Honestly this is very frustrating! It always happens every year around the same time. Etiolaion is caused by lack of sun and too much fertilizer, but what about if your plants are in unshaded unblocked full sun from sunrise to almost sunset with little water and no fertilizer? I honestly have no idea what is causing this. The only common factor i can think of between this year and last year is not watering for a long time (3-4 weeks) in august.
Some examples:
Biggest stem showing most of the thinning.
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Obviously very etiolated. You can see where it was cut last year due to etiolation
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20180906_123843-768x1024.jpg (141.35 KiB) Viewed 2569 times
Any ideas?
There are more cacti in heaven and earth, Horatio, than are dreamt of in your philosophy.
One of the few cactus lovers in Lebanon (zone 11a)
All plants are growing at night hours. But I don't think you plants got ugly ethiolated, ElieEstephane. Especially first one. Many columnar cacti first produce narrow tips and then ribs unfold and later these stems get thicker. The Opuntia also has stems shaped close to natural. Maybe just the Rebutia has not much spines on the top but sometimes it happens at the end of season or later on. I would restrict watering of the last plant.
Maybe your trees shade these cacti?
If your cacti mess in your job just forget about the job.
°C = (°F - 32)/1.8
7george wrote: ↑Sat Sep 08, 2018 2:23 pm
Maybe your trees shade these cacti?
These are on the roof with so shade whatsoever. Almost 70-80% of my cacti are now on the roof and rhey didnt exhibit this kind of behavior.
It may not show george but the thrixanthocereus is pretty etiolated. I've had them for 4 years since they were in 2" pots. They only thinned down and stretched in the past couple of months.
Here's a photo of daiv's plant. Notice the shspe of the tip. :
t_senilis_600 (1).jpg
As for the opuntia, i wouldn't say it's normal since the main stem is now only 1cm wide as opposed to 5cm. Even the opuntia linguiformis in your photo does not show this kind of extreme thinning.
The rebutia is a rebutia donaldiana which has a globular shape and it kept it until early summer where the offsets and the plant itself started growing cylindrically. I couldn't find this behavior in any photo of rebutia donaldiana.
There are more cacti in heaven and earth, Horatio, than are dreamt of in your philosophy.
One of the few cactus lovers in Lebanon (zone 11a)
Hmmm now that you mention additionnal sources of nitrogen, besides birds and insects, the tap water i use to water the plants (different than the home's water) could also contain nitrogen...
There are more cacti in heaven and earth, Horatio, than are dreamt of in your philosophy.
One of the few cactus lovers in Lebanon (zone 11a)
7george wrote: ↑Sat Sep 08, 2018 2:30 pm
Is this an ethiolation? No, just natural shape of that Opuntia pads.
No, locally they call this "linguiformis". Either a variety or sub-species. Sometimes called a good species. They grow in habitat around Phoenix, right next to more "normal" plants, so I don't think it's environmental. They are generally Opuntia Englemannii and have a wide distribution across the American southwest.
Common names include Cow Tongue cactus, cow tongue prickly pear, lengua de vaca (cow tongue), etc, etc...
Disclaimer: I'm in sunny Arizona, so any advice I give may not apply in your circumstances.
Etiolation under conditions of full sunlight all day long does not make much sense to me. Plants do, however, exhibit straggly, abnormal growth, if they are in too close proximity to each other, as I noticed with my smaller cacti occupying the same cactus bed as the taller prickly pear cacti.
They tried to stretch out toward the light, but the sun was only hitting their crowns.
If you do supply fertilizer, you could select one with the first value of the NPK formula being the smallest, and of course, also apply fertilizers sparingly.
I am using a fertilizer with the N (Nitrogen), P (Phosphorus) and K (Potassium) formula 15-30-15 and apply it sparingly, only 1/3 to 1/2 the recommended dosage. During my master's thesis work, studying chili peppers, I did find out, that lowering the nitrogen level had the biggest effect, reducing top growth the most, followed by reduction of potassium. Assuming similar plant response for cacti, my fertilizer does have the correct ratio for achieving low top growth. Of course, watering sparingly should also reduce top growth.
I have never had etiolation problems with cactus growing in full sunlight but, I also do not like to over fertilize my cactus unless they were grown in a greenhouse. I live within a hundred miles of the desert and can see cactus growing in natural environment. I do not see any etiolation but I also do not see anyone watering or fertilizing the cactus. I like to grow my cactus with very little fertilizer and like the compact look unless I am growing them from seed.
. Maybe your plants are growing at night 
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