Home > Great Big Greenhouse Blog > August 2014 > TROPIC and SUBTROPIC


In my childhood, I spent hours poring over a large atlas that had been my grandmother’s.  I had also inherited from her a 30-year collection of National Geographic magazines, many of which contained large, folded maps related to one of that issue’s articles.  My love of exotic place names was firmly established once I had access to a desk-top globe with the world in relief that my brother received as a birthday present. And then there were Tarzan movies, Shangri la in Lost Horizon, Green Mansions (a book I read and re-read…very romantic), etc.  Even today, when listening to the news, my attention is diverted from the import of the story to the name of the country or town where the action is taking place.  The most magical names—Rangoon, Bora Bora, Fiji, Madagascar, Tahiti, Jakarta, Morocco—are places in the tropics and subtropics.  Even Shangri la was a little pocket of warmth in the high Himalayas.

Think back to your grade school geography book with a map showing the equator running straight across the middle, with a line above it labeled Tropic of Cancer and one below labeled Tropic of Capricorn. The word “tropic” comes from the Greek tropikos, meaning solstice, with the summer solstice marking the time when the earth is closest to the sun and the winter solstice the farthest.  The Tropics of Cancer and Capricorn are the furthest points north and south of the equator at which the sun is ever directly overhead at noon. These geographical lines relate to the number of hours of light each day.  Tropical areas have uniform day lengths throughout the year, about 12 hours, unlike our temperature and light fluctuations from spring to summer to fall to winter. 

While day length is consistent, temperatures depend upon elevation, rainfall, and other factors.  High mountains along the equator may be snowcapped, while the beach on a tropical island may never be any cooler than 70⁰F.  Generally, the mean temperature in the tropics is 65⁰F.  By comparison, the average low temperature for our part of Virginia is 48⁰F and the average high 70⁰F, over the past 30 years.  Although our summers certainly qualify as tropical, during the year we experience dips into the teens and highs in the upper 90’s.

If you look at a climate map, there is an irregular zone between the tropics and the temperate zone (our part of the world) both in the northern and southern hemispheres.  This is where water and ocean currents, along with other meteorological conditions, have created a mild climate known as the Subtropics.  Think of the Mediterranean with very infrequent frosts and abundant sunlight.  Florida, from which we receive the majority of our houseplants, is a subtropical climate.  As it is frequently cooler than the tropics, many of these plants are given winter protection in the form of greenhouses or space heaters within open shade houses.

As many as three quarters of the world’s land plants come from the tropics, more than 200,000 species.  Now within the tropics, there is a wide variety of habitats.  Desert, savannah, and rainforest vary widely in the plant life they support.  Most of our houseplants come from tropical rainforests.  Our common philodendron and pothos vines don’t seem so mundane when you picture them making their way up from the dark steamy forest floor into the upper regions of the jungle, climbing on trees to reach the sunlight, where their leaves get so large we hardly recognize them.  Orchids, anthuriums, bromeliads, and other epiphytes cover the upper branches of the forest, their roots anchoring them on tree bark so that they can catch rain and nutrients.  Plants on top of plants, epiphytes create gardens in the air.  Tall graceful, feathery palm trees sway in the breezes of tropical islands instead of being anchored firmly in terra cotta pots on your screened porch.  Even the common snake plant’s wild cousins can be seen growing along the edges of the forests in central Africa like grass from a Dr. Seuss story or a Tim Burton film.

All of this information is by way of explaining why you cannot leave your houseplants outside all year.  Plants adapted to uniform day length, consistently warm temperatures, wet season/dry season, etc., cannot cope with the cold, wet winters we experience each year.  I don’t cope with it very well, either!  

Posted: 8/13/2014 by Margot | with 0 comment(s)
Filed under: MargotGunn, TheGreatIndoors
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