Egg Plants are the only things that can be planted in Dark patches in Ur. It's different on the homestreets but in Ur, dark patches only support egg plants.
It won't work where other trees are not supposed to grow; but the dialog just says you need to be near a tree, which isn't all that helpful when you are in front of one. Something for us to look at. :)
Edit: Ha, I had this open in a browser for awhile but Ayasta beat me to it. :)
Okay, then I'm puzzled. Are Egg Plants now able to grow in the light areas? I've found several in Groddle above ground. I always thought they only grew in the firebogs or in caves...
ETA: I probably missed an update somewhere, or I'm unobservant...