Nerd Alert: Watering - Sep 3 Update!

[greenknight]

It's a common misconception that evaporation from porous terracotta pots is good for cacti, it really isn't. Dissolved nutrients wind up deposited on the outside of the pots. The cactus roots grow to the terracotta and cling to it, instead of spreading evenly through the potting mix. This makes a less-efficient root system, and makes it harder to get the plants out of the pots, with more root damage.

The advantage of sealed terracotta or glazed ceramic over plastic is purely esthetic, what you're doing is fine.

As for "growing like weeds", mikethecactusguy is in Southern California - perfect climate for growing cacti.

[Steve Johnson]

If your pots are around 1 kg and you use a scale with 1 g resolution you can monitor 0.1% relative weight changes. That's of course much better.

I have a somewhat different take on this...

Some people are able to feel the weight of their pots by hand so they'll know if their cacti need watering. I never developed that talent, so opting for a scientific approach (albeit not as sophisticated as yours), I purchased a 20-lb. digital postal scale about 6 or 7 years ago. The scale reads in pounds and ounces or grams down to .1 gram, so accurate enough either way. Here's the procedure I followed:

• Weigh the pot filled with a cactus and your potting medium of choice. Make sure the mix is completely dry, and write down the dry weight.

• Give the plant a good, deep watering so that the mix is completely saturated, and write down the wet weight

• Weigh the pot every day until it reaches its dry weight, and write down how many days it took for the mix to go from wet to bone-dry

While my collection is small (only 60-some-odd cacti), I didn't have enough time to weigh every single pot, so I weighed various pots by the different sizes representing what I have in the collection. Based on the weighing I did, I determined that it takes about 7 days for my soil-less pumice and granite gravel mix to dry out completely. A couple of things did surprise me, though -- first, drying-out time is the same regardless of pot size. Second, it's the same in spring and summer. How is that possible? I can thank a temperate and fairly humid Mediterranean-type coastal climate for summers which aren't blazingly hot and bone-dry all the time.

Generally speaking, desert cacti under cultivation do best if their roots are allowed to completely dry out between waterings. For people like me who prefer a set watering schedule, knowing how long it takes for the mix to go from wet to dry is kinda important. Although weighing pots seems like quite an exercise, IMO it's worth taking the time -- and you'll need to do it only once unless you change your mix or you move to a substantially different climate from where you live now. As to watering schedules, this is something less-experienced hobbyists have to determine as they learn more about the habits of the cacti they're growing. Good news is that a lot of species are remarkably tolerant of growing practices which are less than ideal, and a time-tested watering "rule of thumb" will help -- when in doubt, don't.

I was under the impression that terracotta pots are largely preferred here and elsewhere as it allegedly allows faster drying. But what advantage would non-porous/glazed/sealed terracotta pots have over plastic pots?

While I've been writing this post, Spence beat me to it, but I'll amplify what he just said...

There are a few advantages, although I need to address the terracotta issue first. Terracotta pots fired at a relatively low temperature are porous, and using plain porous clay pots to dry out the potting medium is a beginner's trick done by inexperienced growers who are using soil-heavy commercial mixes. Now we have two problems in one -- too much soil suffocates the roots of our cacti, and evaporation through porous clay sets up a competition between the pot and the roots for water that should be going only to the roots. Healthy rooting behavior is eventually compromised, and I can tell you that for a fact when I saved a Haworthia that was close to dying after 8 years of being in a plain terracotta pot. I moved the plant to a glazed ceramic pot in April of last year, and I have photos set up for a presentation detailing my experience. It's low on the priority list, but hopefully I'll post it on the Cultivation forum by the end of the year. For now, long story short -- the difference between growing in terracotta and glazed ceramic is pretty amazing.

If you seal terracotta with UV-resistant acrylic paint or something similar, that solves the porosity problem. The advantages we have with sealed terracotta/glazed ceramic over plastic are durability, aesthetics, and a greater variety of pot sizes. Going with nonporous pots, we need to solve the other problem -- a soil-heavy mix. Maybe okay for succulents, but completely unsuitable for a lot of desert cacti. The only solution is to lean out the mix with mineral gravel. Pumice is best, the other options being Perlite if you have no other choice, crushed granite poultry grit, or calcined clay cat litter. With the exception of granite, mineral gravels hold a pretty good amount of water, so if your mix contains a lot of soil, I'd recommend leaning it out to give you a 50/50 mix of soil and mineral gravel. If you'd like to go a bit leaner, tilt the mix in favor of 60% mineral and 40% soil.

By the way -- in case you're interested, there is a cactus club in Berlin you might want to check out:

https://www.kakteenfreunde-berlin.de/

The best advice we can get comes from local growers, and this should be right up your alley (or Platz as the case may be).

[7george]

I’d be curious to hear your thoughts about all this. Assuming this is representative, do you think a bottom soak every 10 days is a good choice? But also, do you think the decline in moisture is fast enough? Visually, the top layer of the substrate looked very dry on day 2 after watering (Aug 7). But of course, there is still a lot going on internally.

Weight decline can be associated also with water loss from cactus body during hot seasons. So it is possible you desiccate plants at some degree with 10-day intervals. I don't rely on day counting but on that how thirsty that plant looks to me. A cactus shrinks and gets grayish when it is dry and under sun. Intervals would be different for different pots and plants so I these days water almost every day because "the time has come" for next group of plants. So I never tried to determine a fixed interval of 3, 5, 7 or 10 days even for an unified season but rather according to plant needs. Sometimes I weigh a small pot by hand if in doubt. Some guys I new did watering in 7-days because plants was held in the summer house and visits were in these intervals so let the plants adapt to them and that is.

[From0to10in2weeks]

It's a common misconception that evaporation from porous terracotta pots is good for cacti, it really isn't. Dissolved nutrients wind up deposited on the outside of the pots. The cactus roots grow to the terracotta and cling to it, instead of spreading evenly through the potting mix. This makes a less-efficient root system, and makes it harder to get the plants out of the pots, with more root damage.

The advantage of sealed terracotta or glazed ceramic over plastic is purely esthetic, what you're doing is fine.

Thanks for the clarification. That makes a lot of sense.

[From0to10in2weeks]

If your pots are around 1 kg and you use a scale with 1 g resolution you can monitor 0.1% relative weight changes. That's of course much better.

I have a somewhat different take on this...

I am a bit baffled about your take here as it seems exactly as mine as far as method and motivation. Maybe you intended to address someone else.

But I am curious about the fact you didn't see a difference in drying behaviour between pot sizes. But of course that depends on their relative sizes/volumes. If they are larger then the relative differences in volume are smaller than between small pots. Mine are currently between 5.5 and 8 cm diameter. So, volume differs quite a bit.

Thanks for the feedback about the terracotta pots. It makes a lot of sense.

And yes, I know about the cactus club in Berlin. Due to Covid there is not a lot of activity as far as I can tell. But I'll surely follow up.

Best.

[From0to10in2weeks]

Weight decline can be associated also with water loss from cactus body during hot seasons. So it is possible you desiccate plants at some degree with 10-day intervals.

Yeah, I came to that realisation in the course of the experiment. So far, the cacti don't look obviously wrinkly or greyish after a 10 day period. I also didn't want to suggest that I'll now use always a 10-day interval. In fact, I think weighing the pots before watering and then again every few days is a good way to monitor and adapt to their respective watering needs.

[DaveW]

Plastic pots are in general use in the UK for cacti and have been since they were introduced in the 1960's. It is hard to find Cactophiles that grow entirely in clay pots in the UK these days since plastic pots clean easier, are lighter and far cheaper. Plus not all clay pots are now porous because in order to make them frost proof and stop them delaminating in the cold they are now being fired at higher temperatures than in the past, therefore are no more porous than ceramic or plastic pots. If your clay pot does not produce that white efflorescent deposit on the outside that you have to clean off periodically it is not a porous one. I would think both by amateurs and commercially 80%-90% of the worlds pot cultivated cacti are now grown in non porous plastic pots.

https://www.trustbasket.com/blogs/how-t ... -clay-pots

We never had the incentive to grow in fancy pots in the UK, because up until recently the pot never featured in the judging in shows provided it was clean and plastic cleaned easier than terracotta or clay pots. Unlike the USA where fancy designer pots often influenced the judges in shows and got points. Maybe as well since most UK serious collectors have to grow in heated greenhouses in winter with the plants pots crammed together rim to rim for the best utilisation of expensive heated space, ornamental posts would not usually be on show anyway and often take up more space, unlike you who can grow your plants out in the yard. The windowsill growers of course use more fancy pots for their looks, or place plastic ones inside other containers.

[From0to10in2weeks]

If your clay pot does not produce that white efflorescent deposit on the outside that you have to clean off periodically it is not a porous one. I would think both by amateurs and commercially 80%-90% of the worlds pot cultivated cacti are now grown in non porous plastic pots.

Unlike the USA where fancy designer pots often influenced the judges in shows and got points. Maybe as well since most UK serious collectors have to grow in heated greenhouses in winter with the plants pots crammed together rim to rim for the best utilisation of expensive heated space, ornamental posts would not usually be on show anyway and often take up more space, unlike you who can grow your plants out in the yard. The windowsill growers of course use more fancy pots for their looks, or place plastic ones inside other containers.

Thanks for the additional perspective. Best.

[bartab]

Haven't had summer rain in the SF Bay Area in years it seems. But wake up at 4 this morning with thunder and lightning and rain coming down. Hope my plants enjoy it. None of them are covered. I take pains in the winter time to keep them covered during the rain, but not doing that now.

[mikethecactusguy]

sorry for taking so long to answer back. We all have different situations when it comes to our plants. All of mine are in terracotta. I like how it looks and the plants are happy. 95% of my plants are in direct sun most of the day since I moved them upstairs to my upper balcony. What I have found is that some of the oldest plants that have been in the pot the longest , will develop a soil coating and the dirt turns bad around the perimeter of the pot. Only a few of my larger older pots show any mineral build up. Probably because I move most of my plants up a pot size very year and I water until I get a full flow of water out the lower drain hole.. I'm now in the habit of renewing soil every year. Because the soil mixes I use are very porous, there is very little moisture retention in the 3" and 4" pots. So in this intense heat we are now enjoying, the pots dry out in 24 hours. As for root growth, I have not found many that only have grown only around the pot edges. I normally have nice solid root balls.
I water all my plants just after sun down. So during the cooler night weather they can soak up as much water as they can. I was following all accepted guide lines when it came to watering but noticed the plants were not growing . AS I increased summer watering the plants started to really grow. Where I get into trouble is winter. I lose a few plants every year to rot. I'm not able to protect all my plants from the rains and cold so some suffer.
As I said, all our situations are unique to our location and our habits.

[From0to10in2weeks]

Haven't had summer rain in the SF Bay Area in years it seems. But wake up at 4 this morning with thunder and lightning and rain coming down. Hope my plants enjoy it.

Hah! They may not realize what hit them, but they probably will enjoy it. Best.

[From0to10in2weeks]

We all have different situations when it comes to our plants.

I was following all accepted guide lines when it came to watering but noticed the plants were not growing . AS I increased summer watering the plants started to really grow.

As I said, all our situations are unique to our location and our habits.

I think these are some very good points and I come to accept that everyone has to develop their own routine. That’s why I think at least for me weighing the pots may be a simple way to optimize that.

For instance, I noticed that the flowering baldianum (#5) I mentioned above struggled growing it’s last flower (And it has one more bud that hasn’t developed at all). In fact, it showed significant growth only after the last two waterings, and it finally opened last week although the bud formed already in mid July. The previous 4 blossoms grew much faster.

I have three plants of a (likely) Pilosocereus glaucochrous that I obtained from one pot after repotting. I may do an experiment where I water one faster than I’ve done so far but keep it at the same location. And another I can place in late afternoons on a West facing outdoor(!) window sill to catch some more direct sunlight (not filtered by windows with energy saving glazing).

Anyway, thanks for sharing additional info and your experience.

[keith]

hanks. I have noticed that some mineral deposits are forming at the top of the soil. Is that the reason you suggest to do a top watering from time to time? "

yes that's right. A few other things but remember we all have different methods that work for us. I think it depends on what kind of plants you grow some cactus and succulents can take allot of water and some they just rot. I just transplanted about 20 four year old Lithops out of a mostly pumice soil because they were rotting off one by one and planted them into a mostly sandy soil , dry sandy -pumice- gravel mix, and they are growing in bone dry soil. I mean it must not be bone dry but it looks dry to me. Same for some cactus like Ariocarpus , turbinicarpus , Echinocactus horizonthalonius, etc they will rot if the soil stays wet too long believe me I know. Almost any cactus you find at non specialist nurseries you can water away because commercial nurseries they wont grow hard to grow cactus not worth it. I sometimes see Lithops at big box but they are usually on their way out of this world.

I water depending on the weather and season usually every 3 weeks in Summer and way more for seedlings often every day. Easy to do now that I'm working from home . Your method of weighting pots and calculating moisture based on weight I think is a good start. You do this long enough you can just look at a succulent and tell if it needs water based on how fat it is . And often you have to resist watering the plant even if it looks dry safer that way.

[From0to10in2weeks]

Almost any cactus you find at non specialist nurseries you can water away because commercial nurseries they wont grow hard to grow cactus not worth it.

Hah. That's a great insight.

You do this long enough you can just look at a succulent and tell if it needs water based on how fat it is . And often you have to resist watering the plant even if it looks dry safer that way.

That's the plan. As I saw somewhere else: "if in doubt, don't water" or similar.

Best.

[From0to10in2weeks]

Hi,

I just finished the second round of monitoring the weight of 13 cacti pots for 10 days after watering. Overall, the results are quite similar to the first run but there are some notable differences. Here is a plot of the average pot weight relative to before watering for all pots (dotted) and separated for small, medium and large pots.

Kakteen Averages 20200824.jpg (40.19 KiB) Viewed 1259 times

First, due to a change in my bottom watering process the initial water uptake for each pot was smaller than in the initial run. I had changed to a larger tub to place the pots in but added the same amount of water. Well, that of course dropped the water level which seemed to limit the amount of water the pots could absorb.

Then, in the first run more than half of the pots had a relative weight below 100% after 10 days. My hypothesises then were that either the cacti itself were also loosing water weight. Or that the soil before watering wasn't as dry as it was at the end of the 10 days due to the warmer weather.
Now, only 2 pots drop slightly below 100% although the weather was quite similar as in the previous monitoring period. To me that suggests that this time the soil before watering was indeed much drier than in the previous run. I have some additional preliminary data that support that as well but I want to repeat those due to the mishap re the watering process.

One more thing: this time I paid more attention to the changes during the 30 min bottom soak. The top surface of the smaller pots look "wet" pretty quickly whereas the larger ones change very slowly. Arguably, they aren't even as "wet" looking after 30 min soak. So, it's more and more likely that the 30 mins soak is not enough to saturate the larger pots. That would explain of course why they show a smaller initial water uptake than the medium ones.

Hope this is of interest to someone. Best.