Chapter 19. Chapter 19 Graphic Content

The dark purple region represents the months during which long-day plants typically flower: late March until late May. The height of the dark purple region represents the length of the night during that time period. Between late March and late May the length of night decreases, meaning that the days are getting longer.

The light purple region of the graph indicates the length of the night across all the months of the year.The yellow area represents the length of the day across all the months of the year. For this reason, when the amount of purple gets smaller, the amount of yellow at that point is greater; if the nights get shorter, the days must get longer.

Each graph includes a large amount of information. (1) The length of the day and of the night across the entire calendar year (and the pattern with which it changes). (2) The time of the year during which long-day plants flower (shown by the dark purple region in the top graph). (3) The time of the year during which short-day plants flower (shown by the dark purple region in the lower graph). (4) The rate and direction of change during the periods in which long-day plants flower and during which short-day plants flower.

It appears that two factors are responsible for long-day plants flowering: (1) that the length of the night is decreasing, and (2) that the length of the night falls within a particular range. Similarly, it appears that two factors are responsible for short-day plants flowering: (1) that the length of the night is increasing, and (2) that the length of the night falls within a particular range. These conclusions could be undermined if it turns out that some other environmental variable also has a similar pattern of change. If, for example, the amount of rain/precipitation varies in the same pattern throughout the year as the length of night, it may be the case that it is the amount of rainfall and the rate/direction of change in the amount of rainfall that triggers flowering in these types of plants. Or perhaps it is the temperature and the rate and direction of change in temperature during the year that is actually triggering flowering.

The reversed pattern of day/night length in the southern hemisphere relative to the northern hemisphere should alter the pattern of the light purple and the yellow bands in these graphs. In the southern hemisphere, the low point of the purple (corresponding to the shortest nights) would occur during late December, and the high point would occur during late June. This would lead to the timing of flowering in long-day and short-day plants to be six months later/earlier for each.

In order for landscape architects or gardeners to design gardens so that flowers are blooming at desired times during the year, they must understand the flowering patterns of various species and plant them accordingly. They may want to change their flower species throughout the year to showcase those that are best at different times, or they may want to plant both long-day and short-day species together in a garden.