Question
Hello,
   I am writing concerning a cactus that I am worried about because it has started to turn black at the top. My husband is very fond of this cactus and so I have searched the Internet but I haven't found any information as to the cause or solution to this problem. So I turn to you, an "expert", in hopes that you can advise me as to what can be done in effort to correct the problem and save this cactus.  The other issue I am curious about is that it doesn't seem to be growing very straight and some areas are "skinnier" along the length, is this normal? I don't know anything about cacti so I am including some links to  pictures that may be of help to you. They show close-ups of the problem, the potting arrangement, as well as, an idea of the location and also a full length shot so you can identify the type of cactus.

http://image-hosting.hostwire.com/images/monkegrl/cactus2.jpg

http://image-hosting.hostwire.com/images/monkegrl/cactus1.jpg

http://image-hosting.hostwire.com/images/monkegrl/cactus4.jpg

http://image-hosting.hostwire.com/images/monkegrl/cactus3.jpg

Thank you for your time,
                     Leah

Answer
Dear Leah,

Great pictures, they always help. The cactus you are referring too is actually a Euphorbia (an African succulent). The dead part at the top is usually caused by one of two things, either sun burn or frost. It will sprout arms just below the damaged area as long as there is no rot involved.

From your pics, I notice it is the closest one to the window and there are leaves on the ground outside, which might indicate that you may have had some frost. Where the plants outside? The other plants in the pot are both South American cacti, which are much more tolerant of frost and may not have been damaged if frost is the culprit.

The plants shape has to do with a lack of light however. When a cactus is deprived of intense light, they become thin and lighter in color. Euphorbias are less affected than a cactus would be, which explains why your cacti have been more profoundly affected. Cacti are generally tougher than other succulents when it comes to sunlight, heat, cold, and drought. However, when they do not get those extremes, they start to grow funny.

Your plant should be ok, just be aware of the environment around them. You seem to be doing fine as far as watering goes. Keep them in as bright a light as possible without putting them in direct sunlight. If they were not raised in direct sunlight, they will all sunburn. Hope this helps!

Sincerely,

Greg