It's one thing to say goodbye to your clients the last week on the job. If they ask why you just say that you are staring your own landscaping business.
The ball is in their court to ask if you would take them on.
Some may say there is no difference in telling the customers you are leaving the XYZ and are are starting your own business. Then you are leaving XYZ, starting your own business and you want their business. This is a fine line. Though as with all lines it is best to not cross a line.
I will end with once you have your business established would you want one of your employees to directly take away as many of your customers as he can?