Tanis doesn't burn the plants, Kes does.
She isn't angry about it because if you noticed, it gave her the equivalent of sexual release. Notice how she fell to the floor, sweaty and panting. Notice at the end of the ep. she tells Tuvok he powers bring out her dark side and she is frightened of the evil in herself because she is also thrilled by it. This is foreshadowing of how her abilities are dangerous.
I enjoy this ep. because we finally get to see a idea of Kes' full abilities while finally giving the character inner conflict. What I don't like is the ending with Kes saying she'll never explore her abilities further. I feel it writes the character into another dead end by ending any further exploration of this side too the character.
She says she will never explore her abilities further, yet she is excited at the opportunity to continue her studies with Tuvok. I was kind of hoping that she would eventually try to hone her abilities to help the crew. Her ability to enhance life could have been a saving grace for the ship's food supplies!