Nerd Alert: Watering - Sep 3 Update!

[From0to10in2weeks]

Hi,

As a newbie I was at the beginning not sure about the watering schedule for the cacti I bought in June. After reading a bit here and elsewhere, I settled on a 10 day bottom soak protocol. But it bugged me not knowing if it was too long or too short. And frankly, with the exception of one plant I wasn’t sure if the cacti were growing at all. So, before the last watering I decided to weigh the pots, and then weight them again every 24 hours to monitor the change over a 10 day period.

Processes associated with weight changes relative to the weight before watering are:
- Evaporation (loss)
- Plant uptake (gain)
- Plant growth (gain)

Here, is a plot of the pot weight relative to before watering on Aug 5 over the last 10 days for a total of 14 plants. The weather during that period was rather warm (30s Celsius) and sunny. The plants are indoors on an east-facing window bank getting plenty of morning sun, and they’ll catch direct unfiltered sunlight for about 1-1.5 hours when I open an adjacent south-east facing window around noon.

Kakteen all.jpg (56.53 KiB) Viewed 1875 times

Number 5 in this plot is an obvious outlier among the cacti as its weight changes very differently from all the others. This is a G. baldianum that has been producing flower buds since I bought it in June. So, it clearly gains weight in addition to losses from evaporation during this time period. Further, this plant is the only one that is still in its original soil. All the others have been repotted in a mixture of Seramis Kaktus Substrate and a peat-free soil (2:1). So, the drying behaviour is certainly different as well.

Then, closer inspection shows that the change in relative weight among all plants (excluding #5) can be grouped according to the pot size. Within one group, the plants all behave pretty similar. So to simplify, the following chart shows the average relative weight for all plants (dotted line) as well as for plants in small (2A-C, 3, 8A-B, 9A-B), medium (4, 6, 7) and large (1, 10) pots. Again, #5 was excluded from the plot for the small container size because of its obviously different behaviour.

Kakteen Averages.jpg (40.67 KiB) Viewed 1875 times

EDIT: 08/25/2020: Please ignore the following discussion about why the larger pots show a smaller relative weight gain compared to the medium pots. That turned out to be a red herring. Clarification is in a later post in this thread. Summary: initial water uptake scales with volume of the pots.

What was surprising to me is that the medium pot sizes showed the largest initial water uptake, even larger than the bigger pot sizes. How can that be? The only explanation I can come up with at the moment is that I was doing a bottom soak with a fixed time (30 mins). I had assumed that this is enough to saturate all the pots but that may not be the case for the larger pots though the soil on top certainly looks wet in all cases after the watering. It also raises the question if I possibly should shorten the soak time for the smaller pots to avoid them sitting unnecessarily in water.

But the chart clearly shows that pot size matters for drying behavior. Surprise, surprise. The smaller pots reach 100% relative weight on day 5 after watering. In contrast, the medium and larger pots need 8 days to reach 100%. Does that difference justify adjusting the watering schedule according to pot size? I am not sure and I am certainly open to suggestions. For logistical reasons, I’d prefer to do all plants at once of course.

Then, as you’ll notice from both charts there are a number of pots that dip below 100%. So, they weigh actually less than before watering them 10 days ago. There are two possibilities in my opinion:
1. The weather was certainly warmer during this monitoring period (Aug 5 - 14) than in the 10 days before the last watering (Jul 26 - Aug 5). So, it could be that the substrate was still holding more moisture on Aug 5 before watering than it is now.
2. The cacti start loosing water from their body now.

Of course, a combination of both may also be possible. Unfortunately, this also suggests that monitoring weight is not a good way of judging plant growth. Or mine indeed are not growing…

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.

I may do another experiment series with dummy pots without cacti to monitor how the substrate itself changes weight over time.

Best.

[keith]

Pots weight dry after 10 days I think that's pretty good. In the growing season water cactus when dry so 10 days looks about right. I would water from the top once and awhile to flush the soil. Nice graphs BTW.

[mikethecactusguy]

I water my plants when the soil is dry 30% below the surface and even more during hot growing season. The plants will also indicate when they need water. Some like opuntias get watered every other day right now. Smaller pots get watered more often than larger pots. Currently all my 3" and 4" pots get watered every other day. The plants are growing like weeds right now.

[bartab]

Wow. I'm at 14-17 days. I've got a lot of rot prone plants so I let them go quite awhile. Maybe I looked past it, but what is your growing media, and what kind of pots are being used? My plants are outside, and when I watered the other night I had some plants that were just too plumped up to water, so they are going on 3 weeks. I'm one valley over from the coast, so it is warm with cool nights. I'm using basically 60/40 pumice/grit.

Adding that I see where you mentioned the media.

[Aiko]

A scientific approach like this you don't see much in the world succulents on forums like this or in succulent magazines. Unless it is an investigation on germination rates. On watering schemes I have not seen this before.

I don't know how helpful this really is, though. I prefer to use my common sense and experience. Many of my plants are in a loam mixture. The mix will be almost as solid as cement when dry, so that is a pretty good indicator for me on how much moisture is left in a pot for many of my plants. I just need to squeeze the pot and see if the sides of the pots give way yes or no.

[bartab]

I just ordered a darn scale.

[From0to10in2weeks]

I would water from the top once and awhile to flush the soil. Nice graphs BTW.

Thanks. 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? I don't have access to rainwater and need to use tap water (acidified to ph 5.5 - 6, Hardness 14).

[From0to10in2weeks]

I water my plants when the soil is dry 30% below the surface and even more during hot growing season. The plants will also indicate when they need water. Some like opuntias get watered every other day right now. Smaller pots get watered more often than larger pots. Currently all my 3" and 4" pots get watered every other day. The plants are growing like weeds right now.

Thanks for your perspective. I never heard the expression "growing like weeds" in connection with cacti. Are your plants outdoors? I suspect that one reason mine are not "growing like weeds" is that they are behind windows with some e-glazing. It definitely makes a difference how much light is hitting them. I have an idea how to quantify that. Stay tuned.

[From0to10in2weeks]

Wow. I'm at 14-17 days. I've got a lot of rot prone plants so I let them go quite awhile. Maybe I looked past it, but what is your growing media, and what kind of pots are being used? My plants are outside, and when I watered the other night I had some plants that were just too plumped up to water, so they are going on 3 weeks. I'm one valley over from the coast, so it is warm with cool nights. I'm using basically 60/40 pumice/grit.

Adding that I see where you mentioned the media.

It's really puzzling how different the watering schedules can be. One poster waters every few days, while you wait 2 - 2.5 weeks... This vast variability was in part inspiration to do this study.

The plants are currently in plastic pots. I know about the benefit of terracotta pots but haven't gotten around to it...

[From0to10in2weeks]

I don't know how helpful this really is, though. I prefer to use my common sense and experience. Many of my plants are in a loam mixture. The mix will be almost as solid as cement when dry, so that is a pretty good indicator for me on how much moisture is left in a pot for many of my plants. I just need to squeeze the pot and see if the sides of the pots give way yes or no.

Well, if that works for you great. I imagine that with time and more experience my common sense in this regard will become better as well. In the meantime, I'll use whatever simple approaches are possible to advance my understanding.

[From0to10in2weeks]

I just ordered a darn scale.

One caution about the scale. Depending on the weight of your pots you may need to choose a scale with 1g or 0.1g readability. One shortcoming of this study is that I used a kitchen scale which reads in 1g increments. But my smallest pots weigh around 140g. So, a 1g resolution of the scale corresponds to 0.7%. In other words, changes in relative pot weight smaller than that can not really be measured.

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 now bought a scale with 500g max capacity and 0.1g readability. Good luck!

[bartab]

Thanks for the advice on the scale. I have ordered the 1 gram version and that will be fine getting me going into the winter. I should have worked on this before but have put it off. I've lost a few plants to rot this year so my periods between watering have increased. I've stuck a few trays out of the way, and a few pots scattered around that I have forgotten to water and they are doing fine. I did water a bunch of things tonight at the 12-13 day mark. I look forward to getting the scale. Thanks, and best of luck.

PS. Don't think you will find a lot of love for terra cotta pots in these parts unless they are sealed. I've got a few really small plants in 2.5 inch deep terra cotta but it is the the newer less porous type and sealed them with granite sealer. You will hear both sides, as some do great with the terra cotta though, so I guess it is really a mixed bag.

[greenknight]

If it's reached the point where visible mineral deposits are forming on the surface, you need to do more than just water from the top once in a while - you need to thoroughly leach out the accumulated mineral salts. Here's an article that explains the problem:

https://extension.umd.edu/hgic/watering ... ouseplants

Salts will build up more rapidly with bottom watering - if you water from above until water comes out the drain holes, and discard the drain water, it will flush out some of the salts with each watering. Leaching will still be needed, but less frequently. Watering with rain water or distilled water will also help, but if you fertilize, periodic leaching should still be done.

[From0to10in2weeks]

If it's reached the point where visible mineral deposits are forming on the surface, you need to do more than just water from the top once in a while - you need to thoroughly leach out the accumulated mineral salts. Here's an article that explains the problem:

https://extension.umd.edu/hgic/watering ... ouseplants

Salts will build up more rapidly with bottom watering - if you water from above until water comes out the drain holes, and discard the drain water, it will flush out some of the salts with each watering. Leaching will still be needed, but less frequently. Watering with rain water or distilled water will also help, but if you fertilize, periodic leaching should still be done.

Thanks for elaborating on Keith's point. The mineral deposits are still minor. But I will start using distilled water, and do an occasional top flush/leaching.

[From0to10in2weeks]

PS. Don't think you will find a lot of love for terra cotta pots in these parts unless they are sealed. I've got a few really small plants in 2.5 inch deep terra cotta but it is the the newer less porous type and sealed them with granite sealer. You will hear both sides, as some do great with the terra cotta though, so I guess it is really a mixed bag.

Really? 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?

For clarity, while I have the plant and substrate in plastic pots I then place them in white ceramic or plastic planters. So, the heating of soil and roots by light absorption of dark plastic containers is eliminated/reduced. But the main reason is aesthetics.

Best.