While I knew that cool nights weren’t good for sweet peppers, I didn’t realize how early those cool nights were able to influence pepper size and development
Turns out that a cool night when the blossoms are forming or about to open (yes, that early in the development) causes the ovary to swell up and disfigure the pepper. Some never develop properly at all
The night-time temperature of deformation was around 12C (56F) so you really do want warm nights or great protection for this crop.
The use of frost-fabric right through the first flowering stage would seem to be the trick here, particularly in a cold season.
From HortScience 2011;46 396-401