If you can, try to see if things can be worked out between both you AND her husband. Perhaps it is just a bad case of miscommunication.
However, if you are sure that he is not competent enough to perform what you need....Then I would just gently let your nanny know that you have decided to go w/another person.
And leave it at that. No big explanations needed, your words should speak volumes to her.
If she does insist on knowing why you won't be using her hubby, simply respond that you just found someone else better suited to your landscaping needs.