Why are some people regarded as horticulturalists and others as gardeners? The smart explanation is that the former is merely a gardener with a panel van. Cute, but . . .
Go back far enough and it is suggested that gardening is the oldest occupation. (Occupation, please note; a pair of professions have been claimed to be the most ancient . . .). As with a surprising amount of the English language, horticulture finds its roots in Latin, the ancient Romans knowing more than a thing or two about the subject. Hortus then is a garden coupled with cultura or cultivation. So here we have, by extension, explanation for both horticulturalist and gardener--and are no further ahead.