I have heard a lot of folklore concerning a lot of things; I think our ancestors try to make sense of a lot of stuff by attributing the supernatural to it. But as a scientist, I think the vegetation deity is just when all condition (water, oxygen, warmth) necessary for the germination of seedlings is in place; a seed will germinate. If the ground is fertile enough, it will do well, and we would have a bountiful harvest. But for our forefathers with little knowledge of agriculture or how things grow, it is the work of a deity. It works for them then, but now we know better, we can save the drink used for libation for that hangout :)