Engadget hit on this one, and it's technically possible to do it with a Windows PC, but considering that Mac users are greener than most folks, and lovers of both plants and cool tech DIY, we're posting it anyway. Botanicalls, which is a project to help your plants make phone calls, has posted a how-to guide on getting your plants to twitter when they need water, got water but not enough, or are even being overwatered.
The whole rig requires an Arduino board, and it's not exactly for Mom to do on her own (unless of course your Mom is an electrical engineer, which is entirely possible), because it also will walk you through making your own moisture sensor. Once you've got everything plugged in, the Arduino software will talk to the unit via USB and ethernet, and a terminal script will actually send the Tweets out. Unfortunately, despite a quick search on Twitter, I wasn't able to find any plants actually tweeting, but if you have one, let us know.