Smith is being quite on the nose about what he means. This isn't some indirect allegory about religion, race or specific historic events, as you're suggesting with your question. Smith is explaining exactly what he says.
Before we get into what Smith means, you have to keep three things in mind:
Smith is built for a specific purpose, and that ...
The word tselina (no soft consonants as normal) means "celery" in Bulgarian. You don't need to look for metaphorical connections in other languages: given that we see this written on a greenhouse, it's pretty safe to assume that the only significance it has is to denote what's growing in there.