I would probably first approach it like making an inventory cheat.
Somewhere in there is probably a little table for each screen or something saying where, what and possibly some other info on the "item" in question. You would find this and figure out a way to remove the weed entries from it. It might be easy (especially if you have an if this then that cheat command) or it might be slightly more annoying/something you are better sorting from a memory viewer.
Such a thing would also be the basis for a more permanent herbicide hack. You would find whatever does the generation algorithm (don't know whether it will be each time you enter the screen or just the first time you enter that day/week). When it is generating the random numbers for the weeds you would add on an additional if this then that so the weed command becomes say a rock or something more interesting (or nothing if that is necessary). Don't know if this game has a tornado/storm feature but you might also have to alter it for that.