Changing day length affects plants

Published 5:34 pm Saturday, September 22, 2018

By Terry Rector

Now that we are officially into fall, tradition okays us talking about days growing shorter.  Granted, the daily length of sunlight has been decreasing since June 21, but it’s hard to think that way when the heat keeps on right into the night because we like to think of shorter days and cooler weather together.

But changing day length does change some things in the plants we grow, both ornamentals and the eating kinds.

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Day length plays a role in numerous plant growth processes. Plant activity due to ongoing changes in daylight and darkness is referred to as photoperiod response.   In some species, photoperiod affects the initiation of blooming.

In some it does not. Plants are classified as long day, short day or day neutral in regards to their beginning of blooming.

Each long and short day species and varieties within those species has a critical day length. Long day types will not bloom until day length exceeds the critical requirement. Some of the plants we grow during the shortest days of fall and winter are long day in bloom dependency. Mustard greens, turnips, spinach and lettuce are all known to bolt by sending up stalks that bloom and produce seeds in late winter.

At some point since the days began getting longer after Dec. 21, day length for each variety gets long enough to trigger blooming, i.e. bolting.

Short day bloomers are so designated because they will bloom until day length gets shorter than their established critical day length.

Some summer blooming, hot weather species like cotton fall into the short day category.

And although they bloom much later than cotton, both fall chrysanthemums and poinsettia are short day plants because they quit putting on new blooms when the days get short enough.

Many of our favorites are day neutral, meaning their bloom initiation is due to things other than the length of daylight and darkness.

Everything else being okay, temperature and genetics are the determining factors for blooming with these.  That’s why tomato plants that were blooming prior to the longest day of the year in June are still blooming now in much shorter days provided thirst, bugs or fungi didn’t take them out.

And I can vouch for numerous roses blooming in spring and, if kept happy, re-blooming in fall.

Genetics certainly plays a role in the rose re-bloom regardless them ignoring day length.

Some varieties don’t re-bloom at all, some do to a degree and some outdo their spring blooming in October.

Folks heavy into the day length phenomenon tells us we actually have it all backwards. It’s one of those “after further study” deals.

Long after early scientists concluded it was the period of light each day that caused plants to do this or that, more modern researchers determined it is actually the period of darkness that brings on such changes. But since the world was already used to “day length” instead of “night length”, the terminology outside of diehard scientific research was left as is.

Terry Rector is spokesman for the Warren County Soil and Water Conservation District.