I will add in my two cents. The questions Sean listed should obviously be asked in every interview. I have sat on hiring boards in the past and interviewed probably over a hundred potential hires and here is my insight:
1. If they can't look you in the eyes when talking to you, something is wrong. I won't elaborate, but this is generally true. Could be a personality thing, but I look at it as a negative.
2. Understanding the interview in this case is for Lawn care, if they dress like a slug for the interview, their attention to detail traits are probably not that good and anyone coming to an interview for a job, regardless of what the job is for, should take special care in their appearance.
3. Look at their vehicle if possible, inside and out. Are there stale burgers from 2 weeks ago inside? Does it appear they live in the vehicle? Don't care what people say, this says something about the person. I said this a while back in a previous post, but an old cop buddy told me that his father, who managed a multi-million dollar company, would send someone out to look at an interviewee's vehicle while he was being interviewed and if it was a mess, that person would not be hired.
4. Ask them if they have ever been arrested. This person will have access to homes and businesses on a regular basis. How detrimental would it be to your business for them to commit a theft? I would run a criminal history as well.
5. Ask them who is their favorite person and why? This may reveal something positive or negative.
6. Ask them what accomplishment during their life are they most proud of and why.
There are many more things you could do regarding a potential hire. I realize this is lawn care, but when you deal with the public, if you are not professional, you will pay for it in one way or another, sooner or later.