Darren:
I killed 57 groundhogs last year; I kill 70-80 most years.
For hunting over a garden and around buildings, I typically use my .22 Rimfire (RF).
The ammunition you want is either the .22 CCI Stinger HP, 32 gr. bullet at 1650FPS or the Aguila .22 LR Super Maximum 30 gr. HP at 1750 FPS. Aim right for the ear (or any where else on the head) and the game is over. Both work well in any .22 RF, although sometimes the first Stinger from the clip can be persnickety to feed (the Stinger case is a smidgeon longer than the typical 22 LR case)
Both the Stinger and Aguila shoot considerably flatter than the typical .22 RF High Velocity so you will likely have to re-sight your scope if it was sighted in shooting .22 Long Rifle Standard or High Velocity.
If Mr. Groundhog shows at 4 PM then it will likely show the next day at 4 PM, so ask your sister to keep her watch handy. [You want to be there at 3:30 and set-up before he comes out] And, they will start to show when the first crops are an inch up after planting...and can chew off the whole row in one feeding.
If a groundhog is eating the garden they will do so all year until the garden is ALL gone, so go get em'! And, there will likely be more than one...so be vigilant after the first kill.