gezzy, I think you're referring to "Attack of the Killer Tomatoes", a 1977 film by John DeBello. This particular film is considered by some to be untopped for awfulness, though it spawned not only a cult following but a series of sequels: 1988's "Return of the Killer Tomatoes", 1990's "Killer Tomatoes Strike Back", and 1991's "Killer Tomatoes Eat France", all directed by DeBello. The sequels, deprived of their progenitors originality, pale by comparison, and are merely run-of-the-mill terrible movies. My pick for all-time worst movie, however, goes to Ed Woods' "Plan 9 From Outer Space, now nearly 50 years old and usurpassed.
Killer Tomatoes have their own website:
http://killertomatoes.com/
timber