A golden shovel. :kusatchi:
But it just still doesn't, and never, works!
If you want to guarantee any tree to grow, then don't plant it randomly somewhere in your town.
There are certain places where trees will never grow in your town.
For example, Cedar saplings will not grow in the southern (bottom) half of your town and Coconuts will not grow in the northern (upper) half of your town.
Also there are certain routes that the various animal characters walk when they are out around the town and trees will not grow if you plant them on these routes.
A tree will grow if you plant it in the exact spot where another fully grown tree was standing.
So if you want to improve your chances of growing a money tree you will need:
1. A Golden Shovel - you must have one to grow a money tree
2. A Bag of Bells - the more bells the better - I have success with 60,000 bells - anything less in my town will not fruit
Then:
3. Choose an existing sapling, cedar sapling or fruit tree.
4. Shake it first to make sure there's no money bag, bees nest or to remove the fruit.
5. Cut it down with an axe
6. Dig out the stump with your golden shovel
7. Bury the bag of bells in the hole
8. Fill in the hole with your shovel
Wait three days for it to grow to full size. If you planted enough money you will see the money bags instead of fruit.
If you didn't plant enough money, you will need to try again with a higher amount of bells.
You do not need to water trees when you plant them. If they are going to grow, they will grow. If they are going to die, they will die. Watering them makes no difference.
It is not about taking perfect or good care once it is planted - that makes no difference - it's all about where in town you planted it (the position) as described above.