How plants tell the seasons apart

Plants have a built-in calendar. This calendar can only distinguish three seasons — summer, winter and spring/fall.

The plant’s calendar is based upon the day length. Day length is determined by how long the sun is shining — or, more accurately, how long there is no sunlight. During the spring and fall, the day length is the same, so the plant has a hard time telling the difference between spring and fall.

The calendar is based upon two pigments in the plant; let’s call the two pigments light and dark.

When sunlight hits the dark pigment, it converts into the light pigment. During the night the light pigment converts back into dark pigment. During the summer the plant builds up a high concentration of the light pigment, while during the winter the plant builds up a high concentration of the dark pigment. These higher concentrations let the plant know it is winter or summer.

This internal calendar helps the plant in many ways. The most noticeable is the formation of flowers. We sometimes fool the plant into thinking it is another time of year by furnishing extra light or making it dark.

Poinsettias normally bloom in January and February. However, in fall, the poinsettia growers cover the poinsettia with tarps to make the days seem shorter causing the plants to bloom earlier.

Some plants tell the difference between spring and fall by using cold winter temperatures. If the right day length and temperatures follow a long cold wet period, it must be spring. The flower buds for spring blooming plants form during the summer this includes fruit trees. Inside the flower buds are special chemicals called inhibitors that prevent the flower buds from blooming in the fall.

During the winter, the cold temperatures break down the inhibitors and when the light and temperatures are right the plants bloom in spring. However, some of the flower buds miss the spring blooming period and when fall rolls around the flower buds react as if it is spring and they bloom.

Fall re-blooming of plants removes energy from the plant that would be used the next spring, thus reducing the quality of the spring bloom. It is normally recommended to remove the fall flowers, in order to save that energy for the next spring. The day length and cold temperatures also affect spring blooming bulbs.