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: 
After doing some testing, it seem to be quite random. Either random between all the plant (Survival endless) or random between a row (in the adventure).

What worked best was to play the adventure however and ever. In the level that correspond to row of the plant I am going, there is reasonable probability that I will have it after getting a few. Not sure, but improve the odd over playing survival endless.
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 :)