Question
I have 5 inch Cacti on which the needles towards the top half are turning black. It is just the tips of the needles. Other than that the needles are white. Do you know what the problem could be?

Answer
John,

Assuming it is a cactus, there are quite a few varieties that have black spines that turn gray as they age (the newest spines are at the the top of the plant). Now, it could be a Euphorbia (often mistaken for a cactus), if that is the case, the same rule can apply, but it also could mean that the plant is rotting. Cactus spines are like fingernails (dead cells), Euphorbia spines are partially living tissue and can rot. If it is possible, please send me a photo to [email protected] and I will try to identify for you.

Thanks.

Greg