What period had dinosaurs but no plants?

Plants evolved before dinosaurs, and dinosaurs depended on plants for food. The closest there came to "no plants" during the time of the dinosaurs was the mass extinction at the end, when plants died but left seeds behind that germinated when the dust thrown into the atmosphere by an asteroid impact settled and sunlight reached the surface. During that period of months or a few years without live plants, dinosaurs starved and died out.