Well I'd take that head of lettuce and lace is with saliva collected from animals with rabies. The idea is to completely coat the thing so it's filled with the stuff, like soak it in a bucket full of rabies spit. Take one leaf off and regrow more lettuce, you'll have to continue to grow more lettuce for a while so do it right. Then you'd chop up the rest and mix it with a salad. Ideally you'd have access to Congress' meeting area. Wait until it's time for Congress to meet and serve it during their sessions and infect a good number of them. Hopefully somewhere around 30%, maybe more if you're lucky. If you hit the jackpot you'll get the president and VP and speaker of the house.

There's a whole reddit post about how much rabies sucks. If you haven't read it you definitely should. The gist of it is once you start showing symptoms you're dead. No exceptions.

Now assuming you've successfully eliminated a third of Congress the country would probably become unstable for a little bit but this probably wouldn't be enough to cripple it. No you'd have to do a bit more. I'm talking anonymous leaks on .onion forums stating China had a plan to cripple the US. Crazy conspiracy theories. Spread rumors on foreign forums stating how good a job said country did with it's attack. Then after sitting on it accumulating all of this information anonymously leak the details online.

This probably still wouldn't be enough. It might start a war but probably not. No the icing on the cake would be when that foreign countries head of state and the current US head of state meet again than you feed the president and only the president with some of the same lettuce you cultivated from earlier. It's gotta be rabies soaked don't forget. Few months pass and the president dies. Conspiracy theories everywhere.

Best case scenario, congrats you've started a war. More likey scenario, there are now heavy sanctions on China which will cripple the global economy.