Simple Definition

Most houseplants are tropical plants. That’s why they do so well indoors, at temperature levels humans find comfortable in their homes, around 60 F to 90 F.   Of course, here in Colorado our low humidity levels can be problematic.  More technically, tropical plants are defined as all vegetation growing in a wide band around the equator between the Tropic of Cancer and the Tropic of Capricorn. Just north and south of that band are the subtropical areas, also rich in plants of interest to our group. When most people think of the word ‘tropical,’ they think of the tropical rain forests, where rainfall is over 80 inches per year, but there are many dry areas in the tropics and subtropics as well, with an extraordinary variety of cacti and succulents

Complex Definition

In a recent article in From the Pawpaw Patch (v13, no. 2, Fall 2006), USDA horticulturist and plant breeder Har Mahdeem distinguished between the temperate, tropical, and subtropical climate zones.  As you can see the subject is far more complex than many of us thought.  What follows is a summary of a part of his article.

An overly simplified and nearly useless definition of the climate zones is a geographical one. In this definition, the temperate zone lies between the lines delineating the Polar zones (latitude of 66.5 N or S). According to this scheme, all of Florida, even the Keys, falls into the temperate zone. “At the other extreme, ivory-tower climatologists from very cold climates, impressed by the warmth of southern

Europe and the southern U.S. have come up with a definition of ‘subtropical’ that ends around Washington, D. C., with the temperate zone only beginning above there!” According to naturalists, Florida’s vegetation gradually changes from subtropical to temperate near Cape Canaveral and further south in inland areas. This roughly represents the line between USDA Zones 9a and 9b.

Mahdeem’s “working plant person’s definitions” are as follows: 

The tropics are warm places without freezes.  The subtropics are places where freezes do occur, but on the average of less than once a year.  Milder parts of the temperate zone have freezes once a year or more often. Though these definitions are fairly straightforward, little climatalogical data is available showing the

number of freezes. There are a lot of monthly temperature averages, median low temperatures as in the USDA Plant Hardiness Zones as well of record lows, but not on the number of freezes. Plus, almost all climatological data is collected in lowland cities and is reported as the average number of “frost-free days per year” without details about how this data is arrived at. Is it average last frost date to average first frost date? Or is it record last frost date to record first frost date? And, to complicate matters, books on the flora of different regions usually give the range of altitudes where a species has been located and do not otherwise address climate. 

Tropical Defined

Tropical Plant Society of Denver

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