Refers to a relationship between a Western man and his Asian girlfriend. The man will usually display one (if not all) of the following characteristics: White, old (often much older than the girl), fat, bald, often generally unattractive.
In most cases the woman does not speak English fluently and can be seen either wrapped around the man in public, smiling at him emptily, or staring off into space while the man hangs out with his English-speaking friends. She will likely be scantily clad and display hooker-ish tendencies although she may or may not be a prostitute.
Lemon trees are most frequently found in Southeast Asia, particularly Bangkok Thailand.
The phrase originated from the need for an innocuous term to acknowledge this phenomenon without the subjects knowing you are talking about them and/or judging them mercilessly.
(walking past the couple) "Did you see that lemon tree?"
(standing next to the couple) "The lemon trees are in full bloom this time of year."
(in an area with many lemon trees) "I didn't know there were lemon tree groves here."