It depends on whether or not you want an office manager or an overall business assistant.
An office manager manages the office - answers the phone, takes messages, keeps a schedule for the owner, opens and sorts the mail, data entry, billing, maybe even handles payroll, or at the very least reports payroll numbers each week to the payroll service being used.
A business assistant is more involved. That person could handle all of the tasks listed above and then some....such as:
preparing estimates/proposals, paying bills, communicating with clients, communicating with vendors, scheduling, routing, dealing with employee issues, customer complaints, assist with marketing and advertising, assist in the budget process, etc...
Obviously the more you expect, the more qualified the person will need to be - not necessarily with previous landscape business experience, but certainly being an integral part of running a business and you can obviously expect to pay this person more as well.
Whether you want this to be the case or not, their personality and demeanor will matter as much as anything, because if you are looking for a business assistant/manager, you will be spending a lot of time with this person and not always seeing eye to eye.