Plants, in general, represent knowledge, ideas, and intellectual concepts -- things of the mind. Plant the best and most beautiful of those in one place, give them plenty of water (truth from the Lord about day-to-day things), plenty of sunlight (the Lord's wisdom) and warmth (the Lord's love), tend them to keep the weeds (false ideas) out, and you get a garden. So it makes sense that a garden in the Bible represents the intellect, where with proper care we can nurture the best and most useful ideas and ways of thinking.

(References: Arcana Coelestia 98, 100, 1588; Heaven and Hell 111)