After completing Adventure Mode, this will become purchasable for $10,000. To grow the tree one foot requires one $2,500 bag of fertilizer specific to the Tree of Wisdom so I hope you have plenty of money laying around. If you don't here's my advise, though it's not entirely quick like some endless survival guides, I found I had plenty of money before even unlocking that.
While working to unlock the "Alive and Planting" trophy, you must clear all 10 survival levels, in each level you will get plenty of zen garden plants; WATER THEM. Water and fertilize them and fill in your garden. as soon as they give you diamonds for being grown fully, sell them. Normal day plants give you $8,000 for being sold at full size, even after giving $2,000 for growing it all the way. Mushrooms fully grown give $10,000 when sold.
Basically don't hoard your plants, sell 'em for quick cash, you can go from 17k to 200+k quite easily when you sell an entire screen of plants, just be patient, then buy all the fertilizer you want!