It used to be you had to find a golden glowing spot on the ground. Then you buried the shovel in it and the next day (or few days don't remember) a tree grew with a golden shovel. I think you could also grow money into a tree as well.
You can grow money anywhere a weed or tree formerly was, since that spot "grows" things. Try it with a thousand bells, but if you follow my fruit market idea, you'll be able to reap in 300,000 bells from one tree!