Did Walt Poison Brock?

With this in mind, Jesse starts to feel bad being with Andrea, and decides to end the relationship and stop seeing her and Brock ("Hazard Pay"). Jesse later finds out that Saul had Huell steal the ricin cigarette from his pocket, and that Walt was indeed responsible for Brock's poisoning.

How did Walt give the poison to Brock?

It's an evil trick that works. The truth is that Walt did poison Brock — just not with ricin. Instead, he used a Lily of the Valley plant which was growing in his backyard. The effects of ingesting the flower mimicked the ricin that Jesse assumed Brock had eaten.

How does Jesse find out that Walt poisoned Brock?

Jesse, realizing these pieces fit believes him and helps Walt get the information he needs to finish Gus off. However at the end of Face Off it is revealed that Brock was actually poisoned by a flower called Lily of the Valley, later revealed to be in Walt's backyard, thus implicating Walt in the poisoning.