If you're going to market your skills to the general public, then most people will want to know what qualifications you have - but letters after your name or qualifications on a CV will do that.
A professional 'title' will only show the level of the least skilled unless one has 'ranks', e.g. Horticulturalist 1st Class, etc.
My degrees are in fine art, I am an artist, that doesn't preclude anyone else from producing a work of art and calling themselves an artist. However if I apply for a job or tender for a commission then I will present my qualifications and experience in my cv.
There are a few professions where it is illegal to falsely claim to be one (does that make sense?) and one of those is Social Worker - for obvious reasons. A lot of damage could be done. However, I think that for the majority of trades to limit employment to those who have a qualification is overly prescriptive.
My other thought is that Horticulturalist is such an all-encompassing term - as you say it can include landscapers and tree surgeons. When I employed a tree-surgeon recently I checked out the relevant qualifications online and invited estimates from suitably qualified people, and then checked their references before engaging someone to do the work. If he'd had the word Horticulturalist after his name in his advert I would still have done all of that so what would it change?