What is wrong with my dog tail cactus?!

[AmberSomething]

Hello everyone!

I have just recently started purchasing some cactus, one of which is a dog tail cactus. I have had it for a few months and seemed to be fine until a few weeks ago. The tips are turning black and sort of shriveling. The rest of cactus seems fine, just the tips. I have it on the windowsill, but maybe it's getting too much direct sun? I water it about twice a month. Is that too much, too little? I really love the cactus and am afraid I've killed it? Does anyone know what may be the problem? I am attaching a picture that will hopefully help. My pic doesn't show how black the tips are, but they are definitely much darker than you see in the pic.

Thanks,

Amber

Dog Tail Cactus.jpg (119.39 KiB) Viewed 5852 times

[hegar]

Hello Amber,
your cactus does look OK to me. I am not an expert when it comes to "jungle cacti", but those kinds of plants do best in slightly different environments than those that are classified as "desert cacti". This said, your problem with the discoloration of the tips may indeed be caused by too much light. When chlorophyll is being reduced, the underlying pigments become visible. Something similar happens, when fall arrives and the red and yellow pigmentation becomes dominant. I would move the plant into a location that still does have a good amount of sunlight but with less intensity than what your plant is experiencing now. You will need to experiment a little, because all plants will grow in an unnatural fashion, when their light requirements are not being met. With too little light, the color of the plant will become a lighter green and the thickness of the stem pieces will decrease. This is called etiolation.
As far as watering is concerned, you should watch out, even with this kind of cactus. It will grow in richer and moister soil, but should not be kept in a constantly soggy growing medium. I hope, that somebody, who also grows cacti in pots and has experience growing the type that your purchased, will chime in and give you good advice. Our goal on this thread is to help you keep your plants healthy. It would be a shame, if you were to lose this beautiful and large specimen. I wish you good luck with its cultivation.

Harald

[teo]

They look like Hylocereus undatus seedlings and it looks like new growth at the stems. The different color is a sign of (some) stress (I think) but I'm not sure of why.

[greenknight]

Selenicereus testudo looks very similar, and is one of the species commonly called dog tail cactus. It's not a desert cactus, in any case; it needs more water and less direct sun than desert cacti.

[AmberSomething]

Hello Amber,
your cactus does look OK to me. I am not an expert when it comes to "jungle cacti", but those kinds of plants do best in slightly different environments than those that are classified as "desert cacti". This said, your problem with the discoloration of the tips may indeed be caused by too much light. When chlorophyll is being reduced, the underlying pigments become visible. Something similar happens, when fall arrives and the red and yellow pigmentation becomes dominant. I would move the plant into a location that still does have a good amount of sunlight but with less intensity than what your plant is experiencing now. You will need to experiment a little, because all plants will grow in an unnatural fashion, when their light requirements are not being met. With too little light, the color of the plant will become a lighter green and the thickness of the stem pieces will decrease. This is called etiolation.
As far as watering is concerned, you should watch out, even with this kind of cactus. It will grow in richer and moister soil, but should not be kept in a constantly soggy growing medium. I hope, that somebody, who also grows cacti in pots and has experience growing the type that your purchased, will chime in and give you good advice. Our goal on this thread is to help you keep your plants healthy. It would be a shame, if you were to lose this beautiful and large specimen. I wish you good luck with its cultivation.

Harald

Thank you so much for your reply, and advice!

I am new to the world cacti, and must admit, I thought all cacti would thrive in the same conditions: heat and not a lot of water. I had no idea there were different types of cacti (jungle and desert), so I appreciate you clarifying that for me. I have a golden rat tail, and just assumed both would do well on the window. My rat tail seems to enjoy the heat, but when I saw the dog tail's tips changing color and shriveling, I freaked. After reading your reply, I moved the dog tail away from the window and also repotted it. I hope repotting it doesn't cause further damage, but assumed it needed soil betted suited for a cactus. When I was repotting it, I did notice that the soil was very wet, which I know with plants is not a good thing. I hope this new soil, plus less exposure to direct sun will make her happy again! I'll keep you posted...

Once more, thank you so much for your reply

[AmberSomething]

They look like Hylocereus undatus seedlings and it looks like new growth at the stems. The different color is a sign of (some) stress (I think) but I'm not sure of why.

Thank you for replying! I imagine the "stress" was caused by me. As I mentioned in my other reply to Harald, I wasn't aware that there were jungle cacti and desert cacti. I just figured all cacti loved the sun. Hopefully, with all your advice and help, I will relieve some of the stress.

[AmberSomething]

Selenicereus testudo looks very similar, and is one of the species commonly called dog tail cactus. It's not a desert cactus, in any case; it needs more water and less direct sun than desert cacti.

Thank you!

I really love this cactus and don't want to kill it. More water and less direct sun, indeed - thanks

[ElieEstephane]

They look like Hylocereus undatus seedlings and it looks like new growth at the stems. The different color is a sign of (some) stress (I think) but I'm not sure of why.

I agree. Im growing some from seeds for grafting use later and they look exactly the same

[greenknight]

They do look the same at this age. The difference should become clear when it gets older - either Hylocereus undatus or Selenicereus testudo will get quite a bit larger, but the stems of S. testudo get much wider as it matures (they still have a slender, tapered end, which as what inspired the name "dog tail cactus"). Either needs partial shade in hot climates, produces large nocturnal flowers - those of S. testudo are not quite as large but are more freely produced,

As Harald said, its soil should not be constantly wet - but it doesn't like to get completely dry during the growing season, either. Check the soil, when the top inch or two gets dry it's ready for water. Don't water strictly on a schedule - with plants where I can't get my finger into the soil, I judge by the weight of the pot. In the winter keep it somewhat drier, a little cooler is also good. This winter dormancy will encourage flower bud formation and also keep the plant from making weak, etiolated growth when there's less light available.