Tropical Gardening


Different plants like different types of soils.


Some plants will only grow well in acidic soils, others only in alkaline soils. You can tell whether a soil is acidic or alkaline by testing it with a pH test kit or probe. Organic material such as mulch or manure will make a soil more acidic (rainforests often have acidic soils). Lime, shell grit or coral will make a soil more alkaline (alkaline soils are often found on coral islands, or beach soils containing lots of shell grit). Where plants mentioned in the plant descriptions of this book have specific requirements for acidic or alkaline conditions, it will be mentioned under cultural details for that plant variety.



Many tropical plants need good drainage. Without it, the roots tend to rot very easily and rapidly. In general they prefer well drained soils on slopes, sandy soil, or well structured (ie. friable) soils.



Heavy rains in tropical areas will leach nutrients from the soil faster than in most temperate climates.



High temperatures and humidity often cause fertilisers to 'dump' their nutrients within a very short time. As such, so called "slow release" fertilisers may actually release their nutrients very quickly.




Before deciding how to, or even whether to improve a soil, you need to know if the soil is good, bad or in need of improvement.

Drainage can be tested easily by observing the way in which water moves through soil. Place a sample of the soil in a pot and water, watching how quickly or slowly the water passes through. Keep in mind that this is a simple test. When soil is disturbed by digging, its characteristics may change. However, this test will give you a general picture of your soils drainage capacity. Periodic sampling of the soil will enable you to understand its drainage characteristics (e.g. is a lot of water being retained, does water drain away quickly, etc.).

Soil nutrition is, to some extent, indicated by the vigour of plants growing in a particular soil. Nutrition can be easily improved by the addition of fertilizers, and over the long term by the addition of organic matter.

A well structured soil usually has a crumbly nature, with plenty of pore space (voids) between the small crumbs. These types of soils are generally easily cultivated, have good drainage, and good aeration (oxygen is need by plant roots). Anything that destroys this crumbly structure, such as over cultivation, regular trampling, or traffic of vehicles should be avoided.

Soil pH, which is a measure of how acid or alkaline a soil is, can be easily measured using a simple colorimetric test kit, available from many nurseries and garden centres.


Ways To Improve Soils Include:

Adding sand to clay soils to improve drainage. This is only practicable on a small scale (e.g. garden beds) as a lot of sand would be required to have a reasonable effect, and it would also need to be well mixed.

Adding clay to sandy soil to improve its ability to hold water. Much less clay is required to improve the water holding ability of sand than when adding sand to help improve drainage in clay. Thorough mixing is also important in this case.

Adding organic matter to any soil, except those rare ones already high in organic matter, will help improve soil structure, and hence drainage. It will also improve soil fertility, soil moisture holding capacity, and will provide a buffer against sudden chemical or temperature changes in the soil.

Adding soil ameliorants - Lime can be added to help improve structure in clay soils and/or to raise pH levels in acidic soils. Gypsum can be added to help improve structure in clay soils, without affecting pH significantly, or to help improve saline-sodic soils by displacing sodium ions from soil particles so that they can be leached out of the soil.

Using acidifying fertilizers (e.g. ammonium sulphate, ammonium phosphate) will help lower soil pH, as well as provide valuable nutrients. Repeated applications of organic matter may also have an acidifying effect over time. On a small scale, sulphur dust can be used to lower soil pH, but this is quite expensive.

Mulching will help protect the soil from erosion and compaction, control weeds, and help protect plant roots from temperature extremes. Organic mulches will add valuable organic matter to the soil as they decompose.


Generally, water plants well when growing and reduce water when growth slows or becomes dormant. Excess water may result in the roots rotting. (A particular problem with true tropicals when grown in cooler climates, even under cover). The heat of a tropical or subtropical climate will cause water to evaporate quickly from the surface of the soil, so frequent light waterings may achieve very little. Irrigation methods must be designed and used to get the deeper layers of the soil wet. This can be done by:

Watering slowly for a long period, so that the water soaks in (eg. a drip irrigation system ).

Drilling holes beside plants (eg. with a narrow auger), being careful to minimise any root damage, inserting a pipe into the hole to stop it collapsing, and then watering into the pipe, so the water begins to enter the ground from deep down, rather than near the surface.

Additions of soil ameliorants such as liquid soil wetters aid in water conservation. These products allow soils to hold additional water so that less frequent irrigations are required.

Where high levels of humidity are required to maintain healthy plant foliage, apply frequent light applications of water directly to the foliage, by such methods as overhead sprinklers, or misting systems (using nozzles or sprinklers that give small droplet sizes).



Light requirements will vary considerably from one tropical plant species to another. Those that grow naturally in open sunny conditions, such as in savannah, or in the upper canopy of rainforests generally require plenty of light. By comparison the large percentage of plants that live beneath the protective upper canopy of rainforests generally do best in filtered sunlight or partial shade. Many of these plants have large, broad leaves to maximise the amount of light they receive, and are very decorative. It is from this group of plants that many of the plants grown indoors in cooler climates are derived.

When deciding how much light your plants require, the best indication is to find out their normal growing conditions in the wild. If you find that, when growing certain plants, they are becoming tall and spindly, it is likely they require more light. If you find they are getting a bit sunburnt, or their leaf tips are perhaps showing signs of scorching, then you may need to provide some sun protection.



The first step in planting a garden is to choose the right plants. This can be a difficult task, even with expert advice. There are so many choices, and these it often comes down to personal preference.

To avoid creating problems, consider the following:

Avoid placing plants that have invasive roots near buildings, paved areas, water features, drains, and septic tanks (eg: eucalypts, ficus ).

Don't use plants that have the potential to become weeds. This includes plants that seed prolifically, are prone to sucker, or are rampant creepers (eg: wandering jew, lantana, even jasmine).

Consider how big each plant is likely to grow. You should allow sufficient space for future growth. Avoid planting large trees in places where they will eventually create problems (eg: shade out lawns or other plants, damage buildings).

Don't use trees that are likely to drop branches (eg: many Eucalypts).

Choose plants which are likely to be trouble free (ie. withstand local storms, pests & diseases, or poor soils).

Select plants suited to the tropics or subtropics. Many people become disheartened when a beloved plant dies - when all too often it was a plant that had little chance surviving in the climate.



The best way to approach planting is often the simplest.


Keep plants together in compatible groups. Keep foliage plants together, herbs together, orchids together, or alternatively plants that require similar amounts of watering, or similar amounts of shade.



Let the fittest survive.



Mass plant areas (ie. several of the one variety together) to form hedges or eye catching displays.



Be prepared to get rid of plants which don't work (either don't look good or don't grow well). There's no harm in being ruthless.



Use reliable plants that neighbours are growing successfully, unless you wish to experiment. If so research the requirements of your plants well, or be prepared to lose a few along the way.


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