Monday, September 30, 2013

Fall Equinox

Note: This program first aired on September 28, 2013.

It’s fall in Maine. We’ve just passed the autumnal equinox, that point in the Earth’s orbit where, mathematically we can think of the spin axis of the Earth as a tangent line on the orbital ellipse. What this means in practice is everywhere on Earth has a day of 12 hours of daylight, 12 hours of darkness, more or less. After that, at least here at our middling  northern hemisphere latitude, the darkness increases dramatically, making the light sensitive among us go into mourning, and the snow lovers among us optimistic.

I recently received a weather alert on my phone, telling me that frost and freeze advisories had been suspended for northern Maine, as they had officially reached the end of the growing season there. The alert added that frost and freeze warnings would continue in my neck of the woods, coastal Maine, until the official end of the growing season here, noted as October 10, or earlier if we received a hard freeze before then. It made me wonder, what exactly is the growing season? How is it calculated and what does it really mean? How do we know the exact date when it ends?

Here in Maine, midway between the equator and the north pole, there are three components to growing plants, which when we talk about the growing season, is what we mean. We need light, water and we need warm enough temperatures. The light aspect is very predictable, at least in terms of day length. We know to within seconds how much time the sun will be above the horizon on any given day of the year. Temperature, however, is harder to predict. We can look at averages over time; the date of the first frost for a region, averaged over a period of years for example. I suspect that is where the October 10 deadline came from. Water is, for the most part, not a significant issue here, as we usually have plenty of it, with one major exception. In winter, there may be lots of water around, but it is frozen, and thus not available for use by living plants.

Information for farmers about the growing season is all about mitigating the effects of cold temperatures, really, unless you want very powerful grow lights, temperature is the only aspect of the growing season we can control, and only on a small scale at that. When it comes to light, 10 hours seems to be the magic number. Below that amount of daylight, plants can simply not fix enough of the sun’s energy to meet their metabolic needs and grow. They may be able to maintain, but not get any bigger, or reproduce. For that reason winter is a time of dormancy, a holding period that plants simply wait through. Here in Maine, our true non growing season runs from the beginning of November until the beginning of February, based on that magic 10 hours of light. Wild plants obey the same rules as their domesticated cousins, if my observations are correct. Trees lose their leaves, annual grasses and forbs die back to their under ground roots, or over winter as seeds. Even the hardy ever green trees are limited in their abilities to photosynthesize over the dark, cold winter months by low light, cold slowed metabolism, and lack of available liquid water.

We are tropical animals, living in a temperate climate. Even in the tropics there are patterns of growth and rest, wet and dry, exuberance and senescence. As the calendar rolls towards October 10th, the last day they will bother to warn us about a possible frost, or the beginning of November, when the daylight dips below the 10 hour mark and stays there for three whole months, take heart that this time of year plays a valuable role in the yearly cycle of plants, and people. Take this time to show your other, hidden colors; draw in and hunker down. The dark provides our excuse, our opportunity, finally, after the excitement of summer, to simply stay home and regroup. So don’t fight it. Look around you, are the trees in your yard, the weeds in your garden, or the ferns in the woods arguing with the solstice? Neither should you. Happy fall everyone.


Interesting info from Maine’s own Johnny’s Seed company

A frost free dates for my neck of the woods, from the National Climatic Data Center, featured on a gardening website: