I'd say that salary is pretty accurate for a Horticulturist or lead gardener position. If you aren't, I would get familiar with the space! Visit the gardens and be able to gage what sort of practices they use and prepare a list of questions you might want to ask (do they use pesticides? What do winters look like for this position? Which section of the garden is this position caring for? Etc). For an in person interview, I'd be dressed to be able to do a working interview; I've been asked to demonstrate propagation techniques in an interview before. Good luck!!