God is not visible ... Because there is no god
Not everything that exists is visible to the eye. As for prayer I believe some forget that prayer is for people. Most prayers are for blessings or forgiveness. People pray for their selves and for others Prayer has always been for the soul. I don't know what you mean with the flowers bit so instead I leave you with a quote.
“Faith is about doing. You are how you act, not just how you believe.”
― Mitch Albom
The whole "Faith" thing is based upon NOT seeing . . . Hence why faith is the biggest con-game the man made religion could come up with.
There is no other part of your life where you rely on faith . . . Only belief of that which is not.
If God is Omniscient as the Bible states that He is, then prayer would be futile at best.
God wants us to come to Him based on the free will He gave us.
If He were visible, you would not be able to freely choose to come to know him-.
In such a case, how could you longer think that He does not exist?---you would have no choice but to believe in Him.
I infer from one of your comments you think that anything that exists must leave evidence of its presence.
Some persons with severe "colorblindness" see only shades of gray.
I wonder what value their description of a rainbow is for someone who can distinguish many colors of the spectrum.
(That is the general problem I have when I hear an atheist list his / her problems with the appearance and qualities of the God I believe in---how could they possibly know what they are talking about? He contains all colors and they can see none of them---so they just stopped looking for Him.)
Your question can only be answered over time as you search for the answers from God Himself. You will also find out why you think that God "needs" prayer and flowers and whether or not He does.
I think that search will be for you a wonderful adventure, with knowledge and appreciation of things that you never imagined could exist.
Enjoy