Why Mulching Needs to be Part of your Organic Gardening System

The word “mulch” comes from the old English word “melsc” – meaning rotten hay. In today’s language it has come to mean any material that covers the soil to preserve moisture content, prevent soil erosion and inhibit weed growth. For organic gardening I choose materials that will break down over time, feeding my plants and contributing to the amount of humus in the soil.

Many materials are suitable to use as mulch, such as: leaves, straw / hay, sawdust, gravel / rocks, paper / cardboard, grass clippings, carpet underfelt and even plastic. Each one has its own benefits and disadvantages.

Dark mulches warm the soil, whereas light coloured mulches will keep the soul cooler. In a cool climate a light straw mulch will hold back the development of many hot season vegetables – so take care with your choice of mulch and the time of year you apply it.

I’ve heard it said that mulches can be a refuge for problem garden pests, but nature balances this with enough predators to consume any rise in pest numbers.

Leaves are the most natural mulch of all. However most of the nutritional content has been taken from the tree before the leaf falls to the ground. Many leaves contain tannins and some have growth suppressants (eucalypts & pine needles for example), so it’s better to either add them to your compost heap or place them in a wire container and allow them to decompose for a year or so and become leaf mould, them use as mulch.

Straw / Hay is my preferred method of mulching in my organic food garden. The main advantage over many mulches is that it slowly releases nutrients to feed the plants it surrounds. One disadvantage is that hay may contain weed seeds, but they are usually easy to pull. That is why I prefer pea straw – usually the only weeds are peas and they add nitrogen to the soil. Another problem can be that it may become water repellent. But this is not a problem if you trickle or flood irrigate your food plots.

Sawdust is probably best used by composting it before laying as a mulch as it may rob the soil of nitrogen if your soil is poor to begin with. Also, it can become water repellent. However if you have a good supply it makes an excellent soft, natural looking covering for pathways.

Gravel / Rocks are best used outside of your veggie garden unless you live in a cool climate area and use them around warm climate plants, such as pumpkins and tomatoes. Rocks store heat from the sun during the day and slowly release it through the night.
They can also be used in arid areas around larger plants and trees. Water condenses on the underside of the rocks as they cool during the night helping to keep plants moist.
The disadvantage with rocks is that weeds will grow around them.

Paper / Cardboard are both quite useful as mulches. I often use thick layers of newspaper (which I wet before laying) underneath pea-straw or pine bark. The layers need to overlap about 15cm to prevent weeds from coming through. Don’t use pages with coloured ink as they may contain heavy metals.
Cardboard can make a great mulch under young trees. You can secure it with rocks in a decorative way in addition to straw or bark. Using cardboard beneath sawdust for your garden paths will prevent most weeds.

Grass clippings can be utilised as a thin mulch under trees and shrubs that will feed your plants as it breaks down. Take care not to pile on too thickly though as you will end up with a water repelling, smelly blob!

Carpet underfelt makes an excellent mulch in your organic garden. It won’t blow away, it’s easy to cut to insert your seedlings, it allows air to penetrate and it holds moisture very well. You must make sure that it is the older underfelt though, not the modern foam type.

Black plastic has the advantage of being cheap, easy to install and a great weed suppressor, but its disadvantages are many. It doesn’t feed the soil, it deteriorates with direct sunlight and doesn’t allow the natural gas exchanges between the air and soil.

Whatever your choice of mulch, your organic garden will be more productive and well balanced if you choose a feeding kind of mulch. Remember too that mulches should not come into contact with the stems of you plants as this may cause them to rot.