Mulching: The Way to Achieve a Lush Tropical Garden

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Mulching garden beds preserves moisture in the ground and helps plants survive a long dry season. It’s an effective way of keeping weeds under control too and encourages microbial activity in the soil underneath. Image: © David Astley

If you’ve ever wondered why some tropical gardens look lush and healthy, while others don’t, there is one gardening practice that is almost always part of the reason – and that’s mulching. Other factors such as proper feeding, adequate watering in the dry season, and pest control are important too, but without mulching it’s unlikely that you’ll ever have your tropical garden looking as good as it could be.

Mulching is the process of covering the soil surface with materials to prevent it drying out, controlling weeds and keeping the roots cool. Surprisingly, in some tropical countries mulching is not a widely practiced gardening technique, but you only have to compare gardens that are mulched with those that aren’t to see the benefits that mulching provides to plants.

Types of mulch

There are two types of materials that can be used for mulching – organic and inorganic. Organic mulches are by far the best to use in the tropics because as the mulch breaks down it improves the structure of the soil and encourages microbial activity in the soil. This naturally improves the fertility and aeration of the soil, thus promoting healthier plant growth.

The most common types of organic materials that are used for mulching in the tropics are wood chips, rice hulls, nut shells, coconut fibre and sugar cane bagasse. In highland regions pine bark may be available too, but this is not as widely available in bulk as it usually is in temperate climates. If wood chips are fresh, they should be weathered for a few months in a pile, watering and turning regularly, before they are used on the garden.

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The best organic mulch for tropical gardens is a well-weathered mix of chipped garden prunings and shredded dried leaves. Image: Manfred Richter

Sawdust should not be used unless it has been well-weathered or composted and preferably mixed with some coarser organic materials such as wood chips or dried leaves. This is because sawdust on its own can become compacted and deprive the roots of oxygen. Fresh grass clippings should not be used for the same reason. It is best that they be composted for a few months and mixed with coarser materials.

Sugar cane bagasse however can be used straightaway because its fibres have been crushed and it’s a fairly coarse material, but it may not always be readily available even in sugar cane growing areas because bagasse is also in high demand as a biofuel. Any type of straw that is a bioproduct of agricultural activities makes good mulch, but sometimes farmers prefer to burn the straw to fertilise their fields, rather than baling it up for sale as a garden mulch. This is a common practice with rice straw.

Many gardening books talk about using compost as a mulch. That’s undoubtedly the best mulch of all, but it is time consuming to produce and more productively utilized in potting mixtures. Compost can be bought in bags from nurseries and gardening stores, but that would make it a very expensive mulch.

Rainforest leaf litter is another excellent mulch, but as most rainforests in tropical countries are in national parks, collecting leaf litter from the forest floor is illegal. Even if it is not illegal, it is not ethical to do so. Buying bags of forest floor leaf litter from roadside or market stalls is also not recommended because it’s encouraging an ethical environmental practice.

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Wood chips make an attractive garden mulch but check the source of the wood to ensure that they are free of chemicals like creosote or arsenic which may leach into the soil and harm your plants. Image: © David Astley

It’s better to replicate what you find on the forest floor using whatever organic materials from your own garden are available – because that’s what mulching is all about: trying to copy what happens naturally in nature. You only have to lift the leaf litter from the forest floor to see how much life there is in the soil. Insects, worms, fungi and all sorts of other micro-organisms. That’s what we want to achieve in the garden with mulching.

Inorganic mulches will not achieve the same level of microbial activity in the soil as organic mulches, but they are still useful to keep the soil from drying out and to help control weeds. Landscape gardeners generally use inorganic mulches where they want to achieve a particular design objectives (usually coloured rocks or pebbles are used for that purpose) or where bulk quantities of organic mulches are too expensive.

How to mulch

Before laying mulch on the garden, it is advisable to remove as many weeds as possible. Although mulching will often smoother weeds, some types of weeds such as nut grass will quickly grow through the mulch. It’s better to remove what you can, including the roots, before placing the mulch to reduce weeding chores later on. When weeds do grow through a layer of mulch, they are much easier to remove because the soil is kept moist underneath.

Organic mulches should be laid 5-10 cm (2-4 inches) thick but should be not be that thick around the trunks of trees or shrubs. A shallower depression should be made around plants with a trunk so that the mulch barely touches the trunk or stem. That ensures that fungi or other organisms that might cause stem rot are given a chance to invade the plant.

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Never pile mulch up around tree trunks like this. This will enable termites to more easily attack the tree and leave it susceptible to fungal diseases as well. Image: © David Astley

Straw-type mulches can be laid thicker because they will be compressed when you are walking on the mulch when tending to the garden. Straw-type mulches can be placed closer to plant stems because there is much more air circulation in the mulch compared to more compact materials.

Inorganic mulches such as crushed rock or lava, decorative pebbles or gravel can be laid at about half the thickness of organic mulches because they don’t decompose. They can also be laid much closer to the base of trees and shrubs. Black plastic and synthetic weed control fabrics are also sometimes classified as inorganic mulches, but these don’t look as nice in the garden unless other types of mulches are laid over them.

While black plastic under another type of mulch improves the weed control, it’s not recommended to use in monsoon seasons because the rain will not penetrate the plastic and will cause the mulch on top to be washed away. Some gardeners try piercing their black plastic underlay with fork prongs to allow the water to soak through, but in a heavy tropical storm this doesn’t work.

Old cardboard boxes laid flat under the mulch also works well as a weed control barrier and has the advantage of eventually breaking down in the soil. But cardboard can have the same effect as black plastic in heavy rain unless there are a lot of drainage holes in it or is already starting to break down.

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A mulching machine with an 8-15HP petrol motor is ideal for the tropical garden. Electric models are suitable for only small gardens and will not handle large branches. Always wear appropriate safety gear when operating a mulching machine.

In some countries a new mulching product made of chips of recycled rubber has appeared on the shelves of some garden stores. We don’t recommend using those in the tropics because rubber mulches contain a number of contaminants that seep into the soil, and they have been known to catch fire in very hot dry season conditions.

If you have a large garden, then it’s worth investing in a mulching machine (called garden shredders or wood chippers in some countries). Buy one with the most powerful motor that you can afford as that type will enable you to chip thick branches as well as chopping up prunings and shredding dried leaves. Over time, the mulching machine will pay for itself as you won’t have to buy mulch and you’ll be spending less on fertilisers too.

Even for smaller gardens, there are portable electric models available, but these won’t be capable of chipping thick branches. Mulching machines are not only useful for producing mulch, but they can be used to shred materials for composting. Putting vegetable scraps and dried leaves through a mulching machine before composting results in faster breakdown of the organic materials.

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