The parsley garden represents a world where they are at home, at peace, and can be in control of their lives; they are their own masters within the realm of the garden. Al is plunged into the world of commerce and money when he takes the hammer; a world where he most definitely is not in control. It is upsetting, and Al spends the rest of the story aligning his perception of the world as a place where if someone needs something, it being there is the right way of things-also represented by the garden-with the real world where if you want something, you have to pay for it, and pay something that it might not be worth. He has to adjust to a world where he doesn't make the rules, and where he can be controlled and mocked. His mother has created a world, in her garden, where things are simpler, more peaceful, and fulfilling because it is theirs and they make the rules. Outside that garden though, the world is different; the rules are made by the store owners, and everything is out of Al's control. He has no say in any matter; this is one reason he rejects the job offer at the end. At least that he can control. They can make him pay for the hammer, but they sure can't make him submit his life to them, even if it does pay well. The parsley garden is a place where he and his mother are in control, and that is comfortable for him.
My answer is a lot simpler.
the Parsley Garden is:
1. a place to invite friends over (e.g. Turkish coffee, cheese)
2. a place with calming effect
3. a sanctuary for Al and his mother from reality