Mar 31, 2015
Study how your crop responds to day length

“Watching the grass grow” is a common idiom for the apparent slowness in which plants respond. I’m sure your average plant finds this insulting. That’s why plants countered with, “Stop and smell the roses.” Idioms aside, no matter how slow plants seem to behave, they are far from the inert objects we often take them to be. They respond to stimuli in their own way and in their own time.

Well-known plant responses are to temperature and water – plants in our part of the world go dormant when it gets cold and all plants die if not given enough water. However, plants also respond to other stimuli to which plant scientists have given some pretty fancy names. They are:

Geotropism: Response to gravity – roots grow down and shoots grow up.
Phototropism: Response to light – plants grow (bend) toward light.
Photoperiodism: Response to length of light and dark periods.
Plagiotropism: The propensity to continue growing in a set direction. Cuttings taken from lateral shoots of certain plants will have a tendency to continue to grow in a lateral direction rather than growing vertical.
Thigmotropism: Response to mechanical touch – tendrils curl around whatever they contact. Some plants catch flies and leaves rapidly close on other plants.

These plant responses have been extensively investigated to understand the various control mechanisms. Photoperiodism is quite prevalent in plants adapted to temperate zone latitudes where light levels annually fluctuate with the seasons as opposed to tropical areas with a constant 12-hour daylength. Photoperiodism was first suspected in the 1850s, but was difficult to study until the invention of artificial light. Prior to artificial light, plant scientist knew certain plants behaved differently when grown under shorter, winter days in a heated greenhouse compared to longer, summer days of equal temperature. Even though we use the term “daylength,” it is actually the length of the night (dark) that is the controlling factor.

Plants responsive to photoperiodism are described as short day (the observed response is due to shortening daylength), long day (the response is to do the daylength getting longer) and day-neutral (the plants have no response to changing daylength). We now know that daylength influences a wide range of plant responses from flowering in hops, bulb formation in onions and garlic, runner development versus flower bud initiation in strawberries, seed germination of some plants and other characteristics. Photoperiodism can also interact with temperature and nutrition level, which gets me back to the point of this article.

Plants with a photoperiodic response for flowering will grow vegetatively until they receive the daylength required and then they pretty much cease growth in height and leaf area and switch their energy into other functions. Since nitrogen is primarily involved with vegetative growth, it is pretty much a waste to apply significant amounts of nitrogen after this time. The goal in nitrogen application for these plants is to have all or most of it applied prior to cessation of vegetative growth since there is often a direct correlation in the size of the vegetative plant with yield.

Hops get the signal to flower in late June, so significant nitrogen applications should be done by early June. Garlic and long-day onions get their signal to bulb in late May, so the last nitrogen application should be done in early May. June-bearing strawberries should have nitrogen applications up until Sept. 1 since they will start forming flower buds when the daylength is about 12 hours and 30 minutes. Excess nitrogen and temperature can influence when the timing actually occurs. For example, warm temperatures in September will cause strawberries to continue to be vegetative, but only until the daylength response is too strong.

Understanding plant physiology helps growers produce higher yields and become better resource managers of nutrients, water and other inputs that go into maximizing yields while minimizing inputs.

Ron Goldy, Michigan State University

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