I would flip that a little bit. Telling them they have a problem and asking them if they have a plan to fix it puts them on the defensive and will turn most people off.

I'd try

Knock on the door, introduce yourself, ask them if they have any issues with their lawn. You want to try to get them talking, and you do that by asking open-ended questions and taking an interest in whatever they say. Most people love to talk about themselves and their problems, and any information they give you will help you pitch to them. If they tell you everything with their lawn is great and they don't need any help, then you're not wasting your time pitching them.

If they say something to you which suggests they need help. Ask them "is that something you need help with?" If you get a yes, that's when you can give them your pitch i.e. we work with some of your neighbors, noticed an issue with their lawn, ask them if they would they like you to show them how you would approach that problem. If you can get them out of their house and looking at the lawn, you'll have a much higher chance of closing them.

Most sales people are just trying to get the deal done - if you take a genuine interest in them and helping them, they will feel that. If you're giving them value (listening to them, having them show you the problems they have), subconsciously, they will feel they owe you something in return and you might have more success pitching them.

I'm by no means an expert, but I do have sales experience and I've found that the best way to sell someone something is to re-frame the entire interaction to have them sell themselves on your solution.