Some plants only flower at specific times depending on the photoperiod, or the amount of time that light is available during the day. These plants can be divided into two groups: the short-day plants and the long day plants. Long day plants flower when the days are, you guessed it, longer. These plants need more sunlight to flower, so they grow in late spring or early summer when the days are longer. Spinach is one example of a long day plant. Spinach will only flower if there is a minimum of 14 hours of sunlight a day.