If they get rude with you for whatever reason, you can reply "I'm surprised you think customers would return after being treated like this" so they actually have a chance to reflect on why customers hate the the experience so much.
You don't owe them your money. But you don't want them to outright shut down and leave the place vacant for the next year or so while there is so little interest in starting new storefront businesses.
In my old 'hood there was a florist that we really liked. They were clearly really proud of what they stocked, and treated their customers in a way that made them glad to return. Especially when they recognize you from previous visits. They understood the value of not making big-box shopping more palatable.
So No. I wouldn't buy from a business that was rude to me. Sorry.
Even more intimidating are timber/hardware suppliers. There's a small local firm I use for timber purely because the guy is so helpful.
Rude people are usually irresponsible. That's not a good trait but it can be acceptable. I'd buy flowers from someone who is rude, but I've turned away accountants for this.
But if you really sincerely want to explore this, then you should at least consider the other scenario: that you go back and buy flowers, and they're taken aback. They know they were rude to you, and they also know that you came back and gave them a second chance anyways. Theoretically, that could also be a humbling experience for them that also teaches them a lesson...
I just think about that old saying that everyone you meet is in the middle of fighting a battle. (Off the top of my head: someone they know may have just died. Or they may know that their business is doomed, and this is their last week before they have to pack up...) Er, this is assuming that the person selling the flowers is the same person who owns the business.
Another option is sort of the "anger management" technique. You go back later, at a nice neutral time when you're in a good place and hopefully they are too, and you try to have a friendly, non-confrontational conversation about it. "Hey, I wanted to talk to you. Not jumping on you, but I was in here the other day and you seemed really upset with me. Maybe we could talk about it a little...?"
But I guess the real truth is, this situation will probably take care of itself. I just remember two different people at a company where I worked who started being snippy to people out of the blue -- and it turns out they were in the midst of leaving the company. People tend to be rude when they're mentally already halfway out the door (so they know that it doesn't matter)...
Not too long ago, people could encounter a grumpy store clerk and forget about it by the time they got home. Sometimes they'd even turn it into a joke [1]. It was a simpler, less angry time.
I'd find another florist and forget about him.
(Don't find excuses to businesses that treat you badly)
Slow business is the time a store wants to go the extra mile to sell, to build good will and a reputation, and to build long term relations.
Again, do not try to find them excuses, you do not owe them anything. If anything, they've shown you their true colours: if they treated you badly when they arguably needed you most then how will they treat you when business is good and they don't really need you? Move on.
That florist doesn't owe you a smile or pleasantries. He has as much of a right to his emotions as you do.