A forcing shed was a building used to provide favourable conditions for the growth of rhubarb. Martin Chisom owned a forcing shed which he lit with candles to achieve the necessary low light.
A red metal bug took up residence in Chisom's shed and its influence allowed the rhubarb to grow rapidly and even constrict people, as it did Chisom. The bug initally miscommunicated its wishes, claiming the planet Earth, before correcting the statement to claim the shed - a demand the Third Doctor thought altogether more reasonable.
Chisom and the bug settled a trade agreement wherein he would provide the bug with manure in return for "the tenderest rhubarb in the galaxy, grown at a rate unheard of on Earth." (AUDIO: Lost in the Wakefield Triangle)