GardenShed is a quick little addon I wrote that deletes gardening tools from your inventory if you spend more than 30 seconds or so outside Sunsong Ranch, The Halfhill Market, or Halfhill.
There's currently zero configuration available or required; just move out of the Sunsong Ranch area with gardening tools in your inventory and they'll be deleted. The addon reports status messages in your default chat window when you move into or out of the gardening area and when it performs the deletions.
Disclaimer: While there are no known issues, this is an alpha version. It may destroy your computer, murder your dog, shard your epics, or implode the universe. The probability of any of these events is very near to zero, however. Not tested on non-English clients, but should work. (Please post in the comments if it doesn't.)
50001.3 - 10/17/2012 - Fix support for non-English clients.
50001.2 - 10/16/2012 - Add The Lazy Turnip (inn at Halfhill) to the list of whitelisted subzones.
50001.1 - 10/16/2012 - Minor restructuring, and removed dependency on LibBabble-SubZone, drastically reducing file sizes. 99% less fat, 100% delicious flavor.