Plants Know

Plants know naturally the science of seasons
By Dan Gill

In just a few days, on Dec. 21, the 2009 winter solstice will occur.

Here in the Northern hemisphere, we are tilted farthest away from the sun on that day. That means the period between sunrise and sunset is shorter than on any other day of the year, making the night the longest of the year.

The length of our days and nights vary from season to season because the Earth’s axis of rotation is tilted in respect to its plane of orbit around the sun.

The winter solstice marks a turning point: While days have been getting shorter and shorter and the nights longer and longer since the summer solstice in June, after Monday, the days will gradually begin to lengthen and the nights shorten. From ancient times until today, humans around the world have marked this time of year with various celebrations, festivals and religious rituals.

Plant sense
What does this have to do with a column on plants?

Well, I could mention that for thousands of years certain plants have played a role in human observances of the winter solstice.

In Europe, plants that stay green during the winter often had special significance. They were a reminder of life in the midst of freezing cold and leafless, dormant trees and shrubs. Evergreen plants such as holly, English ivy, mistletoe, and conifers such as fir, spruce, cedar and pine, are still used today to decorate our homes, along with winter-flowering plants such as poinsettias.

Speaking of poinsettias, have you ever wondered why these colorful plants bloom now, rather than for the Fourth of July?

Here’s where the horticultural lesson about the winter solstice comes in. It is important for gardeners to understand that the changing length of days and nights from season to season has an effect on the way many plants grow and what they do throughout the year.

Just like us, plants living in temperate climates where major temperature changes occur during the year need to be able to tell when the seasons are changing.

Two ways that plants do this are by measuring hours of darkness that occur in a 24-hour period, and by measuring how much cold they have experienced.

Counting the hours
The fact that seasonal changes in light during a 24-hour period have an effect on plants was researched thoroughly back in the 1900s, and the term photoperiodism was created to describe the phenomenon.

In 1920, two employees of the U.S. Department of Agriculture discovered a mutation in a type of tobacco called Maryland Mammoth that prevented the plant from flowering in the summer as normal tobacco plants do. Maryland Mammoth would not bloom until late December.

Experimenting with artificial lighting in winter and artificial darkening in the summer, they found that Maryland Mammoth was affected by the relative length of light to darkness in a day. Because it would flower only when exposed to the short-day lengths that naturally occur in winter, they called it a short-day plant.

Once this behavior was discovered, it was found to take place in many kinds of plants, such as chrysanthemum, poinsettia, Christmas cactus, camellia and kalanchoe.

Other plants, such as spinach and radish, flower only after exposure to long days and short nights, and so are called long-day plants. Still others, including many annuals and vegetables (such as the tomato), are day-neutral; their flowering is not regulated by photoperiod.

As it turns out, the terms short-day and long-day are not quite accurate. It is not how long or short the period of light is, but the length of the darkness. Photoperiodic plants actually need a sufficiently short or long period of darkness to develop a response. However, once people start using a term and get familiar with it, it’s hard to get them to change, so we still use the terms short-day and long-day plant.

Ready for a change
Plants don’t only determine when to bloom by measuring the length of night. Increasing darkness also plays a large role in some plants’ ability to anticipate the coming of the winter and respond.

It’s why, for instance, shade trees drop their leaves in November and early December, even if the weather is not intensely cold. Because the nights have been getting longer, they know colder weather is on the way.

What mediates this remarkable response are various pigments, called phytochromes, which allow photoperiodic plants to measure how many hours of dark they receive in a 24-hour period. The phytochrome, in turn, can trigger the release of various hormones or growth factors that may cause the plant to bloom or to drop its leaves or to begin forming a bulb.

How do plants know when spring is arriving, so as not to be deceived by an early warm spell?

Some photoperiodic plants can perceive the shortening of nights to know spring has sprung. Many others are able to measure the amount of cold that has occurred to determine when winter is over. When a sufficient number of chilling hours accumulate, they are triggered to bloom or send out new growth.

As the winter solstice approaches, it’s interesting to note how remarkable plants are. They have abilities to sense the world around them and to respond to it.

It might not have occurred to you that it is just as important for a plant to know when it is time to bloom or drop its leaves, as it is for a farmer to know when its time to plant a crop.

And just as we have used Earth’s movement around the sun to develop calendars that allow us to do this, many plants can also determine the time of year based on similar perceptions