I tend to kill plants, unintentionally, and so in April I bought a kill proof plant - a pincushion cactus. Today, I've just noticed the cactus is dying. A brown withering of the nodules is spreading from the base of the plant up. I keep the plant on my windowsill to get light and I don't water it very often. The weather here has been very unpredictable - hot and sunny one day, rainy and overcast the next, but it's quite humid here and the plant survived winter fine. What am I doing wrong? I love plants but if I kill even cacti I'll need to give up in disgrace.