croton (Codiaeum variegatum pictatum), New Providence Island, Bahamas

Anyone with more than a passing interest in plants should visit the tropics. A few hours of plant watching in the tropics provide an appreciation for the conditions that promote – or inhibit – plant growth anywhere on earth.
Recently, I was on a Caribbean cruise ship that stopped in the Cayman Islands, south of Cuba, and in Cozumel, an island due west of the Caymans off Mexico’s Yucatan peninsula. The Caymans and Cozumel lie just below the Tropic of Cancer, an imaginary line that runs 23-1/2 degrees north of the equator and is the unofficial northern border of the tropics.
There are two major types of climate on Earth: tropical, where the weather is more or less constant year around, and temperate, where distinct changes in weather occur, depending on the time of year. Temperate zones have seasons; tropical zones do not.
The unchanging climate in the tropics is due, primarily, to the consistently high position of the sun in the sky. In the more temperate U.S., the position of the sun changes from season to season; in the summer, the sun is highest in the sky, whereas it dips toward the southern horizon as winter approaches. Awareness of the sun’s changing position is important when we design our gardens. A full sun exposure during the summer may change to part sun or even shade during the winter, depending on the proximity of tall trees and buildings.
In Los Angeles, the sunniest exposure is southern. Eastern and western exposures get equal amounts of light, but of different quality; eastern (morning) sun is brighter, but western (afternoon) sun is hotter. Northern exposure is the least sunny.
As you reach the tropics, landscaping decisions made on the basis of exposure or light requirement lose significance. In Los Angeles, annual vinca (Catharanthus roseus), a heat lover, does best planted on the south or west side of a house or building. In the Cayman Islands, I saw vinca growing on all four sides of a building. I also saw plants thriving in full sun – such as pothos, croton and syngonium – that, in Los Angeles, are only grown indoors. On Cozumel, pothos (Epipremnum areum) leaves reach 18 inches in length.
You soon realize that when our light-loving indoor plants fail to grow outdoors, it is not because of too much sun or heat, but because of too little humidity. In the tropics, the sun shines longer and with greater intensity than in Los Angeles. The difference is that the humidity – the amount of water vapor in the air – is significantly higher in the tropics. The higher the humidity, the less a plant is stressed.
The soil in the Cayman Islands and on Cozumel is sandy (nonretentive of water) and poor in minerals, but high humidity compensates for both of these horticulturally unfavorable conditions. High humidity means that a plant’s leaves can stay moist without having to rely solely on water brought up from the roots; high humidity also causes rapid decomposition of fallen leaves and other plant debris, creating a thin, but steady supply of minerals – a kind of slow release fertilizer – around feeder roots near the soil surface.
Air temperature is another factor responsible for the luxuriant growth seen in the tropics. Rarely, even at night, does the temperature fall below 60 degrees Fahrenheit. Under such conditions, plant growth is nearly constant, and dormancy may never be experienced.
Two groundcovers in the commelina (wandering Jew) family are seen everywhere in the Grand Caymans and on Cozumel. Purple heart (Setcreasea purpurea) has long, purple elliptical leaves and pink flowers; it is found in many Los Angeles gardens, where it requires good light, protection from harsh summer heat, above average water and no fertilization. Rhoeo spatheca, another common tropical groundcover, has leaves that are dark green on one side and purple on the other. In Los Angeles, it can easily be grown indoors in any well lit room.
Tip of the week: If you have been planning to seed a new lawn or overseed an old one this fall, don’t delay any longer. As temperatures get colder, it becomes increasingly difficult for grass seed to germinate.