

I am currently trying to design a self-watering pot for my largest plants and am hitting a bit of a snag between functionality and visually pleasant design.  I suspect I will have to put everything in the bottom of the pot, "hidden" as it were from the outside.  But for my container ranch this would only be on 25 gallon or larger pots, of which I only have 1 at this time.  So I have time to plan and scheme.  I will need to come up with something eventually, though, otherwise my largest palms will require watering daily and I would like a system like this to give me breathing room.

So, anyway, that's my 2 cents.


(EDIT:  If you're using your own palm soil mix I have no doubt that there will be enough capillary action to get water up to the roots.  New unestablished palms will likely need more top-down watering at first, but will soon catch up.  And anything repotted into the correct upsize of reservoir pot should have no problems getting the water it needs.)

Edited by Funkthulhu
  • Upvote 2

"Ph'nglui mglw'napalma Funkthulhu R'Lincolnea wgah'palm fhtagn"
"In his house at Lincoln, dread Funkthulhu plants palm trees."

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Thanks for sharing, it certainly is of great help.

25 GALLONS WOW! You can't carry that to a bathtub, it IS a bathtub!!! Indoors, 9L (2.4G) is the biggest I needed, it will reach the ceiling in such a pot.

I understand you make sure water doesn't reach soil level but that's not what I meant by avoiding keeping it wet. What I mean is that if you take a plain pot with flat bottom and hole(s) in the bottom and water until drips from bottom and then wait until it doesn't drip, the bottom part will be very wet - you always have this phenomenon in pots, but to a lower degree if pores are bigger:


So roots at bottom will be wet, even more if you don't empty saucer properly (a good reason to keep one layer of LECA I think) and there often is a lot of roots there.

(In English this often is called "perched water table", which is a bit weird since that term normally applies to groundwater and doesn't really apply to water in pots; rather I'd just say it is an unnatural phenomenon due to surface tension, and balance between gravity and capillary action.)

Saturation decreases gradually. If you let the bulk soil connect via a wick, and the wick is long enough (I'm thinking maybe 5 cm), then the saturation will take place within the wick, and not in the bulk soil. So that way you would avoid wet soil.

It is also too easy to get dry spots at bottom when plant is a bit rootbound. With any self-watering pot this is solved too.

I've been thinking some more too:

  • Maybe forget about the wick ideas I had and copy the Octopot idea, and have some kind of cloth (maybe a floor cloth of micro fiber) surround the entire root ball's side surface. Even when rather root bound the soil higher up could still suck through this, and so could the roots at the bottom sides. Maybe have a single central wick too that penetrates soil though. Or would bottom go a bit dry then?
  • Not sure I like if roots grow into water, partly because hard to repot, but even more because I want to aim at my wick creating the perfect non-wet non-dry soil. If roots go into water container I'd have to worry about things like oxygen level of water. And I know how easily roots penetrate holes even if quite small, but a flat, very fine-meshed strainer made of aluminum or stainless steel I think should work.
  • I think I'll use 50% peat moss-based soil mix and 50% vermiculite (Seramis has properties very similar to vermiculite, but is more expensive I find, unless you live in Germany or Austria). This actually is almost identical to one of the basic Cornell mixes from 1963.
  • I will not use water container 50% size of soil volume just because it would get too big to look nice. More like 25%. Which means I would definitely like a pump when I reverse the system for leaching. I just leached all my plants this Sunday, took all day.. And would be good to do at least 3 times per year. With these pots one could do them all in parallel in one hour, without any lifting! :-)
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  • 8 months later...

I did a lot of experiments but haven't had time to write about it here. Half of them are on the topic of soil mix and I wrote about them here. Conclusion for me is to use a mix of coir (good quality, fine particles sifted out, and rinsed so that not too salty) and seramis. And that I add some Ca and Mg to my normal fert if I use coir.

Other than that I also investigated wicks quite expensively. First I tried to use other peoples knowledge but found mostly experiments on how moisture moves within textiles in jackets etc (and in plaster/gypsum walls) which in the end isn't directly applicable to plant wicks (since they standardize the test to measure time & distance for hoizontal & vertical wicking but this turns out not to reflect which flow you will get at a given height). Then I did some less successful experiments, e g checking ability for different materials to work as siphon, but it turns out these experiments do not reflect at all well plant wicks (probably because most materials are not internally arranged as perfect capillary tubes but rather like lots of short criss-cross capillary tubes which are not in perfect contact and the holes cause dripping). I also checked experience from gardening forums but they (at best) tend to say "I use this material, it is really good for my plant X", not deeper than that. But at least I got inspiration for which materials to try out. And I tried wicking from above which worked better than expected but (as expected) risk for flooding and not useful. In the end I settled for an accelerated version of a setup that is very similar to the real situation:


To the left a heating fan at a fixed distance. Sifter is filled with moist soil mix to the brim. Wick runs along the bottom right side of sifter and out through a hole at the bottom in the middle, then goes into a glass cylinder with a mL scale. Candy jar and pink plastic bag are needed to assure hot air heats only soil not wick. Wick was wet at start but I let it stabilize before a new test started (since at the beginning it will naturally drain itself a bit). For each graph below there are around 10 measurement points, and for each point I ran a 29 minute test. Lots of culprits along the way. For longer wicks the setup looked like this:


And these are the results for 9 different wicking materials (I only tested synthetic materials since I don't want them to rot):


As you can see "H" can wick to much greater heights than any other material without dropping much in flow, it is useful for heights up to even 3 or 4 dm (below pot bottom), it is an elastic round shoelace, you can even buy it from the manufacturer here. C (another round shoelace from Tobby), K (2 strands of acrylic yarn), D & F (two flat shoelaces from Springyard, of which F is this) can compete with H at heights of 1 dm, maybe 2, and maybe also J (micro fiber cloth used e g for wiping cars) can be placed in that category if you use two layers of it. Complete crap are G (so-called Paracord, originally used for parachutes, sold all over the world) and A (a "starting cord", a quite dense thread used for starting e g lawn mowers), and B is not much better (a plain thin 8 strand loose nylon rope, universal usage, e g wrapping packets).

Regarding flow requirement my two quite thirsty plants Rh Baueri (9L) and Lipstick Palm (~5L) need aboyt 5 and 3dL/24h respectively so with the better materials even one wick should suffice. However I planted 2 or 3 wicks (lately usually material H) per pot. My most extensive experiment on real plant was with my Lipstick and material H, 1-3 wicks stuck in through a hole in the bottom or 1-2 wicks running along the pot edge from top to bottom. It has a peat mix and is slightly root bound but very healthy.

So there you have my conclusion on how good a wick can be and what the best material is. However as a whole, I have given up on wicking, I don't believe in it for palm trees. Mainly because it proved very hard to get anything resembling an even moisture level even with 3 wicks, but also because roots will grow into the wick and suck directly and cause an even greater unbalance. The self-watering pots you buy usually have wicks made of soil, I wanted to avoid that since you get the drawbacks of one or two roots growing into the water container and after that being the only real supply of water plus they live under water permanently which can't be good (risk for rot etc) and you can't repot without cutting these roots. So I was hoping for a better situation with these textile wicks but it isn't good enough for me to trust the plants will be healthy. I think they will get an uneven moisture level with some areas very wet almost constantly and some almost dry quite often, and I also found it too hard to feel sure the soil will actually stay moist (or wet) for x weeks: Sometimes it did (but with rather big variation after a while) sometimes it succeeded so badly I didn't want to let it continue.

I know I am diving deeper into stuff than most care to follow but since I started writing about this I thought I should end it. Let me know if you want more details on any subject. I also did some interesting small related experiments, like how quickly water evaporates depending on how well the top of the contaner is sealed. And had some fun ideas which could have been of interest as part of the wick setup, like how to overcome the fact that the top of the water container needs to be under the pot bottom by using a water dispenser for dogs:


You know how it works? Water is accessible at the bottom level since vacuum at top of container keeps water from coming out the bottom as long as the level in the blue bowl is above the outlet. So basically you could dip your wick in the blue bowl and keep the pot elevated to just 5-10cm.

It might have worked better with a plant that is not at all root bound, but still, one would like this to work especially when (slightly) root bound since that is when they need watering the most often, and also I fear how any inhomogenity in the soild will cut parts of the soil off from water.

I would still like to have a working and reliable self-watering system but the way I see it, it would have to water from above, and until water comes out the bottom, and take care of the excess water, and be able to determine when to water next time. Just the way I water manually today.

Edited by David_Sweden
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