Question
Hi there, im unsure of the type of cactus I have but it is
approximatley 7ft tall, has long branches that grow upwards
from a center trunk. At the top of each of the branches grow
slender leaves. Its an indoor plant and I ahve had it for well over
10 years. This year the top pf these branches ahve turned
brown, as though it is dying. Im unsure if its lack of water or
over watering.  

Answer
Dear Rowena,

What you have is a Euphorbia, not a cactus. It is an African version of a cactus. Anyway it sounds like under watering to me. Usually, plants that die from the top down either have root damage or are under watered. The trick to watering cactus or succulents (Euphorbia) is to water well, then let the soil dry out between waterings.

Some people say to only give a cactus a teaspoon of water once a month, that is incorrect. The only other explanation is that it was frost bite, however, if it is an indoor plant, that is not possible. Hope this helps.

Sincerely,

Greg