I think you've behaved really badly OP.
Writing an anonymous note to be petty and nasty is disgusting behaviour in itself.
Those people are doing you no harm, you know nothing about them and you have no idea if they have let the garden go through lack of ability, lack of finances, poor health or any number of other reasons.
Your friend was retired and had the time, money and ability to take care of the garden as she saw fit, but this is a young couple who might not be in the same position and should not have to be. It's their garden, not yours and not your friends anymore.
All you've done with your anonymous note is make someone else's day that little bit worse, just for the sake of being cruel. And perhaps not just their day, receiving an nasty, anonymous note will play on their minds for a long time, make them wonder who it was and suspect others around them.
If you make a habit of this it could come back to haunt you too, they could make a complaint about malicious communications. You might want to get some sort of professional help if you feel this is an acceptable way to treat people.