Lesson 4 - Mulch
A mulch is a material used to cover the soil surface and include organic materials, such as bark or straw; mineral materials, such as coarse sand and gravel; and fabric or sheet materials, including both natural fabrics and plastics. Mulches provide three main benefits. Firstly, they increase the retention of soil water by reducing evaporation, which in turn can reduce irrigation volumes over drier parts of the year. Secondly, mulches supress weeds by creating a physical barrier that limits weed germination and emergence, particularly annual weeds, reducing the time and inputs in weed control. The third main benefit, particularly with organic mulches, is their ability to regulate soil temperature by effectively insulating the soil surface and reducing temperature extremes in the rootzone, improving plant growing conditions.
There are other benefits as well, often specific to the types of mulches being used. Organic mulches can, over time, significantly improve soil health, by adding organic matter as they decompose, increasing soil biological activity, reducing compaction and improving infiltration and soil structure. Mulches can also reduce erosion by preventing rain and wind from wearing away the surface of the soil, an outcome aided by careful planting, especially on sloping sites. Finally, many people simply like the look of mulches, especially in combination with planting.
Diggers tip
Keep your cardboard packaging! Remove any tape and staples and use these for no dig gardens and to lay on the ground and cover with wood mulch to suppress weeds.
Organic mulches are those derived from natural materials, and include bark, wood chips and leaves, as well as processed crop wastes, such as straw, coir (coconut fibre), rice hulls and bagasse (sugarcane fibre). As noted earlier, organic mulches decompose but the rate of decomposition varies based on their woodiness and particle size. Some wood and bark wastes can take many months or even years to break down completely, particularly if they are mainly chipped from branches and wood. Organic waste mulches, particularly those from herbaceous plants such as straw, bagasse and rice hulls, break down much more quickly, generally within a year of application. The particle size of mulches also affects the rate of decomposition with smaller materials (i.e. 2-10 mm in size) breaking down more quickly than larger particle materials (20-50 mm).
Mineral mulches are made from mined and crushed minerals including different gravels and sands. They are processed by screening which means they have a defined particle size or range, such as pea gravel (5-10 mm), decomposed granite (10-20 mm), scoria aggregate (7 mm). Mineral materials work best as a mulch when they are large and relatively uniform so they can assist with weed suppression and water retention. Mineral materials with a large proportion of ‘fines’ are great for driveways but they compact readily and will not function as a mulch. And while mineral mulches are expensive and won’t provide benefits to the soil as organic mulches, they are very long-lasting and have strong aesthetic appeal. In growing food their use is mainly restricted to woody plants such as fruit trees.
Fabric or sheet mulches include those made from natural fibres such as jute or recycled cotton or synthetic products including those made from polypropylene or polyester. Produced in rolls which are then laid on and then pinned or pegged to the ground surface, they are often described as landscape fabrics or weed matting. They are used mainly to assist with weed control, soil moisture retention and erosion control. Due to the initial costs in materials and installation, plus concerns over plastic pollution, most fabric mulches have limited uses in food growing. Some crops, such as strawberries, are commonly grown with fabric mulches, often in elevated beds with drip irrigation. Fabric mulches can also be useful in preparing a new site for vegetable growing. This method involves treating any perennial weeds first and then fixing the mulch across the site for several months. This will supress any new weeds from emerging and help prepare the site for cultivation for a new garden bed.
Diggers tip
Try to establish a closed mulch loop in your garden. Perhaps invest in a chipper so that any larger branches can be turned into mulch and returned to the garden. Make sure the mulch is aged for at least weeks before applying it to your beds.
In most food growing situations organic mulches are the best to use, but some care is needed. If using wood wastes, especially fresh chipped green waste from arborists or municipalities, it’s best to compost this material for about 6 weeks first. If you can’t compost, then age the material in a pile for 6 months, or even use as a path surface for 12 months, before reusing it as a mulch. These methods help to remove phytotoxins and improve the C:N ratio, reducing the effects of ‘nitrogen drawdown’ from the mulch. For microbes to successfully decompose carbon-rich materials like fresh, woody mulches, they consume nitrogen, temporarily ‘drawing it down’ and making it unavailable for plant use. This nitrogen drawdown is temporary and as the woody materials decompose available nitrogen starts to return to the soil. However, it can result in plants growing in these mulches to show symptoms of nitrogen deficiency, such as leaf yellowing or growth stunting. A situation even worse when those plants have high nitrogen needs, such as most vegetables. It’s the reason you should never use fresh high-carbon mulches when growing these types of plants. Woody mulches can be used in other food growing situations, such as around young fruit trees, and applying a high nitrogen fertiliser, such as urea or blood and bone, to the soil surface before applying the mulch can help to compensate for the nitrogen lost by decomposition microbes. There is no problem using woody mulches around mature trees.
Some care is also needed when using very fine mulches, where most of the particles are less than 10 mm in size. These materials pack down after application, reducing water infiltration and sometimes causing wettability problems at the soil surface. Fine materials can also readily absorb any water that falls on it, rather than it draining to the soil. Examples include shredded green leaves and lawn clippings (high nitrogen) and sawdust (high carbon). Materials such shredded leaves and lawn clippings are best composted first, while fresh sawdust should never be used as a mulch. It takes years to decompose and uses plenty of nitrogen in the process!
Care is also needed with the presence of weeds in some mulches, particularly in green waste composts and some straws (e.g. Lucerne, Pea Straw). While this can be a minor nuisance, in some cases they can be a significant source of weeds. Hay for example is generally full of seeds which will readily germinate in a new garden bed, including common weeds such as clover and ryegrass. Unless you know the product to be weed free then don’t use it.
The best mulches to use are those where most particles are large and largely uniform in size. The Australian Standard that covers mulches (AS4454-2012 Australian Standard for Composts, Soil conditioners and Mulches) requires that for a high-quality product the majority of particles (80%) should be between 20-30 mm in length and 5-10 mm in thickness, with none exceeding 50 mm in size. A mulch with these properties will ensure that there is good air circulation while still retaining moisture within the soil.
Mulches should be applied to the soil to a depth of 50-75 mm. Never apply organic mulches too deeply (i.e. >75 mm), especially those that are woody and/or have a higher proportion of fines. For herbaceous plants, such as vegetables, always use mulches that have come from a herbaceous source and do not have nitrogen drawdown problems. This includes bark mulches, particularly pine bark, and organic materials such as straw and bagasse. Some barks are also screened to produce a uniform size that meets the Australian Standard. These are expensive but will last a couple of years in most growing situations.
Diggers tip
When growing vegetables, we recommend using sugar cane mulch or pea straw.
Further learning
Further learning
- The Mulch Book by Mary Horsfall (New Holland Publishers, 2011)
- Choosing mulch - Gardening Australia