Surprisingly many countries allow import of cut flowers on a personal basis even if their agriculture requirement are otherwise harsh. For example the USA
https://help.cbp.gov/app/answers/det...-united-states
Carrying them onto an aircraft is no problem, other than other passengers crushing them. Do be careful of placing a bunch of flowers on their side if there is any water-retaining material on the end of the stems, even one that claims to be nonsplillable. In my experience they tend to leak if not upright.
You can, on a shorter flight, take a window seat and put flowers by your knee between your leg and the fuselage. They usually survive that.
I bring flowers from Amsterdam to the UK every so often. The Netherlands has more interesting varieties of tulip flowers for sale than the UK does.