I'd call that person an outdoorsman

You have listed multiple traits of the person and some of them are contrasting. There is no particular word that describes all the qualities in a person at the same time: enjoys to observe nature, does not like to go out, He does like gardening and keeps an impressive garden, and favoritism of flowers etc.,

Out of these traits, I'll address the trait which you are most interested about:

1. favoritism of flowers:

The word that can be found in standard dictionaries like Oxford online dictionary is:

anthomania

Excessive or passionate enthusiasm for flowers.

So, the person can be called anthomaniac.

Internet slang for a person fond of flower is anthophile. According to Oxford, anthophile actually means

An insect which is typically found on flowers, or which feeds from flowers.