Root behavior in porous vs. nonporous pots

Discuss repotting, soil, lighting, fertilizing, watering, etc. in this category.
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Steve Johnson
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Root behavior in porous vs. nonporous pots

Post by Steve Johnson »

I grew up with terracotta pots, and when I had a collection (mostly cacti, but a few succulents too) from 1970 through '91, terracotta was all I ever used. Circumstances forced me out of the hobby for 20 years, but that changed as I started building a new collection in 2011. Nothing wrong with terracotta, right? Well, I came to find out that porous clay pots aren't the best -- I'll explain why in words and pictures...

The story begins, not with a cactus, but a succulent that was a Christmas present from my best friend and his wife in 2010. And here it is, a Haworthia attenuata in its 5" terracotta pot on 4/29/12:

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Haworthias are winter growers, so when the winters here in SoCal are wet, I give mine all the rain it'll get. When the winters are dry, it's deep watering every 2 weeks with acidified tap water. Either way, the plant receives plenty of water to keep it happily growing -- and to the point when it completely filled the pot. In fact, the Haworthia was overdue for a "potting up", so time for the move to a glazed ceramic pot on 4/20/19:

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That was shortly after a wet winter ended, and the plant should've stayed nice and green as it had in previous winters. Instead, we're looking at a clearly stressed Haworthia -- maybe it was just rootbound?

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Chop the whole thing in half right down the middle, and you might be surprised by what you see:

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Roots wrapped around the outside, potting medium (in this case, the California Cactus Center's "custom" mix) on the inside. There, my friends, is the problem with evaporation through porous clay as the Haworthia's roots were competing with the pot for water. After all the water it got that winter, the Haworthia's root system was slowly dying of dehydration. Here it is in its new glazed ceramic home:

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For comparison, we'll take that shot and make it a "before" as we see the "after" exactly one year later:

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And the latest with a winter 2021 shot on 1/10:

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The Haworthia's roots have been growing in the mix, not around it -- the plant's root behavior is different and quite a bit better than it was in terracotta, and we're seeing the evidence with better growth above-ground. The same basic principle applies to cacti as well, and I moved all of my cacti to glazed ceramic pots long before I finally took care of my poor neglected "veteran" succulent.

Expert growers have known for a long time that nonporous pots are the only way to go. However, there are some climates which are too humid for it, so if you live in one of those climates, you may need to stay with porous clay pots. If you don't -- and you prefer the look of terracotta, seal your pots with UV-resistant acrylic paint.
If you just want photos without all the blather, please visit my Flickr gallery.
My location: Los Angeles, CA (Zone 10b)
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anttisepp
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Re: Root behavior in porous vs. nonporous pots

Post by anttisepp »

I use plastic pots for all plants all my life, don't like terracota.
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Steve-0
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Re: Root behavior in porous vs. nonporous pots

Post by Steve-0 »

Thanks for sharing this repotting of your Haworthia with us. Pretty crazy what the roots were doing to survive.

I've seen this with other houseplants before but not with cacti or succulents recently. My Haworthia had the sad last year and was begging for more water and sun. I thought it was a goner. So with a bigger pot, more porous mix, water and Sol's help I was rewarded with pups or branches in short order. Happy, happy!

Now back to your mid surgery image of the vigorous roots encasing the dirt ball. It made me ponder upon other factors than just the plant 'competing with the pot for water'. Do you think, because I'm not sure, just curious about it, that other factors are in play?

What comes to mind are these...temperature, soil mix, aeration and maybe a combination of these.

Does a plant send its roots to warmer areas? Hence the roots covering every millimeter of the pot's inner walls which would receive more heat than the center? I don't know so I'm asking.

Was the soil mix hydrophobic? Or become become that after the roots drew in all the moisture? Which caused the water to run off pretty much as soon as it was poured on top towards the pot walls and down along them versus wetting out the soil .

Was there a better air/soil mix along the pot walls than internally? Is this even a factor, IDK?

You mentioned it being rootbound. Could the mass of the above ground plant parts have factored into the watering and root growth behavior?

Too tight of a cluster on top and the water runs off to the perimeter of the pot? Which factors into the root's growth behavior.

And as you said the roots were going where the water was...getting siphoned off by the wicked, wicking terracotta. Thus the battle over water rights. Lol!

I'm probably way overthinking it, but I have seen plants with this same root growth, and also plant killing compacted/zero root growth (the landscaper who planted the trees and shrubs on my property without removing the burlap and twine before dropping them into barely fitting clay holes , grrr). Yeah, they came back and remedied that.

And of course healthy rootballs which is the goal.
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Steve Johnson
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Re: Root behavior in porous vs. nonporous pots

Post by Steve Johnson »

Those are fair questions to ask, Steve-0...

Way back in 2012, a senior member on the forum mentioned the problem with porous clay, so I gave myself a little project that year -- waterproof the terracotta pots for all of my cacti. When I took them out of their pots, I noticed more than a few roots clinging to the insides of the pots. And there's only one explanation for that -- water evaporation through the porous clay. Bear in mind that I was using the same soil-less pumice and granite gravel mix I'm using now, so temperature, soil mix, and aeration weren't factors. Since the water-based waterproofer I used wasn't going to last more than a few years, I decided to slowly transition everything (well, except for the Haworthia) over to glazed ceramic in 2015. With every single cactus coming out of the waterproofed pots, not even the tiniest root could be found clinging to the insides.

When a cactus or succulent has roots completely filling the pot, that's a rootbound plant. My Haworthia was rootbound in a different way, although it did need a bigger pot. If I put it into a bigger terracotta pot without sealing the pot first, the Haworthia would continue to grow, but I guarantee you that its growth wouldn't be as good over time. Water should go to the roots, not the pot, and here's what that senior member (an expert grower, by the way) said: "Relying on porous clay to dry out the mix is a beginner's trick. If the mix takes too long to dry out in a nonporous pot, you're using the wrong mix."
If you just want photos without all the blather, please visit my Flickr gallery.
My location: Los Angeles, CA (Zone 10b)
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greenknight
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Re: Root behavior in porous vs. nonporous pots

Post by greenknight »

I've seen this many times. Not only is it not good for growth, it makes it harder to unpot the plants. You do more root damage unpotting them, as well - roots left clinging to the inside of the pot = broken roots.

On the other hand, I've had small, clumping Haworthias in plastic or glazed pots fill their pots so full that offsets were literally dropping off the sides of the pot, yet they remained healthy and vigorous if watered when dry - which is pretty often when they're that root bound.
Spence :mrgreen:
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Steve Johnson
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Re: Root behavior in porous vs. nonporous pots

Post by Steve Johnson »

greenknight wrote: Mon Jan 11, 2021 6:25 am I've seen this many times. Not only is it not good for growth, it makes it harder to unpot the plants. You do more root damage unpotting them, as well - roots left clinging to the inside of the pot = broken roots.
Oh, yeah -- I sure went through that while I was still using plain terracotta, and for a few of my cacti, unpotting them felt like an archaeological dig. Taking plants out of nonporous pots is way easier -- and easier on the roots. Thanks for pointing that out, Spence!
If you just want photos without all the blather, please visit my Flickr gallery.
My location: Los Angeles, CA (Zone 10b)
DaveW
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Re: Root behavior in porous vs. nonporous pots

Post by DaveW »

Some claim it is the roots seeking the higher build up of nutrients left when the water evaporates through the terracotta pots walls. How true that is I do not know. Some old gardeners used to sink their terracotta or clay pots into sand beds to stop this evaporation through the walls in the days before plastic pots. This was often done for alpines.

https://www.bing.com/images/search?view ... ajaxhist=0

They of course would not have then used more expensive ceramic pots in general horticulture. Plastic pots only became generally available later.

I don't know about the USA, but in the colder UK many clay or terracotta pots are now sold as frost proof. The old clay pots if left out in winter used to laminate or shale since the water in the pot walls froze and expanded, thus ruining the pot. Modern terracotta pots here therefore are now fired at much higher temperatures and are therefore as non porous as ceramic or plastic ones.

The easy way to tell if you have a porous or non porous terracotta or clay pot is whether it develops the white efflorescence on the outside that indicates water is passing through the walls and leaving a deposit of salts on the outside as it evaporates.

https://www.house-plant-hobbyist.com/bl ... t-it-means

However I gather they are often even artificially applying a white coating to some new non porous clay pots to make them look "aged" like the old porous clay ones! People are even doing it using various mixes from yoghurt, to lime and also salt. However you still finish up with the pot you started with, meaning it still either porous or non porous regarding growing the plant in it, therefore it is merely looks.

https://my100yearoldhome.com/make-it-yo ... otta-pots/
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Steve-0
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Re: Root behavior in porous vs. nonporous pots

Post by Steve-0 »

Steve, thanks for the in depth reply. I think I only have one or two unglazed terracotta pots but will check again. I do prefer ceramics as they just look better but can also understand the economy of using terracotta in large collections...or plastic. Plastic is okay from a store or nursery but the black pots really do heat up fast in my high altitude desert summer sun. I've tested with a soil thermometer and saw some crazy hot temps on my black potted cacti last summer during a week of blistering heat. Temps up to 130F. Yikes. All plants survived though. I did move them into shade that week. This year I'm taking pottery courses with Andy Ward in Arizona and will be making my own pots ( with internal glazing ) going forwards. We'll see how that goes. I started over the weekend.
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Steve-0
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Re: Root behavior in porous vs. nonporous pots

Post by Steve-0 »

And Dave, thanks for your input as well. Always good to have more experienced cacti folks chiming in to mentor newbies.
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Re: Root behavior in porous vs. nonporous pots

Post by DaveW »

In the UK Steve most plastic pots are terracotta coloured and only a few use black plastic ones. I presume black plastic is better to stop UV (Ultra Violet) degradation from the sun in hot countries, but as you say has the disadvantage of heating up quicker. Also in the UK most of our plants are packed closely together on the greenhouse staging's which keeps sun off the pots sides.

The main trouble with most plastic pots in the sun is UV degradation and the polystyrene ones most nurserymen sell the plants in do not last more than a couple of years in the greenhouse in the UK before going brittle. Those who can get them therefore use polypropylene or similar longer lasting forms of plastic pots. Plastic pots have virtually taken over the hobby here since the late 1960's, but a few terracotta ones seem to be making a comeback.
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Steve-0
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Re: Root behavior in porous vs. nonporous pots

Post by Steve-0 »

Interesting. I worked at the Soybean Research Department of North Carolina State University in the mid 70's and the plastic pots I filled by the thousands with their vermiculite and topsoil mix haven't changed a bit since then. Some are thicker and sturdier... others, thin and flimsy... but the middle of the road pots haven't changed in the US in the nursery industry at all as far as I can tell. Most are landscape sizes of two to ten gallon. Probably cheap to make, stack very tightly and are light weight for shipping from the manufacturer to the destinations.

I have a couple of those plastic faux terracotta look pots and they've gone brittle in our high UV climate. So they'll need replacing as well as all the black nursery pots.

The home improvement stores here - The Home Depot and Lowes, are the big boys, sell all kinds of terracotta, stoneware and ceramic at great wallet emptying prices. Pottery shops are a whole other level of expensive.
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Re: Root behavior in porous vs. nonporous pots

Post by keith »

For anything over 4" diameter I use clay pots. Plastic pots break apart unless they are the Thicker black Plastic types . Picking heavy cactus up by the rim of a normal thin walled plastic pot only to have it crack and the plant falls out is more than irritating. I also place plastic pots inside clay pots if they are on the smaller side less than 4". And if they are square and small like 2 1/4" rose pots I pack them together they last longer that way plus they are not heavy so don't crack apart when I lift them.

I use to paint the inside of my clay pots long ago but don't bother anymore with all the pumice I use plus a cooler climate they can dry out as fast as they want which is almost too slow.
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One Windowsill
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Re: Root behavior in porous vs. nonporous pots

Post by One Windowsill »

DaveW wrote: Mon Jan 11, 2021 11:21 am Some claim it is the roots seeking the higher build up of nutrients left when the water evaporates through the terracotta pots walls. How true that is I do not know.
Could it be that the capillary action action of the ceramic means that while there is still some damp compost somewhere in the pot the moisture is drawn up the walls of the pot?
DaveW wrote: Mon Jan 11, 2021 11:21 am I don't know about the USA, but in the colder UK many clay or terracotta pots are now sold as frost proof. The old clay pots if left out in winter used to laminate or shale since the water in the pot walls froze and expanded, thus ruining the pot. Modern terracotta pots here therefore are now fired at much higher temperatures and are therefore as non porous as ceramic or plastic ones.
I was, to understate it somewhat, very disappointed with two terracotta pots I bought about twenty years ago. I had a hankering for me old mum's wholewheat flowerpot bread. It gets such a superb crust all over. You get a little crusty extrusion in the drainage hole. The terracotta pots were not sold as frost-proof. They looked like a normal terracotta pot. It turns out that some are frost-proofed by soaking them in a plastic. A plastic that turns into poisonous fumes when heated - fumes that are unbearable and taint the oven and kitchen for some time after.
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One Windowsill
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Re: Root behavior in porous vs. nonporous pots

Post by One Windowsill »

Steve-0 wrote: Mon Jan 11, 2021 5:46 pm Temps up to 130F. Yikes. All plants survived though. I did move them into shade that week. This year I'm taking pottery courses with Andy Ward in Arizona and will be making my own pots ( with internal glazing ) going forwards. We'll see how that goes. I started over the weekend.
Perhaps you should look into an external glaze with a high infra-red reflectance to keep the roots cool. It seems most papers I could find were not open access but one had a recipe in the abstract:

Complex inorganic green pigments having a high near infrared solar reflectance have been synthesized. Cr2O3 is the host component and mixtures of TiO2, Al2O3, and V2O5 were used as the guest components. TiO2, Al2O3 and V2O5 were mixed into 39 different compositions. It was found that a sample, denoted by S9, with a composition of Cr2O3, TiO2, Al2O3 and V2O5 of 80, 4, 14 and 2 wt% respectively, gives a maximum near infrared solar reflectance of 82.8%. The S9 pigment powder was then prepared as a reflective coating material with different amounts of pigment powder from 4–8 g in 100 g of ceramic glaze. The prepared materials were sprayed on the clay tiles for reflectance measurements. It was found that 6 g of S9 pigment powder mixed with 100 g of ceramic glaze gives the highest near-infrared reflectance value of 74.5%. The newly synthesized pigment is a suitable ceramic roof coating for its high reflectance performance and the durability performing once the ceramic roof installed on house.
https://www.jstage.jst.go.jp/article/jc ... 9/_article

Vanadium is seriously bad for plants (they confuse it for phosphorus) so I would not use that glaze in contact with moist soil. The others might not be beneficial either. Remember that fungi can mine minerals.

Some other NIR-reflective glazes are brown. Boring, contain antimony and not as effective.
https://www.sciencedirect.com/science/a ... 1-main.pdf

Ivory-white, yellow and brick red are possible but involve rare-earth pigments doped with molybdenum or another rare earth. The Chinese have promoted rare-earth elements as plant-growth promoters but I have not heard anything else about that in the last 30 years.
https://www.sciencedirect.com/science/a ... 4811002996

Gold leaf will reflect both visible and infrared and may be cheaper than you think.

Sub-micron layers of silver are also mentioned. This reminds me of a glass blower who got spectacular effects by dusting silver halides (chloride or bromide, possibly) onto the surface of hot glass along with glass fibre. The silver halide would decompose leaving gorgeously coloured layers of silver on the surface of the glass.

The paper also mentions that green military camouflage has to use IR-reflectance as chlorophyll reflects IR.

http://www.academia.edu/download/492480 ... a8plky.pdf

Obviously, leave the advanced stuff until week 2 of your ceramic course. :D
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Steve-0
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Re: Root behavior in porous vs. nonporous pots

Post by Steve-0 »

@ One Windowsill ...Good stuff on temperature control in ceramics. My goal in making my own pots is to learn pottery making in the Southwest Native American style using the same clays, tools, methods and decoration as the ancients did. Andy Ward of Tuscon, AZ is the instructor. His site: https://ancientpottery.how/

And another motive for making pots is to save expenditure on expensive ceramic pots as well as having a complimenting pot for my cacti.

Something like this with the shape adjusted to be more suited for plants. ie - wider neck, drain holes, flat bottom, glazed insides for waterproofing.

PS - I'll keep in mind the visible and infrared thermal protection offered by applying gold leaf to my pots...maybe as an under layer before adding the design glazes. :wink:
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