It’s a bright sunny morning here in Portland…in January, not a real common occurrence in a place where we typically have some kind of cloud cover due to our climate with its strong maritime influences…but today it is sunny, and I’m thinking about the annual cycle of changing day length as we move from our shortest day, on the winter solstice, toward our longest day, on the summer solstice. The solstice result from the tilt of Earth’s axis, which remains more or less fixed, though there is a bit of a ‘wobble’, as the Earth follows its annual orbital path around the Sun, spinning like a top, effectively changing the surface it presents along the way.
Portland lies very close to 45º north, halfway between the equator at 0º and the north pole, at 90º. The curvature of the Earth, literally curves away from the Sun relative to our position as we move toward the poles. Here in the northern hemisphere, on December 22, the winter solstice, we experienced 8 hours and 42 minutes of daylight, between our sunrise and sunset. On the summer solstice ‘daytime’ will increase to 15 hours and 41 minutes, an increase of 7 hours, when the elevation of the sun will have increased from a low angle of 21º to a high of 68º above the plane of the horizon! The length of our days are now increasing and the sun, as it travels across the sky, is reaching higher above the horizon at solar noon. We are receiving more sunlight each day and it is increasingly intense.
In either hemisphere, locations away from the equator will experience greater variation in their day length and the intensity of the light, as you move further away and closer to the poles. Eventually, at the point where you cross just beyond latitude 66º, you pass into the Arctic, where on the winter solstice at no time will the sun rise above the horizon and on the summer solstice, it will remain above. (This precise latitude varies on a 41,000 year cycle approximately 2º…something we don’t really have to be concerned about.) At latitudes greater than this, day length varies 24 hours through out the year, moving from unending darkness to nonstop daylight. Move even closer to the Poles and the period of total darkness in winter, and summer of the midnight sun, increase. In winter the plane of the horizon ‘tilts’ completely away from the sun, the bulk of the Earth effectively blocking the Sunlight. This map shows the difference in daylight hours between winter and summer solstice as you move away from the equator in North America.
This is not just about time, an increase or decrease in exposure leading to a given irraidiance, the amount of energy striking a given area on the surface at any moment changes as well. Our patch of Earth, like all do in the northern hemisphere, received the smallest amount of solar energy of the year on the winter solstice…the shortest day with the Sun at its lowest angle of incidence, the ‘flattest’ angle, meaning the sun had to penetrate through more atmosphere and was spread across a broader surface than it is on any other day! These are those days, which when clear, tend to ‘blind’ as as we walk toward the sun as shading our eyes from the flood of light is difficult without also obscuring what we’re looking at. Duration and intensity of our solar energy gain vary together through out the year. Think about locations directly on the equator where every day the sun rises to 90º, directly over head and each day of the year is exactly 12 hours long. Where conditions along the equator support it, growth is extravagant. Tropical rain forests support massive growth in several layers compared to the much more limited growth of native cold temperate landscapes declining ever more as you move on into the tundra or polar latitudes. Latitude makes a huge difference in the amount of energy any given place receives and therefore, the type and quantity of life possible there. In fact, nearly all of life is reliant upon the sun’s energy either directly through an organism’s ability to convert it photosynthetically, or less directly through the consumption of other organisms who ultimately can trace their ‘power source’ back to the sun. There are some exceptions whose metabolism is based on the oxidation of other elements, but these are a relatively few bacteria, but even these may not be possible were the Earth just a cold, barren rock, without a sun to light or heat it.
While the sun is the primary driver of our weather and climates, it is not their sole determiner. It is good to keep in mind that regardless of the date the sun delivers the essentially the same amount of energy to the Earth on any given moment, differentially heating it. The sun may not always shine in Philadelphia, but it has never for a moment ceased shinning on the Earth. It is us, affixed as we are to place, that experience it’s ‘cycles’ through out the day and season. There is no ‘switch’ that turns it on and off, no ‘dimmer’ to smooth the transition.
Plants which evolved in more tropical locales, especially those tropical desert and high elevation, alpine sites, such as Agave, the general of ‘cushion plants’ that endure in the high Andes, have to bear the short season and cold of their altitude along with the long days of summer’s intense irraidiance. Such plants have evolved a variety of protections to its intensity, protections still in play when we try to grow them outside of these conditions at temperate latitudes.
When we choose to grow plants from different latitudes we must be aware of this difference. It is not just cold hardiness and temperature that will effect our success, it is the duration and intensity of the light you receive throughout the year. Full sun is not the same as you move north or south…bear this in mind when you read descriptions of plants and their growing conditions.
Tropical regions of the world, which closely flank the equator, are more biologically diverse than locations at more remote latitudes for a host of reasons. By far the vast majority of all of the species to have ever lived, have been resident to the tropics. Day length or photoperiod and the intensity of the light, above and beyond ambient temperatures, must be central in your considerations. You will have the greatest success growing plants that come from or near your own latitude, as long as you pay attention to temperature and precipitation patterns and of course the rest of the factors, soil type, the amount and density of shade and the particular members of the biological community that share and influence it.
Choosing to grow plants from tropical and sub-tropical latitudes, those below 25º latitude, require that we recall how different our conditions really are. These plants evolved with both more intense sun and a less variable photoperiod. We can compensate in various ways for differences in precipitation pattern. We can provide protection when our winter temps dip too low, but when the sun retreats too far southward, that can be a much more expensive barrier to overcome, because you will have to provide it artificially. Tropical plants can often be induced into dormancy by drought stressing them, that is if their native regions have regular wet and dry seasons; not possible for those plants that lack this history and capacity. They also, because of their tropical origins, generally lack the ability to go into a cold dormancy. When temperatures drop too far they simply shut down and die, or, in the case of freezes, their cells rupture and the plant collapses catastrophically. Trying to carry a tropical plant through long periods of low light intensity with too short duration, will stress any of them, possibly leading them into etiolation, weakening the overall health and make them subject to attack from a variety of other factors, not normal for them.
Problems will also occur moving plants in the opposite direction, from cool temperate regions to tropical equatorial regions. Changes in elevation can compensate for this in someways. Collecting plants from higher elevations of tropical latitudes, from the Andes or Himalayas for example, can counter some of these problems, but not all. Such plants will likely be more adaptable to our temperate winter conditions, as long as our cold extremes are not too great, but these same plants may then suffer during our too hot summers. Regions that have a more direct maritime influence tend to be more moderate and can thus accommodate plants from higher tropical elevations than those that suffer the large swings in more continental locations.
Plants have evolve particular strategies for success across their native range. They do not possess capacities that do not serve them. Why would any plant ever require the capacity to withstand freezing if its progenitors never had reason to? Why would any plant armor itself against intense sun if it did not evolve with it? True, changing environmental conditions have provided selective pressures that many who study evolutionary biology recognize as being an important factor in the evolution of species and perhaps even the creation of new ones. But these kinds of changes tend to be slow, moving at geological pace. Other species are in the ‘mix’ as well and may be more likely to adapt to these and fill available niches, leaving another species ‘stuck’ and limited to its earlier range and conditions. Over long enough periods though speciation happens, plants can change enough to differentiating themselves from others.
It is thought, that some 50 million years ago when the Bromeliad family first began it was relatively simple, occupying a wet tropical region of northern South America. But changes did occur, over hard to imagine periods. The Guiana Shield rose creating highlands, a massive area of ‘basement’ rock, dense and erosion resistant, lifting up through the Earth’s crust, and in so doing, adding ‘selection’ pressures that began to shape them into the group they are today. Bromeliads took on their recognizable form of today able to trap and contain the water they needed, within their clasping and overlapping leaf bases, despite the relative drying of their region. Their epidermis changed as well developing ‘trichome’ structures on their leaves surfaces that allowed them to take up water and nutrients directly through their leaves, their roots gradually reduced to simple anchoring systems that could cling to rocks and trees. Then the Andes began to arise becoming a massive, continent long barrier to wind forever changing the climate, introducing a degree of aridity hither to unknown by these plants. They ‘climbed’ with the mountains evolving into the much more drought tolerant plants of today including the Puya and Fascicularia. Others followed their own paths becoming Dyckia, which predominate across the southern Brazilian highlands while others were separated becoming the North American Hectia. These generally retained fuller functioning roots along with their characteristic rosette forms. The 650 some species of Tillandsia took to the trees and became epiphytic across most of Mexico on into the American southeast. In their more exposed sites Bromeliads often modified their water absorptive ‘trichomes’ into structures that shield their epidermis, forming an ‘armor’ covering that protects them from the intensity of the tropical and high elevation sun. In other species these retained all or part of both functions.
Other families followed comparable strategies to make their epidermis more resistant, a thicker cuticle or a covering of fine ‘hairs’, which are also cellular, trichome structures, to reduce the received light intensity. Many combine these with modified metabolic systems allowing them to function more efficiently with higher ambient temperatures. All species will pursue those conditions under which they better thrive whether that means exposed sites where they rely upon their own defenses or they seek out those conditions afforded them by growing in close relationship with others, in this case benefiting from their shade. Plants need to be understood both in terms of their individual qualities and their relationships, if one is to grow them, especially when doing so well outside their natural range. We must attempt to provide conditions within which they can thrive. Any species comes as an entire ‘package’. They do not ‘shed’ either their capacities or their limitations when we transport them around the world. Plants adapted to high/intense light situations will keep them, especially when they are bred into them in terms of their own structures and metabolism. These ‘defensive’ characteristics will continue functioning, reducing light in the same manner when these are grown at ‘higher’ latitudes, effectively reducing the light available for their photosynthesis.
Today, in the later Anthropocene, the Age of Man, with our technology and capacity for travel, we have separated ourselves from many of the limits that once held us in place, in ways similar to the world’s plants. Not too long ago, a very few hundred years, most people, travelled little further than the distance they could walk in a day or two from their birthplace. Herdsmen and hunter gathers were among the exceptions, as they followed the seasons looking for better pasture for their livestock, the ripening fruits of the native landscape and wild game. It was a tightly followed annual cycle upon which their survival depended. Today we often travel for its own sake. In earlier times, travel over greater distances, was a trip into the unknown and brought much greater risk. Today we travel, in comparison, almost casually, with little thought to what it means. We take the harvest from around the globe from who ever will offer it. We have become almost completely detached from the Earth’s annual cycles which are still directly linked to place and the Sun’s energy. More and more, we are living outside of the living systems upon which we are still dependent. We build shelter, heat and cool them, as we deem necessary, we wear clothing for these reasons as well, but also to satisfy our egos and to make social statements of who we are. Only the ‘wild’ today remain faithful to the Sun and its natural cycles. Gardening gives us the opportunity to remember all of this and what our place is in the larger cycles of life. While the Sun, in a sense, has ‘retreated’ from us in the winter, it has not left us, it is merely spreading its largess, sharing like portions with all of the other lives across the planet…a message and lesson we would do very good to emulate. With the winter solstice our place begins its return, reawakening the living world. In many yoga practices, one begins each day with Sun Salutations, facing the sun, waking one’s self up to the new day. Our relationship with it is thus celebrated. Whatever we choose to do in our mornings we should do fully and consciously, instead of simply groaning as we get up at the prospect of facing another day of largely unfulfilling work, at jobs that do not nourish our bodies and spirits.
Plants carry far more within there genes than prescriptions for pretty flowers and leaves…so do we! We are all organisms intimately linked to the Sun. Our plants arrive in our gardens with complex histories, limits and capacities that demand of us, the gardener. Sun and place are intimately linked and living well requires that we recognize and honor this relationship. This is something to be embraced. Our gardens afford us the opportunity to better understand the world we live in and what is expected of us if we are to be successful. We rise and fall with the other life on the planet, so grow them all well!!!
I’m listing here a couple sites I’ve found with great charts, graphs and tables offering information on the sun, day lengths, the sun’s altitude, where we are in our orbit about the sun…all kinds of interesting facts.
Time and Date.com: This one shows much of this broken down day by day for Portland, and for many cities around the world giving you a better idea of what conditions are like on the ground where your plants may have originated.
GAISMA: Offers similar info in chart form and includes info for many hundreds of cities around the world. This particular link is to Santiago, Chile.