Etiolation?

[HP22B]

I have this Opuntia subulata f. monstrose outdoors under 30% shade cloth but I'm not sure if it is getting enough light. The Opuntia looks to be narrowing on some branches, but I can't tell if this is from low light or it getting ready to flower because it looks as if buds are forming on some branches (I've never had one of these before). I also have a young Myrtillocactus geometrizans that is light green at its apical tip rather than blue green like the rest of the cactus, I'm not sure if this is the beginning of stretching or not.

The sunlight here is very strong and the plants did not seem happy in direct light (Opuntia branches drooped), so I figured a little bit of shade cloth would help but now it seems like it is stretching.

[hegar]

Your suspicion is correct. Your plant is showing first signs of etiolation. I do have an Opuntia subulata cactus in my backyard. It is planted against the off-white stucco of the south wall of my house. Because of pecan trees nearby the cactus receives mostly morning to midday light. So it is protected from the sun during the hottest part of the day. Our solar radiation is also very intense, especially the UV rays, because I live at 4,000 feet above sea level. The air is oftentimes rather dry here in the desert and the temperatures are mostly in the 90 degree range, with a good number exceeding 100 degrees F.
Perhaps because you do have the monstrose variety, the plant is not as tough as the full-sized plant. You should reduce the shade somewhat. I do not know, if there are shade cloths available that provide a lighter shade, say at the 20% or 10% level.
I am attaching two photos I just took of my O. subulata plants. As you can see, the top growth is as large in diameter as the part below.

Harald

[HP22B]

Thanks for your quick and detailed response! I suppose all I can do is move them to a brighter but not AS bright area as direct sunlight. I can probably find a good spot against the side of the house similar to yours. Hopefully the small narrow bit won't make the plant too strange looking as it matures!

[hegar]

Well, bright light and also the necessary length of it is all that is required for most cacti. There are actually some "desert cacti", that do not do well in the harsh, bright sunlight. I found, that most of my plants - even the natives - do better with filtered light. So it is always a good idea to have some trees around that do not have too dense a canopy. Mesquites, Locusts, etc. and also some shrubs will still allow enough light to penetrate their foliage and your plants will thank you for it. I have actually killed nice, greenhouse grown plants in a day or two, after having placed them in our sunlight. If you go out in nature and look for cacti, you will find, that many of them - and in most cases the best looking specimens - will be growing in creosote bushes or near a big shade-providing rock. Of course, there also would be wild flowers around and other smaller shrubs that would provide a little shade. Just for the germination to occur, the seed has to be in a favorable location.

Harald

[greenknight]

Perhaps give them some morning sun, shade them only from the hot afternoon sun.

[iann]

O. subulata can take all the sun most people have to offer, but isn't fond of very high temperatures. Afternoon shade in hot areas, full sun in cool areas. The monstrose form is slightly more touchy but is still unlikely to be damaged by UV alone. Adapt plants from shadier areas slowly, although it shouldn't be a problem this late in the year.