You are right at giving the dog an appropriate toy to chew on when he goes for the plants. Holding a dog down however can cause problems, so I would stop doing that. If he bites or nips at anyones hands or clothing, do the same with the plants and give him a better option like a toy. Repeat over and over and over, it will go away soon. Your pup is at the age where the puppy nipping thing is just starting to go away, so really just stay consistent and it will stop soon.
Pulling on the leash is a whole different story. Basically don't let him pull you. If there is tension, stop walking and back up a few steps or simply change direction while calling the pup back closer in to you. Only when the leash is loose can forward movement towards your destination be made. Try bringing treats along for faster results and a more motivated pup too
When he is older you may have to try out some other devices. I started off with a martingale when my guy was 4 months and when he turned 6.5 months he switched to a prong. Get into training classes to learn more about getting your pup to walk nice on a leash as well as other behaviors!