Tanis didn't burn the plants, Kes did. But Kes was under Tanis's influence when she did that. Otherwise she would never have burned the plants she loved.
When Tanis was gone, Kes's powers were back to normal. She could no longer boil tea in a cup or be a threat to anyone. In fact, we never saw anything of any dangerous powers from her (unless you don't count that utterly ridiculous episode "The Gift" which was written as a lousy excuse to get rid of the character).
And yes, she realized the danger and told Tuvok that she didn't want to explore her abilities further. A very wise decision which proved that she had learned from the experience.