The flowering in plants is in response to duration period of light or relative length of light. The long day plants need a longer duration of light whereas the short day plants need short duration of light. 

If the long day plant and the short day plants gets the required duration of the light necessary for their flowering they may produce flower simultaneosly in a given plave. If the long day plants is planted before the short day one such that the long day plants gets a longer duration of light , both the short day plant and a long day plant can produce flower simultaneously in a given place