Tropics: Any region within the latitude of Tropic of Cancer and Capricorn, including deserts and high elevations that do not have tropical(never drops below freezing), weather. Subtropics are those areas just outside the tropical zones and are intermediary in climate between tropical and warm temperate climates such as often adjoining mediterannean climate zones. Much of coastal California has a climate that can support subtropical plants, but is not subtropical because of the winter wet/summer dry rainfall patterns define it as a mediterannean climate zone. Warmer areas of south Florida are considered subtropical because they have the more typical winter dry/summer wet rainfall pattern or rainfall generally throughout the year. Tropical regions include alot of territory that doesn't have typical low elevation tropical weather. Vegetation types within all the tropical regions can vary dramatically, allowing us to grow tropical region plants under widely different climatic conditions.

Bahia's definition is the standard geographer's definition. It is one that is independent of climate. By this definition, a person standing on Peru's Pico de Huascarán, one of the coldest spots on the planet at over 22,000 ft. elevation, would be smack dab in the middle of "the tropics". Biologists, horticulturalists, and climatologists often use slightly different understandings of the terms, so context is important. As in any taxonomy, there are lumpers and splitters. One person might see semi-tropical, peri-tropical and upland tropical climates as being very different and needing different terms. Another might use the term "subtropical" for all of them.
Originally, "tropical" meant 10º to 23½º north or south of the equator and was distinguished from "equatorial" (0º to 10º north or south). A few people still maintain this distinction, but most simply use the term "tropical" for any area where the sun's rays can fall perfectly perpendicular to the surface of the Earth. This phenomenon would only occur at the equator if the Earth's axis were parallel to the sun's. But because the Earth is tilted on its axis 23½º, the effective solar equator migrates over a band 23½º north and south of the actual equator during the Earth's orbit around the sun. "The tropics" is the term that geographers use for this band of latitude where perpendicular insolation can possibly occur.

I'd throw in a more "Gardening" definition. Tropical is any plant that starts to falter below temps of 50 F, with major leaf burn or wilt/die-back if we hit 40 F. A subtropical plant is one that handles temps in the 30s all right, but still would sustain the expected damage at freezing. Still can think of many exceptions to these rules just presented.

Very unscientific and ambiguous in this definition, but potentially useful.

We have a meeting coming up and I am suppose to get the difference between tropics and subtropics plants. I thought it would be easier to just explain the difference between the regions. I can see that probably isn't the case now. I need some explanation that isn't scientific. Poiu is closer to what I am looking for. I am glad I did receive the scientific definitions for myself. I always like to get more detail but that's not what the group is looking for.
Is the statement,"Most subtropical plants can be grown in the tropics but most tropical plants can't be grown in the subtropics?" I'm talking about in the ground.
Thanks everyone for your help?

Well, then, the cool weather tropicals come in... like my Salvia chiapensis, from the Chiapas region of Mexico... likes it on the cool side. :o)
Another thing about tropicals, usually the majority of their rain comes in the hot season, not the cool season.

An unscientific definition is the only practical one, and the subtropical-tropical range needs to be seen as a continuum, not two discrete categories. It is also true that some subtropical plants don't grow well in some subtropical climates, and some tropical plants don't grow well in some tropical climates. A few examples are plants from TDF (tropical deciduous forest) and monsoonal climates. Both climates are tropical but have different precipitation cycles that may be critical to a plant's survival.
Accepting the latitudinal definition of "subtropical" for the sake of argument, two important distinctions are the amount of seasonal variation and the total number of annual heat units or growing degree days. Subtropical climates normally stay above freezing but typically have a cool season (which is usually dry) when photoperiod is reduced, plant growth slows or plants go dormant. In tropical climates, photoperiod is relatively static and there is no particular time of the year when plant growth slows (assuming water is available). Tropical plants often fail to show expected performance in the subtropics for that reason.
Defining "expected performance" is one of the subjective variables that affects whether or not one can reasonably say that a given plant "grows" or is "hardy" in a given climate. Plants vary widely in how tough they are, and there are a surprising number of fully tropical plants that can get by in subtropical or even Mediterranean climates. Physiological adaptations to seasonal aridity may also help a plant cope with cold weather, even if that is something the plant doesn't experience in nature. Conversely, some subtropical plants may fail to flower in the tropics because their flowering is triggered by photoperiod. Such a plant might "grow" in the tropics, but it doesn't really exhibit expected performance there (several orchids come to mind).

Bahia's initial point is worth repeating: you can broadly categorize plants as subtropical or tropical based on the amount of seasonal variation they are adapted to, but you can't type a bioclimate by the kinds of plants that humans manage to coddle in their gardens. Even here on the Oregon coast, I'm growing vireyas from New Guinea that naturally occur within 5º of the equator. Trading latitude for altitude, a temperate sea-level climate at 41º north mimics their high-altitude equatorial climate closely enough for them to succeed thousands of miles north of the tropics.

Here's a *very* unscientific definition: if you can walk around naked all year round (day or night) without feeling chilled, it's tropical.

Hawaii is not situated within the tropics, but most people would consider it fairly tropical (microclimates notwithstanding). There are very few plants that we can't grow because of heat requirements, but a lot that we can't grow because of chill requirements. When you have to start worrying about anything not making it through the winter, then you've dropped out of the tropics, and if there's an actually freeze (below 32F) then you're out of the subtropics.

I'm sure there are plenty that would quibble with that, and perhaps rightly so, but that's kind of an oversimplified working definition for me.

Only a few of Hawaii's uninhabited reefs and shoals are not situated within the tropics. Kauai, the northernmost inhabited island, sits right on the 22nd parallel. The Tropic of Cancer is 100 miles to the north.

Oops, you're right, DS! Somehow I had it in my mind that we were just on the other side of the line.