To me, it's like the old story (which I had always heard involved Winston Churchill, but probably is really attributable to somebody else!)
WC, talking to a beautiful woman at a social event: "Madam, would you come to my bed for the sum of a million dollars?"
She flutters her eyelashes and looks flattered, replying "Surely, it's not often a woman is prized in that manner!"
Then WC replies, "Would you come to my bed for ten dollars?"
The woman ceases to flirt and looks downright mean, and highly offended. She yanks herself away from WC and raises her hand like she might slap him silly, and retorts, "You offend me! How dare you! What do you take me for!? I am not a whore!"
WC, completely unperturbed, replies, "Madam, it's already been established that you are a whore. Now we're just haggling over the price."
In other words, stealing is stealing. It doesn't matter whether it's a flower from a garden center ("Oh, they'll never miss it, it's such a small thing, they'll never notice it's gone") or a gazillion dollars from a major corporation, or an audacious old-fashioned bank heist.
Or even out of your partner's wallet, bank account, or sock drawer.
Theft is theft. If it doesn't belong to you and you don't ask, you've stolen it.
That said, if you ask the manager of the flower shop/garden center, she or he will probably be more than happy that somebody wants to do the upkeep and maintenance for free, but you never know. They are, after all, in the flower business to make money.
Apologies in advance if my opinion or my post offends.
Whitewater