For any challenging issues you have in your garden or if you’re only seeking some gardening advice – Ask Malcolm.

Ask Malcolm

Ask Malcolm

Malcolm has years of experience in horticulture and gardening and not only has an extensive knowledge of plants, including their propagation but also has experienced and overcome practically every issue that could confront the home gardener.

We would like to make Malcolm’s experience available to answer your questions related to your gardens or horticulture in general. Please send your questions with as much information about the issue you have, including photos (if that’s possible) to:

Any questions and their answers that have broad appeal, we will publish on this website on the ‘Ask Malcom’ page.

Answers to our clients’ horticultural and gardening questions  

When should I prune my citrus and apple trees? – Mark, Toorak 

It may be easiest to answer this question by advising when not to prune your fruit trees:  

  • In summer as the exposed young branches and fruit can be burnt by the hot sun and cause disease and decay in the tree, 
  • In winter as the deciduous fruit trees are dormant and any large cut made to the tree may not heal properly, again exposing the tree to disease and decay. 

So that leaves autumn and spring as the ideal times to prune your fruit trees although advice from the experts suggest autumn is the ‘idealest’ ideal time.  Commence your pruning when the tree is finished fruiting and all the fruit has been removed from the tree.   

If you are pruning to encourage a vigorous growth of fruit in the next season, then it is important to understand whether the tree produces fruit from spurs or from the tips and then prune accordingly.  Otherwise, if you are pruning your tree to shape (ie. reducing the height so the fruit grows within your picking reach), then ensure you leave sufficient branches and foliage on the tree so the new fruit-bearing growth will have protection from the summer sun. 

I’ve provided a link to an article on the Sustainable Gardening Australia website that discusses this topic in detail. 

Chrystie, September 21, 2020 

My compost isn’t decomposing. What should I do? – Mark, Toorak 

Whilst all organic matter will decompose eventually, the objective of composting is to speed up the decomposition process of organic matter by establishing conditions that are optimal for the worms, bacteria and other microorganisms that break down this matter physically and also chemically into the compounds and elements that are necessary for plant growth. 

Before I rush into your answer, I feel it’s important to explain why composting so importantCompost, when added to soil, will eventually break down further into ‘humus’, a dense, spongy, nutrient rich material that provides an abundant food source for the soil microorganisms allowing them to feed and reproduce.  The spongy or microporous property of humus enables it to retain water and also to trap oxygen within the soil structure enabling the aerobic (good) soil bacteria to thrive. For these reasons, humus is can be considered the ‘life-force’ of soil.   

The major (or macro) plant nutrient elements that are produced in the composting are: 

  • Nitrogen (N) – from the decomposition of food waste, grass and leaves (otherwise known as ‘green’ or ‘wet’ waste) 
  • Carbon (C) – from the decomposition of the wooden or structural parts of plants, including paper. (Otherwise known as ‘brown’ waste.) 

Plants are unable to absorb N from the air (with the exception of some of the legume family of plants) so their roots are used to access N that is present in the soil in the form of Nitrate compounds (salts).  Humus provides a stable supply of Nitrate compounds that is readily available to the plants. 

As plants access (or ‘fix’) carbon from Carbon Dioxide (CO2) in the air via the photosynthesis process (ie. not from the soil), the ‘organic’ carbon produced in composting is captured and retained in the soil structure.  This prevents the carbon being converted into CO2 and released into the atmosphere so compost/humus can be considered a very effective ‘carbon sink’. 

It’s amazing to think that all of these positive outcomes for the health of our soil and our planet resulted from us composting our food and garden waste instead of consigning it to landfill (where combined with all the other rubbish it contributes to the production of methane that builds up in landfill sites. Methane is a greenhouse gas significantly more potent than CO2.) 

Ok, now let me answer your question. 

How do you get from this to this? 

Composting is an aerobic method of decomposition which means the microorganisms that break down the organic matter can only function in the presence of air.  These microorganisms also require the following four elements: 

  • Carbon – for energy. Brown (wood or dry) waste is the high carbon source in composting. 
  • Nitrogen  to grow and reproduce generating more microorganisms to oxidise the carbon.  Green (wet) was is the nitrogen source in composting. 
  • Oxygen – for oxidising the carbon. 
  • Water – to sustain the microbial activity and avoid anaerobic conditions developing in the compost. (Water is rarely required to be added to compost as the green waste releases water as it decomposes.) 

The most efficient form of composting is ‘hot’ composting where the compost is contained in an enclosed space such as a large drum or purpose-built pit to enable the heat from the microbial activity to naturally build up within the compost.  The microbial activity is most vigorous in range 65oC – 75oC.  This is the method for the commercial production of compost, however, controlled external heating systems will be used to keep the temperature of the compost in the optimal range 65oC – 75oC.  a 

Where it is not practical to have this type of composting system, for example houses that only have small yards or apartments that have a balcony as the only outdoor space, a composting system with worms can be used instead.  The worms ingest and break down the organic material in the first stages of its decomposition, producing ‘castings’, a dense brown substance that is rich in the nutrients required by plants. This is then further broken down by the bacteria and other microbial life forms in the compost.  Worms also help aerate the compost via their creation tunnels as they move through the compost. 

It is also important for the compost to be mixed constantly to ensure it is sufficiently oxygenated.  With a worm composting system, this can be done every time a new load of food or garden waste is added to the bin.  Simply use a trowel or small shovel and dig into the compost, bringing the compost from the bottom of the bin to the top. 

Another important condition with worm composting is the amount of moisture in the compost.  Worms may perish if the compost is too wet, a condition that occurs if there is too much green waste in the bin.  To correct, add more garden (brown) waste to the compost and mix through.  Once you have the correct composition in your compost that is ideal for the worms (some trial and error is required), they will settle in and start breeding and you will notice the waste you add to the bin being converted to castings in a matter of weeks. 

For more information on composting and how this contributes to the natural improvement of our soil, I recommend listening to the following episodes from the All The Dirt podcast: 

Episode 69: All Things Composting with Garden Presenter Angus Stewart 

Episode 35: Making The Perfect Compost 

Episode 107: Brent Burns Building Healthy Soils For Our Farmers and Our Future 

Chrystie, September 21, 2020