Think about it, if you were to have say 65 energy for pet games and for gardening (at level 50) how easy would it be do both? I rarely get below 20 energy when gardening, and I use the rest for pet games. With this tactic I have gotten two pets up to ancient, and myself up to a rank 7 (expert) gardener.
I agree its illogical. The two activities seem very unrelated. Perhaps instead of 'energy' they should have called it 'pixie dust' (or whatever) with the explanation you need this special magical dust to do certain activities.
Anyway masterplat is right. Having a second 'utility' wizard is handy. You can zip a second wizard up to level 15 really fast if you focus on main quest and shared-bank stuff to make it easier.
Another tip: If you go to your castle or dorm regularly, place your large garden(s) in a secondary residence so you don't keep activating the plant 'needs' triggers. Then visit your garden only when you will get optimal benifit out of area spells. If you take care of all plant needs before leaving nothing will die regardless of how long you are gone. Don't revisit unless you have sufficient energy to fully take care of your garden.