The photograph was taken by Paul Jasa, of the University of Nebraska-Lincoln. The soybean field pictured lies near the UNL campus, along 84th Street. The green circles extending from the light poles show the effect of night light on the maturity of the soybeans near the poles.

By JEFF BURBRINK
Extension Educator Ag & Natural Resources, Purdue Extension LaGrange County

LAGRANGE — One field my dad farmed for years was along a busy road, a road lined with street lights. Every other year, when the field was planted to soybeans, the beans under those lights would stay green longer than the rest of the field. It was fascinating to me to see semicircles of green beans in fields that were otherwise beginning to turn yellow, ripen and lose their leaves. The corn was not affected like this.

Soybeans and the bean family are a great example of plants that have photo-period dependent characteristics. Scientists used to think that the length of day was the critical trigger to get some plants, like beans, to flower. After all, on the first day of summer, the amount of daylight we experience becomes less and less until the first day of winter.

It was not until the mid-twentieth century that plant researchers first came to the understanding that it is actually the length of uninterrupted darkness experienced by a plant — rather than the length of daylight — that is the most crucial to plant development.

For example, a short-day plant that requires more than 12 hours of uninterrupted darkness to initiate flower production will still produce a flower if it is shielded from light for some time during its 10-12 hours of daylight exposure; it will not, however, produce a flower if it is exposed to light for a period of time during the 12 plus hours of darkness it requires.

Beans are not the only plant that are photo-period sensitive. There are many other plants that depend on increasing darkness or increasing daylight to trigger a growth response.

Poinsettias, the Christmas favorite, must have 13-14 hours of complete darkness beginning in early October, for the plant to be ready for our holiday. What’s more, a single untimely blast of light during those weeks of controlled darkness can reset the internal clock and delay blooming.

Poinsettia growers are very strict with their light regiment, for just one 10-second mistake can ruin weeks of work. Padlocked doors, warning signs, and threats of unemployment to those who do not follow the rules are a part of the poinsettia grower’s routine.

Plants vary widely in their response to length of light or darkness. Some are neutral, some require shorter nights, some longer. Even maple trees can be affected by night lights. I recall a maple planted adjacent to a street light in Goshen. Each fall, the lower branches, within 3 feet or so of the lamp, remained green well after the leaves on the upper portions of the tree were going through the annual fall change.

Come to think of it, I think there are a lot of people that are affected by length of day too!