Hi all,

We came to the end of yet another growing season where I am from and I want to repot some of the larger cacti I have. I also want to use this chance to introduce the plants to some fresh soil and hopefully optimize growth for some problematic specimen in old soil. I had some strange things happenning to 2 of my larger pachanois. 1 pachanoi went a bit yellow under full sun. This plant was planted on the ground, then transplanted into a very soggy location with a lot of clay 6 months later before getting potted up. It has been in a 30cm pot for a year now but it always had that dissapointing growth rate and I noticed it tended to go yellow significantly under full and intense sun. I did a colorimetric pH test (where you mix some soil with pH indicator liquid) and found that the soil has a pH of around 6-6.5. Another spineless pachanoi was frequently going yellower under intense sun as well and the pot soil it was planted in had a pH of 6-5.5. I should add that both of these plants are completely healthy in colder months and show lush green growth except mid-summer. Now it is also worth noting that there was some soil compaction in the potting mixes for both plants, espeically in the case of the spineless pachanoi where I can see the small rootlets on the surface and the soil is basically rock hard but strangely does not include much clay. Also it seems that I have been applying liquid compost way excessively. The instructions suggest using it every 3-5 months but I accidentally applied the full dose every 2 weeks. The long story short, I might have concentrated the older soild with a lot of nutrients which is why I want to repot the plants into much larger containers with a lot of fresh soil in them, and preferably make a slightly more alkaline mix to counteract the somewhat lower pH in the old soil. This brings me to my final questions: how much powdered dolomite lime or powdered limestone (gardening lime) I should add per volume of potting mix to achieve a pH of 6.5-7? Also should I break the compacted soil while repotting the cacti? The main ingredients of the potting mix will be as follows:

2 part sieved potting mix (all purpose)
1 part perlite
1 part sand

I know you might think there are posts out there addressing this question but believe me no one tells the EXACT amount of lime they use on trichocereus.

Also these are the fertilizers I have been using if that gives you a clue about compaction as I heard too much cal/mag may cause soil compaction which is one the suspects as well:

-Synthetic all purpose garden fertilizer at 1/9 strength 1gr/L (N: P:K 25:5: 8. 8 ) + S 4.6 / Mg 0.5 / Fe 0.18 / B 0.005 / Cu 0.005 / Zn 0.004 / Mo 0.001

-Osmocote slow release all purpose fertilizers with trace elements was sprinkled onto the potting mix at half recommended dose.

-Liquid Cal/Mag (5% / 1.4% respectively) at 1 ml/L

- Trace element mixture in powder form. 25g/9L was used since the recommended dose (25g/L=225g/9L !!) seemed unrealistically high for fortnightly applications. Ingredients: (% W/W) sulphur 6.29, Ca carbonate 10.00, Mg sulphate 3.62, manganese sulphate 2.88, iron chelate 2.73, copper sulphate 1.25, zinc sulphate 1.00, boron 0.09 and molybdenum 0.09

-Liquid potash (P 2.0: K 14.0) at 20 ml/9 L

-Liquid compost mixture containing fish and seaweed solution, humate and trace elements 50ml/9L

-Great White micorrhizae mixture in powder form (1 teaspoon/9L). These bacteria were used much less frequently, around 3 times per growing season.

-Organic phosphorus granules (N: P:K 0:12:0). This one was eyeballed unfortunately. Sprinkled over the soil sparingly each year.

Only 2 types of fungicides were used against black and orange spots with little success. Those were a solution of copper ammonium complex (93g/L) at 5ml/L and myclobutanyl at 0.05g/L.

I would appreciate every answer! All the best and keep safe

Ahubaba the yellowing is probably caused by too much sun during periods of hot weather. These cacti benefit from afternoon shade once the temps get into the 90's and above Fahrenheit. In much of their native range the high temps rarely go above the 70's F. High temps combined with full sun all day will cause the cacti to turn yellow and stall out new growth until the temps moderate. But with shade in the afternoon they remain green and continue growing, even when the temps are over 100 F.

Hopefully someone can answer your questions about potting soil. Generally speaking, most cacti are able to thrive in soil that is slightly alkaline. It is common for desert soils and ground water to be alkaline. I can tell you from personal experience that Trichos do great in soil that is slightly alkaline. Although they will probably be fine in neutral soil as well. I'm not sure about slightly acidic soil and how it affects them.

All the best with your cacti. Thanks for sharing your experience with us.

Thanks for you input Grey Fox. Yellowing and a bit of skinny growth is what bothers me the most. Though looking back at it I think I have bigger suspects like hot weather and full sun than soil chemisty. Also kept my plants in a shed for a month too which caused obvious etiolation. Too much going on to blame one thing really. I sure used a lot of organic fertilizer too in this growing season and in the next one I will try fortnightly application with much more dilute ferts. I will try 1/2 tablespoon gardening lime and 1/2 dolomite lime per gallon of potting mix for this next round of repotting and see how it goes. Also thinking of sprinkling half a tablespoon on the compacted soil and mixing it in. Probably will break the hard soil around the rootball too if the larger roots have not invaded the mix already. I always thought that messing up the ideal soil composition is easy while making it fertile again is not so simple so I see these repottings as a chance of revitalization.

Again thanks for chiming in an all the best

I wish I knew more about how best to fertilize and amend the potting soil. But all my experience with Trichos comes from growing them directly in the ground. My trichos get some organic compost worked into the soil at planting, and then some additional compost worked in every spring. But growing in containers is more complicated than that. I hope you get feedback from some other people. Take care Ahubaba.

It's been a while since I raised my concern over this and it seems like even with repotting and application of equal parts of dolomite and garden lime at half strength, there seem to be certain strains and clones more prone to yellowing at high temperatures and full sun. So Grey Fox you are probably right about that. I tried spray painting black pots and it seems to be helping a lot to the sensitive clones actually so that's been a nice improvement. You cannot believe how hot the pots get right around the surface where I find most of the roots of my trichocereus to be resting.

I noticed watering and fertilizing less with kelp extract is also good for yellowing. Don't know why kelp makes them turn pale but I am convinced that it definitely does. Always morbid, patchy yellowing the next day after I apply kelp even at lower concentrations. Now that I have a worm farm and good amount of compost I will resort to these for fertilizing instead of organic seaweed solutions and see if there is an improvement.

Yes I have also noticed that certain plants yellow more in the sun than others. For me it has mostly been Pachanois. The Bridgesii and Peruvianus seem to handle the sun better, with less yellowing. It makes sense, since they have long spines and the spines help to provide some shade on the surface of the cactus.

Did you apply that kelp fertilizer during hot weather? Fertilizing when the temperature is high can cause stress and yellowing. Thats why many gardeners do not fertilize during the summer heat. If you think its absolutely necessary to fertlize during hot weather then its probably best to make sure the fertilizer is extra diluted and maybe try fertilizing at night after the cacti have already been watered.

But this time of year you should not fertilize since winter is coming and you want the cacti to go dormant. Trying to grow them indoors through the winter usually causes etiolation, unless you have a high quality indoor grow setup with lights.

Im wondering if indeed there may be a secondary factor at play other than full sun that results in yellowing, whether it is somethign related to the soil, or maybe just that it was too fast a change to full sun.. How gradual was it moved to full sun?

I've had pedros, peruvianus and bridgesiis in full sun in zone 9b and never had any issue with yellowing or similar, but I've always done it gradually when I first transplanted them,..

Many of my Trichs yellow with too much sun/heat and new growth stalls out as well. Several types of Pach seem to do it the most, along with a couple of types of Bridgesii. I don't have a lot of Peruvianus but I can't say I've ever seen a Peru go yellow in the heat. It's hard to avoid in 7a, but this Summer I put up 30% shade cloth in the greenhouse and it dropped the inside temps about 10 deg F, which I'm sure they appreciated. I'm considering other strategies (adding a Thermal Mass, more shade plants inside the greenhouse, etc) but I think to get the temps down so they'll grow all Summer it would take adding air conditioning of some sort which would be expensive.

The soil I use these days is pretty much any organic potting soil, perlite and vermiculite mixed about 60/20/20, but 70/30 Potting Soil/Perlite works pretty well. I used to use sand but seemed like it tended to pack hard sometimes if I used too much. I sometimes add Azomite rock dust to the soil, or bone meal but the only thing I've added I know they really like is fertilizer, specifically Iguana Juice liquid, mixed half recommended strength.



Ahubaba if by that you mean loosen up the compacted root mass when repotting then I would say yes. I got in a hurry a couple of years ago and left the pot-shaped rootbound mass intact on several and It seems like they haven't grown as fast as I would expect since. The repotting videos I've seen all suggest loosening up the root mass and I suspect it's for good reason.

Its the combo of hot weather and full sun all day that causes the yellowing and slowed growth. They can take temps in the 70s and 80s no problem. But once it gets into the 90's and especially once it gets into the 100's then the problem starts.

Everyone's climate / microclimate will be different. I'm not sure if humidity levels play a factor. But here in the desert I see a lot of banana yellow PC's growing in people's front yards. Afternoon shade solves the problem around here.

The worst I ever get in terms of yellowing is occasional light mottling like in the photo attached here. This is on what I presume to be PC pachanoi. My suspicion as to its cause was also in the direction of kelp-based nutrition and I've been more cautious in its application since then.

The cacti are in a sheltered, south-facing location in northern Europe, hardiness zone 7 but reaching a full-sun maximum of 43°C - luckily with afternoon shade.

As far as lime goes, I've found T. terscheckii to respond poorly to excessive presence of limestone in the substrate.

Thanks downwardsfromzero. I often thought that this might be due to a tobacco mosaic virus or a similar viral pathogen and have been isolating those specimens for a while by now.

I agree with Grey Fox regarding the susceptibility of certain species.I definitely noticed pachanoi and bridgesii do not perform their best under full sun. I would even stretch that to peruvianus but they definitely are more tolerant of sun stress, however still get somewhat yellower under their protective blue coating. Sun here is no joke, it nibbles on you even in the winter and in the summer temps can reach above 40 degrees C.



Endlessness they have been sitting in the same position since early spring when the weather was pretty cool and the sun was mild. However in their current position, I notice that they take up to 10 hours of direct sun from dawn to dusk. I will try to move them near the house for protection from the sun after 2pm or so. They will still face the harshest rays but will have the time to recuperate I assume.

My soil composition is pretty standard as described above but lately I decided not to sieve the bark and the little pebbles in the commercial mix fearing it might cause poor drainage and compaction. Many growers use the commercial mix straight away and I noticed that they have happy plants so not trying to reinvent the fire anymore. But I now add 2 parts perlite, 1 part mushroom compost, 1 part commercial compost and 1 part volcanic rock dust to 6 parts of potting mix. I have been mixing in 1.5 tablespoons worth of 50/50 garden lime and dolomite mixture into 6 gallons of potting mix but I cant say I see much difference. I also eyeball pelletized organic all-rounder fertilizer mainly based on chicken manure, but it still contains kelp unfortunately. This mix is only used on my new plants so I can only compare the cuttings of the older, yellowing ones I planted into this new mixture. They all seem to be doing alright so far but the older ones still keep getting sunburned in their old location. Also there are plants which are exposed to the same organic fert who are doing fine while pachanoi and bridgesii getting tip burned or molted under the same full sun location.

All in all I tend to believe that it is the sun, which might be contributed by the fact that I like to withhold water from my cacti and usually allow the rain to do the deed. I will move them to a bit more merciful location and try watering them weekly, increasing watering frequency during heatwaves.

Still cannot help but wonder how did all that torment affected the plants? I am sure I lost quite a bit of biomass but it would be interesting to somehow measure the effect of intense sunstress on the alkaloid production. Kinda makes me sad to think that some trichs are not able to put the maximum energy they can receive from the sun into good use. Being able to respond with vigor to full sun would be a very valuable trait. Then again this is pretty much what all the ground planted trichocereus do even here so I think us pot growers will have to walk that limbo for the time being.

Ahubaba I wanted to tell you that I planted some Bridgesii in the spring in an area that gets full sun all day. I'm running out of spots with afternoon shade, so I decided to see how they would do with the extra sun.

They yellowed somewhat and slowed down growth in the summer. But as soon as cooler Fall weather started they greened right up and have been on a growth spurt of healthy looking new growth.

I water my Trichos (all in ground) once a week in the summer. But those Bridgesii in full sun got watered twice a week and I think it made a big difference in terms of them adapting to that sunny location. Full sun in hot weather is a stressor for most Trichos, but the stress can be reduced somewhat with more frequent watering than what plants in shadier locations receive. They can definitely survive full sun in a hot desert climate, but more frequent watering is needed to keep them as healthy as possible. Just have to be careful not to over do it and bring on the rot.