In Genesis 21:14, this signifies the state of wandering in the beginning, of people who have been introduced into the doctrinals of faith. (Arcana Coelestia 2679)

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.

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