Question: In Plants vs Zombies, are there strategies that make it more probable to have certain plants (for your Zen Garden)?

How does the game decide which plant is given?

Answer: 
I believe the game hints mention that certain types of plants only appear in certain types of boards or levels.

For aquatic plants, you need to be playing an aquatic level.

For plants for your fungus garden, you need to be playing a night level.
as said before water plants are in pool levels and shrooms are in night levels.you get normal plants on your lawn.in i zombie levels(in the puzzle option) when you eat the brains it might give you a plant but it gives all kinds of plants also replay whack a zombie and bust up as many graves you can and at the end other than collecting the money bag destroy all graves,its also a good way to money too :)