Getting to Know the Keys

The Florida Keys are a unique chain of subtropical islands that extend south from mainland Florida to the Dry Tortugas. The main islands of the Keys total 65,443 acres and are connected by the Overseas Highway from Key Largo to Key West.1 Each of us has our reasons for living in the Keys, and many of those reasons have to do with the surrounding ocean waters. As much as our current environment is influenced by the surrounding waters of the Atlantic Ocean and Florida Bay, the geological history of the Florida Keys and mainland Florida was completely formed by the ocean until very recently. About 95% of Florida’s geological history has taken place underwater. This unique geology offers several challenges to the gardener in the Keys.

Geology

by Jeanette Hobbs

Geologically speaking, the emergent Keys are very young, but their formation began with that of mainland Florida millions of years ago. The Keys sit on some 20,000 feet of marine sediments which began depositing hundreds of millions of years ago.1 Present-day Rebecca Shoals was forming as a young barrier reef about 140 million years ago. But, it was not until about 25 million years ago that mainland Florida began to emerge from the sea and terrestrial deposits began to form there. Since that time, the coastline of Florida has changed numerous times due to rising and falling ocean levels. About 20 million years ago, the Wisconsin Glacier lowered sea level about 450 feet from present-day levels, and the Keys islands were completely surrounded by dry land!2

The Keys themselves are formed of two distinct substrates, but both are types of limestone. The Upper Keys are actually remnants of a coral reef which emerged from the sea about 100,000 years ago (in contrast to the millions of years ago that mainland Florida emerged from the sea), as ocean levels dropped.4  This ancient coral reef formation is known as the Key Largo Limestone formation and extends north into the mainland for some distance. This limestone is actually the calcium skeletons of coral and is very porous, with large solution spaces. These large pore spaces mean that water moves very freely through the limestone in all directions. Therefore, rainwater drains quickly from the soil, but salt water may also rise up into the soil.

The Lower Keys are formed of a substance known as oolite. Oolite is a medium to hard limestone that forms in shallow, warm ocean waters. The higher temperatures and higher evaporation in shallow waters raise the concentration of calcium carbonate particles in the water. These small particles cling together until they are heavy enough to sink to the bottom, where over time they are packed into solid limestone rock.3  While oolite is a porous stone, water cannot move quite as freely as it does through the coral rock of the Upper Keys because the solution spaces in oolite are smaller. The most current information sources place the boundary between the Upper Keys coral and Lower Keys oolite between Upper and Lower Matecumbe Keys.1,4

In most areas of the Keys, the natural soils over our limestone rock are still only a few feet above sea level, so most homes are built on several feet of fill. This fill is usually crushed limestone that was often the dredge spoil of canals. Both the rocky fill and our native soils and rock are alkaline due to their calcium origins. Alkaline soils and salt water are both problems for Keys’ gardeners.  This is the price we pay to live in this unique and beautiful environment.

  1. Monroe County Year 2010 Comprehensive Plan Technical Document
  2. Webb, S. David (1990).  Historical Biogeography.  Ecosystems of Florida.  4, 70-100.
  3. Hamblin, W. Kenneth (1992).  Earth’s Dynamic Systems.  114-116.
  4. United States Department of Agriculture (1995).  Soil Survey of Monroe County, Keys Area, Florida.  3.

Hydrology 

by Joy Tatgenhorst

Hydrology is the cycling of freshwater, originating as rain, moving onto the earth’s surface, evaporating into the atmosphere, and cycling back to rain. The water cycle in the Keys is different than anywhere else in the U.S.

A lot of plants love it here. During the summer, weather conditions are much like a tropical rain forest, steamy and hot. The daytime temperatures and the relative humidity are close to the same. There are frequent afternoon showers. The dew point is reached nightly leaving plenty of condensation on plant leaves. Some of the dew water is reabsorbed during the morning hours. The rest comes from rainfall.

Rainfall

The annual rainfall in the Keys is approximately 42 inches, with the majority of it occurring in the summer and fall during the hurricane season. This amount is relatively less than the Miami/Everglades areas of South Florida. The rainfall pattern in the Keys is caused by our small land mass, where evaporation and cooling from the plant canopy is conducive to afternoon showers.

In the Florida Keys, traditionally, winter and spring are extremely dry seasons with summer and fall being extremely wet. Hurricane season starts in June, but in the Keys, we start watching storms in August, September, and October. During these tropical storms, it is not unusual to have 10-20 inches of rain in a 12-hour period. Water spouts are common over the surrounding waters, but rarely come onto land.

Most of the islands are 4-6 feet above sea level. Maximum elevation is 18 feet in just two locations in the Keys, occurring in Windley Key and Solares Hill in Key West. Because of this, the Keys are extremely susceptible to storm event saltwater flooding. During heavy rainfall events, rainwater and floodwater run off the islands and into surrounding waters relatively fast because the land mass is narrow. The Keys are less than 1/4 mile wide in many places.

Saltwater

The Key’s geology is unique. The bit of land left above the water table is surrounded by salt water: Florida Bay to the north, the Gulf of Mexico to the west, and the Atlantic Ocean to the east and south. Water evaporates from these waters, as well as surface waters, manmade waterways, and plant canopies. 

Fresh Groundwater

Under this string of islands called the Keys, there are two sources of groundwater. The deeper groundwater is the Florida Aquifer which is separated from the above by sand, making this aquifer less permeable to surface water infiltration. The Biscayne Aquifer is the shallower source of groundwater, housed in more permeable Key Largo limestone. The Biscayne Aquifer is approximately 100 feet thick and is housed in Miami oolite. It is the source of the potable water that the Florida Keys Aqueduct Authority pipes down the Keys. 

Freshwater Lenses

There are nine major fresh water lenses in the Keys under the larger islands. Many people on Big Pine Key and Key West have wells feeding from these lenses. Well water is used most heavily in the dry season, which can cause saltwater intrusion. It is important to test well water for salt toward the end of the dry season to determine if salt water contaminates the well water. Using well water for landscape irrigation is recommended (providing there is low salt). However, residents should not use this as potable water for drinking and bathing, due to septic, and cess pool  contamination.

Climate

by Betsy Marlowe and Tina Henize

The tropical climate of the Florida Keys attracts residents as well as tourists. The Keys weather is characterized by warm, humid summers and mild, dry winters. Frost is an extremely rare occurrence in the Upper Keys; therefore, the Keys are classified as USDA Hardiness Zones 10b, 10c, and 11 (sometimes listed as zones 11a and 11b).

The weather in the Keys differs from that of mainland Florida due to their small land mass and the fact that they are completely surrounded by warm waters- Florida Bay and the Gulf of Mexico to the north and west, and the Atlantic Ocean to the south and east. The influence of these waters is to temper summer heat as well as moderate the continental cold fronts approaching from the north during winter months.

Daily variations between high and low temperature are only about 10oF throughout the year. The table below shows monthly average temperatures in Key West (Lower Keys) and Tavernier (Upper Keys). Note that average temperatures are slightly less moderated as you approach the mainland.

The 42 inches of annual rainfall is considerably less than the annual 50-60 inches mainland South Florida receives, since the island chain has a relatively small land mass and no large land masses to the west (with the exception of Key Largo). In fact, average annual rainfall decreases with distance from the mainland, as illustrated in the table below.

The record monthly rainfalls indicate another important characteristic to the Florida Keys gardener. Rainfall tends to occur in connection with tropical storms, and a large amount of rain can occur in short time periods. This phenomenon is common in the wet season, which is also hurricane season (June 1 to November 30), and occurs occasionally during the dry season (December to May). Short term flooding can happen in any month, as can weeks of little or no rain!

The storm surges, heavy rains, and high winds associated with tropical storms and hurricanes can topple plants and damage plants with flooding and salt exposure. Winds of tropical storms and hurricanes can exceed 74 mph. Fortunately for avid gardeners, there are many beautiful plants that thrive in our unique conditions.

Temperature and rainfall for Key West (Lower Keys) and Tavernier (Upper Keys)

Plant Communities

by Tina Henize 

The Florida Keys natural habitat is comprised of varied plant communities (groups of species adapted to the existing conditions of an area). Plants may exist in one or several communities, with soil and salinity as important determining factors. If you are choosing plants for the landscape, consider looking at the plants growing in nonresidential undisturbed areas close by.

Mangroves

The main shoreline community of the Keys is the mangrove community. This is found on out islands often flooded by tides, in coastal areas along waterways, in the flat coastal fringe where there is little wave action but good tidal flow, and in low inland areas where minimal runoff and tidal flow occur. The ground is loose, wet, and saline. The mangroves serve as the nursery for the primary end of the nearshore marine food chain. They also protect, stabilize and over the long term, assist in developing the shoreline. The aware property owner respects the value of their mangrove fringe. Red, black and white mangroves and buttonwoods are all exceptionally salt tolerant trees found here, as well as saltwort and glasswort, and a few other low ground cover type plants.

Beach Berm/Dunes

Most Keys coastal areas do not have naturally occurring beaches and dunes. Beaches and dunes are found only where there is enough current to carry large quantities of sands through openings on the reef to be deposited on shore with stabilizing seaweed. Dunes are always unstable, and the pioneer plants (sea oats, salt marsh grasses, etc.) are extremely important in holding the dunes of the Keys. Further up the berm one may find bay beans, sea lavenders, bay cedar, scrub briers, sandspurs, salt-bushes, golden creepers, beach elders, lantana and sea oxeyes, plus larger shrubs and stunted sea grape, buttonwood, black bead, black torch, seven year apple, and even joewood. 

Transitional Wetlands

These secondary wetlands are inundated only part of the time. They filter storm water runoff and absorb nutrient runoff from uplands, benefiting mangroves and near-shore communities. They also provide shallow feeding grounds for shorebirds and wading birds, and serve as buffers separating uplands from non-developable tide-influenced areas. It is a stressful plant environment of rock and oxygen-poor soil with often extremely high salt concentration. Some of its plants are saltwort, glasswort, sea purslane, Key grass, saltgrass, sea oxeyes, beach carpetweed, joewood, saffron plum, gulf cordgrass, black and white  mangroves, mayten, and Christmasberry.

Tropical Hardwood Hammocks

The Keys are fortunate to still have some undeveloped hardwood hammocks. In general, hammocks exist on the higher ground not subject to tidal action. Where large numbers of palms and cacti exist, the terms palm hammock (found in the Middle Keys) and a cactus hammock (found on Big Pine) are often used. In the Upper Keys, where elevations can sometimes reach more than five feet above sea level, trees grow taller in the “high hammock”. “Low hammocks” are found more often in the Middle and Lower Keys, with shorter trees on lower elevations, often next to transitional wetlands. Generally there is the layered pattern of canopy, mid-story, and understory. Some predominant canopy trees are wild tamarind, Jamaican dogwood, gumbo limbo, poisonwood, willow bustic, and ficus. Mid-story trees includes pigeon plum, blolly, black ironwood, milkbark, cinnamon bark, the stoppers, black bead, rhacoma, and a string of others, many of which can be canopy trees in low hammock. The understory has the full spectrum of trees as saplings and seedlings, plus the shrubs, woody vines and groundcovers. Some of these are wild coffee, randia, false boxwood, snowberry, cheese vine, hogplum, limber caper, and wild bamboo. Around the sun-drenched edges one finds, among others, strongbark, lancewood, soldierwood, lantana, sweet acacia, scarlet bush, wild cotton and quailberry. Underlying all is leaf layer, or humus, which holds it all together and provides the “nursery” for the next generation of plants.

Transition Pineland/Hardwood Hammocks

Here there is a mix of pineland plants with the succeeding hardwood hammock plants. This changeover is due to fire suppression, whether natural or deliberate.

Pine Rocklands

This is the rare Keys variety of pinelands. Only found in the Lower Keys, they are very sunny and dryer than other upland plant communities. The two major components of the Keys pinelands systems are 1) the presence of freshwater lenses within the Miami oolite, and 2) the periodic fires which prevent transitioning to hardwood hammocks and protection from revegetation of other species. In the most general sense, there are three layers of vegetation; the top layer holds the mature slash pines, the second layer consists of young pines, other trees, shrubs and palms and the third layer has many varieties of grasses, wildflowers, and seedlings of the two upper layers. The Keys pine rocklands include large quantities of silver and thatch palms, poisonwood, black bead, mysrine, locustberry, pisonia, wild guava, golden creeper, bluestem, dropseed, three-awn grass, ladderbrake, pine ferns, numerous wildflowers, and a few cabbage palms. (Also, on Big Pine Key, Big Pine partridge pea and wild croton are worthy of a special note).

Freshwater Wetlands

Of all the Keys plant communities, freshwater (nontidal) wetlands are the least known to the general population. The water salinities change from dry to rainy seasons, and the plants have adapted to the saturated soils. Keys wetlands occur due to the respective ability of the two different Keys geological formations to retain rain water. The porous Key Largo limestone’s rare natural freshwater sites are limited to scattered, intermittently flooded basins in hardwood hammocks and a few naturally impounded, seasonally flooded low areas. The less porous Miami oolite holds the rainwater in lenses of freshwater floating above the heavier saltwater beneath the caprock surface, or in ponds where the caprock dips below the groundwater level on the larger islands. Large sprawling buttonwoods, white mangroves, pond apples, sawgrass, ferns, orchids and bromeliads are found commonly in these marshes, sloughs, and basins. Oddly, in some areas, low pinelands are also part of the freshwater community.