When Project Mayhem ramps up, Tyler starts recruiting "trainees" to live in his house. At the beginning of it all they clean up the house, etc., but specifically, one the tasks he has them devote a lot of time and effort to is making a garden. He could have just had his 'army' clean up the back of the house but instead he has them create and maintain a garden. And, it's also worth noting that after the garden is made, pretty much at all times there's someone working on / grooming the garden (or, at least, every time the garden is shown there's someone working in it).
What was the purpose/motivation behind having the garden? Given Tyler's philosophy of not owning things just for the sake of it and that a person isn't defined by what they own, having a garden seems fairly out of place to his overall cause.