The Sarah Jane Adventures - Season 3 Episode 12 -

Rani and Clyde discover rakweed in the school science laboratory. However,they see that the sound of the school bell destroys it so Sarah Jane gets Mr. Smith to re-programme all the electrical equipment around the city to the same frequency. This not only restores Luke but kills any rakweed in the area. Unfortunately some of it has been eaten by Leaf and Tree, who, when they come to Sarah Jane's attic, explode as a result.