The Dirt Issue 7 – Latitude, Day Length and Our Growing Season

Posted on August 1, 2016
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In a previous post of The Dirt, we touched on the issue of climate and its contribution to successful farming in the Skagit Valley. In this month’s edition we are exploring how latitude and day length contribute to the characteristics of climate.

First of all, when it comes to discussing climate, what matters most, for our purposes, is how it affects the growing season. Conventional wisdom suggests, “Climate is what you expect and weather is what you get.” Many of those expectations come about because of the latitude of our location.

The very word, climate (from the Greek klimat), originally stood for a band of latitude, or, in simple terms, how far north or south a location is from the equator. This location/latitude has the single biggest effect on a region’s climate.

Here in the Skagit Valley, we reside in the mid latitudes, a zone affected by both warm, tropical air circulating toward the poles and by cold, polar air circulating towards the equator.

Did you notice the word, circulating? It turns out wind circulation patterns change with latitude. So does the intensity of the sun’s radiation.   

This brings us to the second big factor affecting our valley’s climate, the tilt of the earth. In its year-long orbit around the Sun, the earth always maintains its orientation: the North Pole tilts toward the North Star (Polaris) which happens to be very close (in astronomical terms) to the celestial north pole.

When the Sun is on the same “side” as Polaris (relative to where Earth is in its yearly orbit), we in the northern latitudes receive more sunlight (solar radiation) and longer day length. When the Sun is on the other “side,” the opposite holds true. This constantly changing relationship, caused by the Earth’s tilt, gives us our seasons. No tilt = no seasons = always hot equator; always frigid, dark poles.

Here at latitude 48.4212° N (Mount Vernon) we enjoy a spring through summer period of significantly longer day length—16.07 hours of direct sunlight on June 20 (this year’s summer solstice) compared to 12.09 hours projected for the upcoming autumnal equinox on September 22.

Those extra hours of sunlight play a big role on what grows in the Skagit Valley.

A case in point—and a perfect example of the value of day length—is the highly-valued crop of spinach seed grown in the Skagit Valley. According to the Organic Seed Alliance, “The majority of spinach varieties require a minimum of 14 hours of daylight to initiate the transition from vegetative growth into floral development. If spinach is grown for seed in a location that never gets 14 hours of daylight, only a small percentage of plants will likely flower and set seed.”

As you would expect, greater exposure to solar radiation correlates closely with increased temperature. The combination of greater day length and warmer temperatures are the two primary environmental factors that affect plant growth, or better said, the growing season.

What is the growing season?

The definition is a bit more specific than “when plants can grow.” Here in North America it is generally said to be the time between the last frost of the spring and the first frost of the autumn.

That doesn’t mean, of course, that all conditions are “go” immediately after the last spring frost. Besides our ever-present need for the soil to dry out enough to plant after the winter rains, any farmer, let alone home gardener, can tell you the growing season is not universally the same for all crops. But that last spring frost does indicate the stirrings of the suitable time.

Our growing season is…well, growing.

Here in the western United States, the length of the frost-free season has been expanding. From 1991 to 2012, our frost-free days in the Pacific Northwest have increased by 16 days. And, we’re not alone in that situation. The Central England Temperature (CET) record has been maintained since 1659(!). According to its data, central England’s growing season is now at 280 days; in the late 19th century, it was 244 days. The CET also shows that, in addition to the longer growing season, the number of frost days is declining.

What does it mean for the Skagit Valley?

Look around. Crops are being harvested sooner than we’re used to; famers’ markets have a richer array of produce even at the very beginning of their season; and, farmland is, in some cases, a candidate for double cropping—planting and harvesting two warm-season crops in a single year (we’ve long seen cool-weather crops planted over the fall/winter months). A longer growing season also opens the door to the cultivation of plants previously not viable here, longer-season crops which expand crop diversity, types and varieties.

Simply said, longer day length in a longer growing season enhances the capacity to grow things.

And that’s the whole point, really.


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