I'm not sure at what level CO2 is toxic to plants, but you can be sure it'll be much more than it would to be toxic to humans. CO2 is toxic to humans at 10,000ppb. In other words, you should be fine with four jugs (you and your plants). Giving CO2 to outdoors plants would be difficult but unlike smokinmayne's post, outdoor plants most certainly need CO2, but it would be much more difficult to effectively deliver MORE than what nature already provides. If you're really want to try it, I'd stick a fermenter under every plant, you will probably run into issues with bugs and other animals though. If you find something that works, let us know.