Solstice and Equinox: Astronomy and Plants

A Solstice occurs twice each year and represents the two extremes of the earth's axis in relation to the sun.  Generally in the northern hemisphere June 21 is our summer solstice and December 21 is our winter solstice, and they also respectively represent the first days of summer and of winter. In the southern hemisphere it's the other way around. These dates can shift a little due to slight differences in actual astronomical time and how we measure time with our calendars.    June 21 is the longest day in the northern hemisphere, meaning that the ratio of daylight to darkness on that day is most disproportional in favor of daylight.  Beginning on June 22 and each day thereafter, the sun will rise just a little bit later and set just a little bit earlier, slowly increasing the amount of darkness in each 24 hours. 

An Equinox also occurs twice each year and represents the mildest angles of the earth's axis in relationship to the sun.  Spring Equinox generally occurs on March 20 and Autumnal Equinox generally occurs on September 22.  On these days there are 12 hours of daylight and 12 hours of darkness. 

One of the strongest relationships between the astronomical calendar and summer vegetable crops is how sensitive they are to the ratio of daylight to darkness.  The amount of time a plant is exposed to sunlight, and then the corresponding time it is in darkness trigger different responses.  As a young plant enjoys the ever-lengthening days of sunlight, it grows big and tall, full of beautiful big leaves and then begins to flower; but the moment it realizes the days are decreasing it begins to focus it's energy on one thing: make fruit and therefore seeds. It's a biological clock kindof thing.  The growing season between the Last Frost Date and summer solstice can be viewed as a plant's youth, the season after summer solstice as the plant's sexual prime, and as the season begins to close toward the First Frost Date and Autumnal Equinox - the plant will be in old age and will be fighting for any last ditch effort to leave behind some final prodigy.

So as you plan your garden plantings, you'll want to plant as early as is safe, but many common vegetables can be planted up until the end of May with flourishing results.  Even planting late, your seedlings will be in tune with this astronomical clock and will actually grow more quickly as a result.