How does this sound? (assuming of course that what I have suggested, is true!)
Dear ***gardener,
As you know, I have always appreciated the good work you have done for us in keeping our garden in order.
However, due to personal circumstances, I regret that I can no longer retain your services, and therefore I am giving you notice that we will not require a gardener after (date).
If you should ever require a reference for future gardening jobs, please do get in touch with me.
Many thanks for all your help over the past *** years.