Long-day plants take advantage of photoreceptor proteins so they can flower when days are longest and nights are shortest. Photoperiodism is the reaction an organism has to the length of day/night. The longest day of the year in the northern hemisphere is June 21. This would make sense since plants start to flower in the spring (when the days start getting longer). Pictured are potatoes. Potato plants are long-day plants and grow best in the summertime (they are typically harvested in July).