Mulching is a fundamental part of gardening to plants looking beautiful and productive through the year. If you don’t already know how to mulch, it’s important to learn when to mulch, the right depth for mulch, and the right type of mulch. 

WHAT IS MULCH?

At its simplest, mulch is any material that covers the soil’s surface. In nature, mulch is simply fallen leaves and plant debris. In the garden, mulch can also include compost, wood chips, rotted manure, cardboard, or even seaweed.

It’s only recently that we’ve come to appreciate mulch’s sustainable and ecological benefits. Done correctly, mulching feeds our soil’s living microorganisms with nutrients and the waste from these tiny microbes creates healthier soil structure for plants, limiting compaction.

BENEFITS OF MULCHING

1. Reduces weed growth by keeping light from reaching the soil surface.
2. Reduces water loss from the soil surface, which helps maintain soil moisture.
3. Moderates soil temperatures, keeping it warmer on cold nights and cooler on hot days.
4. Protects bare soil, reducing erosion and soil compaction.
5. Protects plants from the harsh conditions of winter freezes, thaws, and winds.

There are many other benefits of mulch:

• In winter, soil under mulch will be warmer than unprotected soil. This protects plants from the cycle of freezing and thawing (which can heave them out of the ground).
• Prevents crusting of the soil surface. Water moves more readily into soil covered with mulch instead of running off.
• Keeps soil from splashing onto leaves; keeping soil off leaves reduces the chance of plants getting fungal and bacterial diseases.
• Breaks down and feeds the soil (if organic mulch).
• Improves the structure of clay soils and the moisture-holding capacity of sandy soils.
• Slowly increases soil fertility (if organic) and may make micronutrients already in the soil more available.
• Warms the soil in spring, allowing the gardener to plant days or weeks before the soil would normally be ready.
• Keeps plants clean and off the ground, especially tomatoes and melons, to avoid plant disease.
• Limits the chance of damaging trees’ trunks when mulch is placed around them instead of grass.
• Improves plant health and growth (due to fewer weeds and more consistent moisture and soil temperature).
• Makes gardens “spiffed up” and attractive, giving a uniform appearance and rhythm to garden design.

Commonly Used Organic Mulches

• Timber by products - from untreated timber including: sawdust, wood shavings, bark and wood fibre. Wood products can take several years to decompose.
• Straw, hay, sugar cane - The finer cut length the easier to apply and work with. Ideally source from certified sources.
• Coconut husk fibre- can be bought in a bale or a dehydrated block that can be rehydrated in a wheelbarrow and used as a mulch.
• Seaweed – loose only (not growing) collected from the shoreline of unpolluted beaches (permit may be required in some areas). 
• Dry leaves – Collected from gardens, parks or other areas that are not adjacent to roadways. Leaves should be composted first to create leaf-mould before applying as mulch. Eucalypt and conifer leaves have toxins which harm some plants. If unsure, test in a smaller section of the garden.
• Manure – should be well rotted i.e. covered and allowed to mature for a month or more. Composting of manures in this manner turns the free nitrogen and potassium into a usable form that is less damaging to plants and also less likely to leach. Can bring in weed seeds. Should only be sourced from an ‘organic system’.
• Mushroom compost and general compost – sourced only from organic production systems.
• Paper – use under mulch.

*Note: straw, hay and farmyard and horse manures from non-organic (non-intensive) sources may be used in an organic system after being aerobically composted for three months or stockpiled for six months to allow the break-down of chemical residues that may be present.

General Rules

• Most un-composted organic materials (straw, hay, shavings, leaf litter etc.) will draw nitrogen from the soil, starving plants of nutrients. To avoid problems top dress with slow release nitrogen (such as blood and bone). 
• A layer of newspaper underneath mulch, reduces the thickness of mulch needed, avoids contact of un-composted material with the soil and deters weeds.
• Mulch should be shallower around the base of the plant and not in contact with the stem, to prevent collar rot i.e. the stem at ground level starts to die off due to excess heat generated by the composting mulch and fungal infection. 
• Use contaminant free organic mulching materials.
• Light fluffy mulch will need constant renewal as it quickly settles to form a thinner layer. 
• Always water the area before applying mulch. 

Living Mulch

Living mulch is different to a cover crop. Cover crops are slashed and incorporated into the soil before establishing a vegetable crop. Living crops are either established before or after the vegetable crop and grow in association with it to improve soil structure, fertility, suppress weeds and provide habitat for beneficial insects. 

Although other pests such as slugs can increase in living mulch, it should not be susceptible to the same pests and diseases as the crop. Living mulch will compete with crops for nutrients and moisture, therefore it is not recommended for use with shallow rooted, low growing or drought intolerant plants. To limit competition, living mulch should be low growing (relative to the crop) consequently it must be a species that can germinate and grow in shade. It should not complicate or restrict the harvest.

Inorganic mulch: 

Weed matting is considered part of organic weed control methods without the need for chemicals or hard work digging them out. The weed matting is laid out and the seedlings are planted through slits that are cut into the matting. 

The downsides are it has to come out after the crop is harvested and sometimes roots or plants can get tangled in the weed matting and can make it difficult to remove if not removed immediately. Other options are to use weed matting as a permanent fixture under a pathway.