Generally it's better to just exclusively use the small ones. Everything else takes way longer and in the case of the Tough ones they get pegged with so much crap on the approach their extra health over the others is basically nulled. For my mutations I use garlic, lima beans, and celery (the biggest bonuses for each stat). The garlic never wilts, so you can pretty much plant that and leave it there.
"What happens when 2 rockets collide?"-LostProphet24 on TF2 "one becomes a pot of petunias, the other becomes a sperm whale"-scaredofbears
You need to to have the crop planted adjacent to the zombie when you dig up the zombie for the chance of mutation. Also, I noticed that crops that are touching on a corner is still considered adjacent and can cause the zombie to mutate.
Sometimes getting pelted by the thrown things isnt a bad thing. At higher levels when they're throwing things super fast, its good to have a tough zombie walking along at snails pace to take the damage off your little guys.
pinky0926 - CE's resident Scotsman."Only in America." http://i39.tinypic.com/9rnbex.jpg