Photo period, chilling hours, and gardening in the buff
LEON: Back before European aliens brought a new calendar with months named by the Romans to the New World, Native Americans had a lunar-based calendar that was much more pragmatic. What we call October was really “the Moon of Falling Leaves” (the name varied some according to the tribe).
My thought for the month is: how do the trees know when to get ready for winter and begin to drop their leaves? The answer(s) won’t be of much help to real gardeners but are good for coffee talk around armchair gardeners. The short and simple answer is that most trees begin preparing for winter at the summer solstice, when the days start getting shorter. Some trees are really good at measuring time and can detect the day being only a minute shorter, others are a bit slower to learn, and some young trees aren’t old enough to learn.
Gardeners have always known that plants respond to day length, but it wasn’t until 1918 that two U.S.D.A. men decided to experiment with daylight exposure to determine just why the tobacco plant Maryland Mammoth grew so large without flowering. The experiment was very simple. They put the tobacco plants in a dark chamber every afternoon and showed that this “short day” treatment would make them flower. Being new at the game, they called this reaction “photo-period.” Later studies found that it was actually the length of the night, rather than the shortness of the day, that is the key to flowering—though temperature also plays a part. Some people tried to change the name from photo-period to scoto-period, meaning the length of dark time; but photo-period was already in popular use, so it stayed. Now for one more little detail before we get practical: plants do detect light by measuring time, but it is in two wave-lengths of red; so daylight is measured even on dark, cloudy days—red can run through the clouds.
Many other plants respond to photo-periods, but for the moment, I want to stay with trees. As a tree detects shorter days, it begins to go dormant. It quits producing new leaves and the nutrient material (sap) in each leaf drains out, going down into the trunk or root system. The green chlorophyll slowly dies out, leaving the bright colors that were already in the leaves. This brings on the beautiful fall color, especially in areas where the climate is drier and cooler. You will remember that we have pretty good fall color in some trees—sometimes. If the fall is moist and humid, diseases affect the already dying leaves and they drop off.
This shorter day/longer night situation brings up another question. The very far north is known as the land of the midnight sun. In the summer they have no night at all. As we move southward the summer days get shorter until we reach the tropics where the days and nights are of equal length throughout the year. Some of the tropical trees, and magnolias in our area, just drop leaves throughout the year, with no fall color. Before coming to LSU, I had worked for two years with the Firestone Plantations Company in Liberia. Having come from a Wisconsin graduate school, I was full of ideas about the long day/short day responses. Imagine my surprise to discover that rubber trees “winter” once a year. We were almost directly on the equator, and there was hardly any difference in day-length. But all of the trees would lose their leaves and stop growing for about a month. Then they would flower, produce new seeds, and begin growing again. The key to this was the tropical monsoons. For climatological reasons that I don’t understand, there is a predictable time for a short, dry season that signals the trees to drop their leaves and go dormant until the rains return. When LSU sent me to Malaysia for two years, I found that they have two monsoons per year, preceded by a dry period each time. So many of the fruit trees, such as the durian, have two fruiting seasons each year.
I told you about trees going to sleep (dormant) and will give a very short account of how they wake up in the spring. The exact timing varies with the tree; but basically, a tree measures the total time that it is exposed to cold—but not very cold—weather. This is a strange skill that I don’t quite understand, but the tree only counts time at temperatures between thirty-two and forty degrees Fahrenheit—freezing. Anything below freezing doesn’t count towards what is called “chilling hours.” You may already know a little about this in that peach trees bred for Ruston won’t wake up in Baton Rouge. You also know that spring isn’t really here until the pecan trees wake up. Pecans have the longest chilling requirement of which I’m aware. I could ramble on, but I think you only need to know that trees go to sleep by the day length and wake up by the temperature.
Now we can become a bit more practical and talk about other plants that flower according to the day-length. Before the advent of selective plant breeding, most plants flowered according to some combination of day-length and temperature. Those that still do respond will have “date of planting” on the seed packet. The older varieties of poinsettias had serious photo-period problems back when my buddy, Ed O’Rourke, worked with them. They needed long dark periods in order to flower for Christmas, and most of them did pretty well with our nights, but Ed’s greenhouse complex was located near some girls’ dorms, and they had dates at night. Well, it happens that even a short flash of light will break up the dark period, creating the impression of a short night. The poor poinsettias were going crazy trying to figure out when to start blooming! Ed had to rig up a large dark cloth to shield the plants from the lights of cars, but then he ran into differences in temperature under the black cloth so he had to correct for that! Thankfully, most of the new varieties aren’t so sensitive—also the poinsettia-testing program has been moved to a greenhouse at the Burden Center, where people don’t drive around at night.
ANNE: You’ll notice that my friend Leon has avoided the topic of our backyard, what we’ve come to call the Craven Garden Research Center. That’s because no one on our gardening team—being Leon, my husband, our toddler, and me—has wanted to endure the extreme heat for significant periods of time. Lately that has meant about five minutes. We sneak out there to gather the okra, peppers, and snap beans still surviving in our garden, then it’s a race back inside just before our skin actually starts to melt.
Speaking of skin, a website that I check on regularly (gardenista.com) mentioned an article on nude gardening in the recent issue of Wilder Quarterly. Aha! Surely there are people secretly engaging in this niche of the hobby here in South Louisiana—how else could you stand the heat? Let it be henceforth known: anyone whose garden looks better than mine in July, August, or September shall be branded a naked gardener. (FYI: World Naked Gardening Day is the first Saturday in May. The movement’s website made me blush: wngd.org.)
By the time you read this, the temperatures will have dropped, hopefully. As August turned to September, we planted seeds of leafy greens like cabbages, collards, and lettuce, plus crimson clover as a cover crop. We’ll have much more to report on next month. Happy fall!
