The difference between a gardener and a landscape architect is one of size; the size of the land they work with, and the size of their salaries. Well, that and all the extra work. A senior landscape architect did probably once mow the lawn as part of their job, but at the peak of their career they'll be drawing up site plans and cost estimates, and then arranging contracts to deal with work as and where required.
A keen understanding of building materials, planning laws, and labor costs are an absolute must if you're considering a career jump into this role. That's only the foundation, though. You'll also have to have a bachelor's degree in the topic, and at least six years' on-the-job experience before you're likely to be considered for a management position - for which you'll obviously need good people skills, too.