Odds of Hanging On
Hanging on to a stable job with a stable florist is actually easier than getting the job in the first place; job security, on the other hand, is more of a concern. What this means is that hanging on should be no problem as long as there is something to hang on to. The florist business can be unstable for some, and if the shop owner can't balance their books, you'll soon find yourself booking interviews elsewhere.
Otherwise, the hard part, like in all horticultural industries—heck, like in almost any industry—is getting through the first few years with low pay. Anyone who can hang on through that should have no problem hanging on when the pay rate increases. The work is enjoyable enough to limit turnover.
There really aren't many reasons to leave this job, unless you go do something crazy like get married or start a family. There's also the chance that a rash of killer plants could take over your shop, but that's not nearly as likely as needing a better-paying job because you have another mouth to feed.