He is expecting regular raises.  Not every month, maybe not even every year but he expects a raise and he has his own timetable for when you should give it to him. No matter how hard you try to keep to a fair schedule of raises, uncertainty about his expectations together with other random factors mean that at some point you are going to fall behind.

As time passes and no raise he is going to start slacking off.  Maybe just a little bit at first but it’s going to be noticeable.  Now from your perspective it just looks like he is not working as hard as when you first hired him.  You tell yourself stories about how gardeners start out by working hard to get your business and then slack off over time.  You might even consider that maybe he is slacking off because you aren’t giving him a raise but what are you going to do now?  You can’t possibly give him a raise and reward him for slacking off.  If anything your raise is going to come even later now.

And so he slacks off even more.  In fact he has been through this before so the very first slack-off was a big drop because he knew it was the beginning of the end. He’s gonna be fired pretty soon.