Clover will flower if exposed to 8 hours of dark and 16 hours of light. it will not flower if exposed to 16 hours of dark and 8 hours of light. however, if the 16 hours of dark is interrupted with a brief period of light the clover will flower.
which triggers clover to flower?

short dark periods

Explanation:

Based on the information provided within the question it can be said that the Clover will flower with short dark periods. This is completely following the instructions that have been provided since short dark periods allows the Clover to receive enough sunlight in order for it to flower. As mentioned even long dark periods may be acceptable if they are interrupted by brief periods of light which causes the dark periods to become various short dark periods.