Would this be a morning or afternoon once, as a one off?
I suspect you mean a morning or afternoon every week, on a regular and long term basis?
Is your mother well off to an extent where she can very easily afford to pay the useless gardener this very high rate?
I think your time would be better spent recruiting a better gardener for her, if this is an ongoing thing.
As a one off you would be unreasonable to charge her for 3 or 4 hours of your time, but if she is trying to manipulate you into committing to drive for an hour to spend several hours gardening, while she continues to pay the gardener whose work she expects you to actually do, YANBU to say "No, I will not do this, but I will help you find a more satisfactory solution by helping you find a gardener who will do your garden properly".
I disagree with the trite old "she wiped your bum" argument . Its utter tosh. We all chose to have children and knew that doing so would involve bum wiping and other small child personal care
- you take that on willingly and unconditionally, without any wish to be "paid back" when you choose to bring a child into the world. Nobody "owes" their parents in a tit for tat way.
However becoming your parent's employee puts a very odd co-dependant twist on the relationship and doesn't sound healthy.
The correct way to handle this isn't to take on the manual task she can't do and ask to be paid, it is to help her more appropriately out source the work to a non related employee with whom she can have a normal client-contractor relationship OR to willingly take on the work for free if and only if you will not resent it (for example if you love and adore gardening and are looking for an excuse to be around your mum more). In your case the first option of helping her more appropriately out source.