Why use a Garden Cover / Mulch?

Garden covers come in many and varied varieties of material creating a layer on the surface of a growing media.

Mostly they are organic but may also be mineral or synthetic depending on your needs.

Organic covers (mulch) are generally a slow release fertiliser that gives the plants a bit of nutrient bit by bit as it decomposes.

They have four main uses:

  • To reduce evaporation and maintain moisture in the soil
  • Alter the soil temperature
  • Reduce weeds &/or
  • Reduce the compacting effect of heavy rain or irrigation.

Mulch can be:

  • Organic matter that decomposes quickly eg. straw, hay, compost, leaves, animal manures, sea-weed, grass clippings, etc
  • Organic matter that decomposes slowly eg. pine bark, wood chips, peanut husks, rice hulls, sugarcane, shredded bamboo etc.

Garden Covers can be:

  • Mineral materials eg. pebbles, gravel, crushed bricks, etc.
  • Synthetic materials such as black plastic, weed mat and other impervious material.


You will know yourself how soil dries out when exposed to the air.

Evaporation is therefore reduced by shading the soil & reducing the movement of water lost from the soil to the atmosphere.

A 4 cm straw layer will reduce the evaporation by about 70% BUT a layer that is too thick (one more than 10 cm) will hold too much water reducing the amount of water that gets through to the soil beneath.

So your mulch will be wet but your soil won’t be unless you are watering it underneath the mulch layer with say – a drip line system.

Some mulch is great to reduce evaporation but can pack down & form a water repellent layer (like paper and grass clippings). Making holes in the layers to water through, gives weeds an opportunity to take hold and grow.

This can be overcome by applying only a very thin layer of the product or mix it with some course material to keep it open.

Again the problem can be overcome by using a drip line water system beneath the mulch keeping the compacted layers on the top reducing the evaporation while watering the soil beneath.

Mulch also slows down the movement of water ACROSS the soil, while still letting water enter the soil rather than causing erosion of the topsoil layer. Again this depends on what you are using.

Black plastic film will reduce evaporation but also reduce any water & air going down into the soil & hence to the plant roots.

Temperature control

Mulch keeps the soil cooler in summer & warmer in winter allowing plants to grow for a longer period throughout summer & winter.

Organic mulches are great to use for summer as fine feeding roots of plants will grow up into the mulch. The top few centimetres are where a lot of the available nutrients are accessible to the plant roots (remember that plant roots can only take up nutrients in solution).

Back plastic needs to be covered with mulch to stop it from getting too hot and killing your plants. It can however be a great way to sterilise your garden bed of all bacteria and microbes.

Just let it sit uncovered for 2-3 weeks exposed to the sun and the heat generated under the plastic will kill the bugs. Then of course you need to re-fertilise your soil as it has killed everything- even the good bugs.

Reduce weeds

Many weed seeds will not germinate under a mulch layer as they use up all, or most of the food in their seeds, as they try to push through the mulch to reach the sunlight. Those that do emerge will be weak and easy to pull out.

Mulches with a lot of weed seeds in them should not be used (as a general rule) as that will only increase the weed seed supply

(note from Vicki – in my experience using grass clippings full of weed seeds and reapplying them each time you mow, tends to smother any new seeds trying to grow up. I personally have not yet found it to be a problem to date. However, I also get very little time to water so perhaps it is the lack of water rather than smothering each layer that prevents the seeds from germinating. It has also been said by someone, that tossing corn meal over your grass clippings will prevent seeds from germinating. But do not do this in your veggie garden or you may not get anything re-sprouting. I will be doing a test on this – coming soon- but if anyone has tried it we would love to get your feedback)

Nut grass will pierce through almost all mulches including plastic sheeting so needs to be killed BEFORE mulch is applied. (Bush Turkeys are great at removing nut grass – so Vicki says)

Vigorous grasses (eg green couch & Kikuyu), from an adjoining lawn area, need to be controlled before installing any mulch as they tend to climb over the top of any mulch and send underground runners into the muched area.

Paper, as mentioned above, either reduces evaporation, prevents water from entering the soil, or provides spaces for weeds to enter the soil and sprout. (Note from Vicki – I have found it to work really well if you soak your shredded paper for a period of time, spread it around your garden and then cover it with mulch. You will find the worms will come up to eat it and it turns into beautiful soil. But it does take time and needs to be kept moist. And just be sure to leave holes in the layer so water reaches the soil)

Of course never use plants that have recently been sprayed with herbicides eg. grass clippings recently sprayed with weed killer.

Reduce surface compaction.

Mulch reduces the force of raindrops or irrigation landing on the soil, therefore reducing the compacting effect.

Surface compaction reduces water and air movement down into the soil (& so to the plant roots) plus it reduces the dispersal of Carbon Dioxide from leaving the soil.

When an organic mulch is used that is still a bit fresh, you may need to added Nitrogen (such as Urea, Sulphate of Ammonia – about 5g N/m2). This is because the increase of nitrogen in the soil gets the nitrogen eating Bacteria working in overdrive eating up all the available nitrogen and depriving the plant of nitrogen. It is very short lived but the plant suffers as a result.

(note from Vicki – I have noticed in my experiments in the perspex pots that worms will take organic matter from the top layers deep down into the soil reducing the compaction, aerating the soil, and allowing for better drainage. This was especially evident when I used shredded bamboo as it decomposes very slowly. It does not do the same thing with grass clippings as grass clippings break down quicker to become composted soil.)


The ideal mulch thickness is:-

  • Fine compost & fine bark: 2-3 cm
    Coarse compost & coarse bark: 3-4 cm
    Chipped tree trimmings: 4-6 cm
    Bark nuggets, chopped sticks: 6-8 cm
  • Before applying any mulch, the soil needs to be thoroughly wet so the moisture is trapped under the mulch as mulch will often take up any water applied to it before it gets through to the soil.
  • A water drip feeder under the mulch is a good way to keep the moisture in the soil and reduce evaporation with your mulch layer on top.
  • Mulches can also be used to keep fruit off the soil (eg strawberries, cucumbers) so they are less affected by soil diseases.
  • Using compostable mulch is best because you can always mix it into the soil to break down into better soil and then add more on top. You can’t do this with mineral or synthetic coverings.

Today’s Did You Know…?

Using vinegar in your garden and outdoor living area :

  • Clean scissors of sticky material or grime by wiping the blade with a cloth dipped in full strength vinegar. Great for your garden shears.
  • Kill bad smells from the Compost bucket you keep on your kitchen bench – add some bi-carb soda as well to give it a good clean out.
  • Wipe away mildew by using full strength vinegar for heavy mildew & half strength for light mildew on your outdoor garden furniture.
  • Get rid of smoke odour from a bush fire (or burnt steak or oven) by placing a shallow bowl about ¾ full in the worst area or room. Fresh cigarette smoke is also “killed” by waving a cloth moistened with vinegar.
  • Unclog & deodorise your roof down pipes or blocked garden run off drains by using ½ cup of Bicarbonate of Soda followed by 1 cup of vinegar then flush with hot water when the foaming stops.
  • Clean any outdoor chrome & stainless steel fixtures using a light misting of full strength vinegar then buffing with a soft cloth. Great for the Bar-b-q.
  • Clean your silver jewellery after gardening (if you have forgotten to remove them) by soaking them in 1/2 a cup of white vinegar plus 2 tablespoons of Bicarbonate of Soda for 2-3 hours, rinse & dry with a soft cloth.
  • If you own any of those lovely old fashioned brass planter pots, you can clean and polish them by rubbing them with a paste of equal parts vinegar & salt OR 50/50 vinegar & Bicarbonate of Soda. Rinse & polish with a soft cloth.
Mixing Bi-Carb Soda with Vinegar

NB. Do not use vinegar on pearls or gemstones as it will damage them.