you know you're professional if you go to work in someone else's garden and get paid for it. It's nothing to do with how good or experienced you are which I suspect is what you're asking
I'm an amateur but so many people need help in the garden. I volunteered for several projects at one school I could easily have set up a business by the number of times I was asked if I did gardens! A lot of professional gardeners are just glorified maintenance people if you can't use a power tool on it then they're not interested. Fortunately I live in York(Askham Bryan college) so there are quite a few garden services with properly qualified people running them.
Try to imagine such a situation... you speak with a person about some staff that is connected with a garden. But you don't know if he a professional gardener or an amateur. If he a professional one, what kind of words or terms will he uses during the conversation?
Many of the people on here could win mastermind with their knowledge of gardening but a professional is paid for their services so if the person you speakto asks for money that’s a clue!
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In the sticks near Peterborough
In the sticks near Peterborough
https://forum.gardenersworld.com/discussion/996256/is-it-just-a-hobby-or-a-professional-interest#latest
I live in west central Scotland - not where that photo is...