Is the Zen Garden a good investment?

If you are actively playing the game, there's no faster way of getting gold than harvesting it through playing the last stand mini game (and you can still go semi afk on that minigame).

The only reason why you would want a good zen garden is to be able to harvest the coins while you are completely afk. Once you have fully grown your plants and made them happy, they will randomly drop coins over time and your snail can then collect them (give him a chocolate to make him fast for a while to increase the rate).

Here is a youtube video of how to do so:

I'd say the Zen Garden is a good investment. If you give Stinky the snail a piece of chocolate, with a reasonably full Zen Garden you can easily gain $10,000-$20,000 (or $1,000-$2,000 without chocolate) without doing anything.

On the PC version you can just leave the app running in the background. On the iPhone version you don't even need to be running the game, just come back an hour later and it will say "Stinky picked up N coins before falling asleep".

Once you buy the gramophone you get infinite use so you can get your $15,000 back pretty quick. Bug spray is also fairly cheap compare to how much money you'll get back.

You can ignore the night garden for now - the benefit is small since you can only store 8 plants there.

The night garden is worth it if and only if you already have several night seedlings. Each fungi seedling you raise to full growth gives you two gems ($2000 coin). And every happy (glowing) fungus plant you can sell to crazy Dave for $10,000. You also get smaller bonuses as the plant grows worth $10-200 each time.

You can get back your investment with only three seedlings if you are willing to grow them to full. Growing a plant to full at most costs $350 (price of a single bag of soil and bugspray), which is easily offset by the smaller bonuses or sitting and farming for gold.

Ex in the past five minutes I've earned about $1500 from my four happy fungi by letting the game run in the background and pausing to collect coins while I type. The plants give off the most gold right after you bugspray/gramphone them.

Also, the gramphone is expensive to buy, but you get unlimited uses.