Gardening is a sort of housing end game. You need to be on top of it at all times.
Servers aren't actually down for 100% of the time they're offline for maintenance so time will pass for gardens. I doubt SE would be able to suspend the timers over servers merely being offline.
Any time a long maintenance is scheduled, it's best just to stick them in storage if you won't be able to tend just before maintenance begins or logging back in within a couple of hours of servers coming back online.
