Gardening - Why Mulch?

by Craig Elevitch and Kim Wilkinson
Permanent Agriculture Resources
PO Box 428, Holualoa, Hawaii 96725 USA
http://www.agroforestry.net © 1998

Why Mulch?

Agriculture with mulch in the tropics promotes plant health and vigor. Mulching improves nutrient and water retention in the soil, encourages favorable soil microbial activity and worms, and suppresses weed growth. When properly executed, mulching can significantly improve the well-being of plants and reduce maintenance as compared to bare soil culture. Mulched plants have better vigor and, consequently have improved resistance to pests and diseases.

"Mulch" is a layer of decaying organic matter on the ground. Mulch occurs naturally in all forests; it is a nutrient rich, moisture absorbent bed of decaying forest leaves, twigs and branches, teeming with fungal, microbial and insect life. Natural mulch serves as a "nutrient bank," storing the nutrients contained in organic matter and slowly making these nutrients available to plants. All forms of plant life from the ground layer to shrubs and trees thrive, grow, shed organic matter, die and decay, in a complicated cycle of nutrients.

Mulch forms a necessary link in nutrient cycling vital for our soils. When mulch is absent for whatever reason, the living soil is robbed of its natural nutrient stores, becomes leached and often desiccates. Natural environments without a litter layer are usually deserts. Non-desert plants grown in bare soil require constant fertilization, nutrient amendment and water, not to mention the work required to keep the soil bare.

Sheet mulching as described here is a suggested method for controlling weeds and improving soil and plant health with mulch. The process mimics the litter layer of a forest floor.
Basic Techniques of Sheet Mulching

Once you get the hang of it, sheet mulching can be used anywhere plants are grown in the ground. Sheet mulching may be used either in establishing a new garden or tree planting, or to enrich existing plantings. In both cases, mulch is applied to bare soil or on top of weeds. New plantings are planted through the mulch, and a small area is left open to accommodate established plants and trees.

The benefits of mulching justify putting the energy into doing the job right, using ample materials. Collect all of the materials (as outlined below), and complete the mulching process in a day. A reduction in maintenance and increase in plant vigor will reward the initial effort.

Sheet mulch is put down in layers to mimic natural forest mulch: well decayed compost, weed barrier, partly decayed compost and raw organic matter.

How to sheet mulch
Step 1: The Concentrated Compost Layer

To prepare the site, knock down tall weeds and woody plants with a brush cutter, scythe, or simply by trampling the area. Then proceed to lay down the sheet mulch.

Whether you are mulching bare soil or weeds, the first step is to "jump start" microbial activity by adding enriched compost, poultry or stock manure, worm castings or the like at the rate of about 50 lbs/100 square feet. This high nitrogen matter stimulates soil life and gets things going. If the soil is acid, which it likely is if the area has been disturbed recently and treated with conventional fertilizers, add a layer of lime or crushed coral. A soil analysis will indicate the need for adjustment of pH or mineral amendments. This is the appropriate time to add the recommended doses of amendments such as rock phosphate and K mag.

Soak the area well with water when the amendments are dispersed.

Step 2: The Weed Barrier

Most cultivated areas today harbor untold numbers of weed seeds. There are also weed seeds carried around by wind, animals and people. Soil borne seeds are lying dormant and waiting for the right conditions to sprout. Simply pulling or killing growing weeds will not erase the weed problem: more seeds will sprout almost as soon as the soil is exposed to moisture and light. Therefore the next step in mulching is to put down an organic weed barrier. This barrier prevents the germination and eventual emergence of weeds through your mulch.

Underneath this weed barrier grasses and weeds die and quickly become food for earthworms. From now on, the worms turn and aerate the soil, as they do naturally when in the right environment.

Of the four sheet mulch layers, the weed barrier has no natural counterpart on the forest floor. In the forest, weeds do not sprout because there is "no room for them," which simply means a lack of space above and below the ground, and a lack of light. By planting an area properly, there will eventually be no room for weeds. The weed barrier is needed only for establishment of the mulch, and disappears with time. If your area is planted appropriately, weeds will not emerge after the decomposition of the weed barrier.

Materials for the weed barrier that work well are: 4-6 sheets of newspaper, cardboard, burlap bags, old rugs of natural fiber, worn-out jeans, gypsum board, or whatever you can find around. Banana, ape and ti leaves also work if laid down in several layers. Overlap the pieces of the material so as to completely cover the ground without any breaks, except where there are plants you want to save. Around these leave a generous opening for air circulation around the root crown. Care in laying down the weed barrier will save you the headache of emerging weeds later on.

Step 3: The Compost Layer

This layer is on top of the weed barrier - it must be weed seed free. Well conditioned compost, grass clippings, seaweed and leaves are ideal materials to spread over the weed barrier. Any weed-free material mixture at the right moisture level for a good compost will do. This should form a fairly dense layer about 3 inches thick.

Step 4: The Top Layer

The top dressing mimics the newly fallen organic matter of the forest. It also must be weed-free. Good materials for this include leaves, twigs and small branches, fern or palm fronds, straw, coffee chaff, macadamia nut shells, wood chips, sawdust, bark, etc.. The top layer will slowly decompose into lower layers, and therefore must be replaced periodically; it represents reserves of compost. This layer should be about 3-5 inches deep. Many materials suitable for the top layer often have a pleasant cosmetic appearance. What luck! For this reason, there should be no hesitation in using sheet mulch in all cultivation from landscaping to gardening to permanent orchard crops. In fact, as you use mulch, bare soil will begin to seem ugly and undesirable.

When the soil is amended and sheet mulch applied properly, there will never be a need to turn the soil. Earthworms do the tilling. The only task left will be to keep the soil covered by replenishing the mulch.

Warning: Feral pigs love good, moist soil, and will grub in sheet mulch if they have access to it. Do not use sheet mulch if pigs have access to the area; they will be attracted to it and will destroy both your work and your plantings.
Mulching Around Trees

1) Plant tree.

2) Amend soil around tree in a wide ring shape from a few centimeters from trunk out to 1 meter (3 feet) with a light layer of nitrogen fertilizer, such as chicken manure, and other amendments if necessary. Rake or water in.

3) Spread a layer of permeable weed barrier around the tree in a ring shape, leaving about 15 cm (6 inches) diameter around the trunk of the tree for it to "breathe." Make certain there are no gaps in the ring shape through which weeds can emerge. Water the weed barrier layer thoroughly before the next step.

4) Spread compost and/or mulch about 15 cm (6 inches) thick over the weed barrier, again making sure it is several centimeters away from the trunk of the plant.
The Ongoing Process

To make mulching as efficient and easy as possible, use mulch materials which are readily available. With good planning, mulching of gardens and orchards can become a regular part of maintenance-just mulch with handy materials such as grass clippings, plant prunings (chipped or roughly chopped), animal bedding, etc.. Eventually, other tasks such as watering, fertilization and weeding will be reduced. The overall maintenance burden in mulched conditions, when properly executed, is far less than in conventional systems.

Once a plant is properly mulched, its own leaf drop will constantly add to that mulch. But is natural leaf drop enough to maintain the mulch? The answer to this depends on the plant species and also how the plant is growing in relation to other plants. Certain trees produce tremendous amounts of leaf matter which decomposes rather slowly; examples are: avocado, macadamia, lychee, as well as many others. These trees can be expected to generate sufficient mulch for themselves once vigorous growth is attained. Unfortunately, under most conditions many trees do not create enough long lasting mulch for maintenance of their needs. To explain this apparent deficiency, look once again at the forest. Here, plants are "stacked" in the vertical direction in ground-level, middle, and tall vegetation. This means that the ground under each plant receives organic matter from several plants.

There are many ways to produce sufficient mulch at your site. Grass clippings, for example, represent nutrient rich mulch material. Deep rooted, vigorous growing plants that readily come back from hard pruning or coppicing will also work. There are several nitrogen fixing trees which produce copious amounts of green matter. Each should be evaluated for the specific site before planting. Other plants that work well are kukui, hau, desmodium,, various bunch grasses (such as Guinea grass), lemon grass, comfrey, etc.. Also, many water plants such as water hyacinth are good mulch materials. Since plants that produce heavy amounts of organic matter are by their nature nearly irrepressible, extreme caution should be taken not to let these plants escape your management and become weedy.

Sheet mulching should not be confused with composting, artificial weed barriers, or green manuring. Sheet mulching as described here is quite different from these in that it seeks to recreate the organic mulch layer of the forest with a minimum of effort from people. Properly planned, a backyard or orchard system will produce its own raw mulch in sufficient amounts and people are involved only in putting this material back onto the ground where it belongs.

References and further reading:

Molly Curry's article, "Sheet Mulch Now!" in The Permaculture Activist, issue No. 34-A, August 1996. Order from The Permaculture Activist, P.O. Box 1209, Black Mountain, NC, 28711, USA.

Bill Mollison's excellent Permaculture: A Practical Guide for a Sustainable Future, published by Ten Speed Press and available from bookstores.

ECHO's informative, THICK MULCH FOR NO-TILL GARDENS

Ruth Stout's No Work Gardening Book, published by Rodale Press, is an excellent reference but out-of-print and hard to find.

Agroforestry Net, Inc.
PO Box 428
Holualoa, Hawaii 96725 USA
Contact us: email@agroforestry.net