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Organic Soils and Mulches

Note: We suggest chunky mulch as a groundcover mulch for large and commercial landscapes

By Todd Layt

All too often landscape professionals use organic soils and mulches incorrectly. Unfortunately the confusion is enhanced by suppliers promoting the wrong product as ground cover mulch, and inadequately refined products for soil conditioning or soil blends. Australian trials and years of real world evidence show which types of mulch perform. When the right products are used the results can be outstanding.  Don’t worry too much, it’s not that hard, simply follow some basic guidelines when choosing which organics to use in your landscape.

Choosing the right ground cover mulch has one main rule. Only use chunky mulch with preferably zero fines. (See tips below for Roads department specifications). AVOID fines, slivers, forest blends, leaf litter, and raw tub ground mulch from land clearing. All these products are worse for your plants than using no mulch at all. DO USE chips and chunks that have a minimum particle size of 15mm, and avoid any fines. The reasons why this rule is so important are very clear.

Firstly, fine particles act as a seed raising mix. Fine grade organics make the best potting mixes and soil conditioners, which allow seeds to germinate really well. Ground cover mulches’ primary job is to stop weeds. Only chunky mulch types with zero fines do this, as there are lots of air gaps that make it very hard for weeds to germinate, or the depth of 75mm stops weeds from breaking through from the soil below. Commercial landscapes cannot afford to replace mulch every 1 to 2 years, so slow to break down mulches are required, and only chunky style mulch does this. Good hardwood chunky mulch can last up to 7 years; however a top up may be required after about 4 years in many situations. Good chunky bark products last 2 to 5 years. In Victoria Chunky Bark products seem more popular and seem to last a little longer, maybe due to the cooler weather.

Ground cover mulch is used to improve the health of plants. Only chunky mulch does this, and in fact fine grade mulch used as a ground cover will usually adversely affect the plants health. Mulch should reduce drought stress, and chunky mulch does this well. Chunky mulch with its big particles easily lets water flow through it, and then once in the soil the chunky mulch reduces evaporation and keeps the soil much cooler on a hot day, and warmer in the middle of winter. Fine grade mulch acts like another layer of soil.  Firstly soil scientists claim that layering soil like this is bad, but worse research has shown that fine grade mulch becomes hydrophobic and rather than reducing drought stress on plants, the repelling effect actually usually increases the drought stress on plants.  Mulch in general releases organics into the ground greatly improving the soil, but chunky mulch does this in a far more economical way. Chunky mulch breaks down slowly, working year after year for many years, without the need to re-mulch, whereas fine grade ground cover mulch releases more quickly, and is more likely to have nutrients leached into waterways. I liken it to why we use slow release fertilisers.  

The worst consequence for plants when using fine grade mulch is from disease. By piling up a soil like substance, with very little airflow around the base of a plant, you greatly increase the risk of soil borne diseases. In wet weather this often results in plants dying from Phytophthora. Chunky mulch has better airflow, which makes it far harder for these diseases to develop. In humid areas like Sydney and Queensland this is just so important, yet it is Queensland that is one of the worst offenders when it comes to using the wrong type of mulch. In Sydney, a very large trial of 2 mulch types by the Roads Department showed that the one with fines failed on many levels, including weed invasion and plant disease. On the flip side, the Chunky hardwood chip mulch did its job very well. Now 6 years later, it’s even more obvious. The plants used in the chunky mulch are generally still alive, whilst almost all the ones planted in the fine grade mulch are dead. Now that’s conclusive evidence.

Queensland has a nasty habit of using raw tub ground cheap mulch on their projects. The reason always quoted is “this mulch is made on site, and is far cheaper, or free, and we have to get rid of it anyway. We have been told we have to use it.” Well don’t. It’s better not to mulch at all. Pay for the right stuff. It will cost far more money to re-mulch and replant, which will usually be necessary not too far into the future. This poorly composted raw mulch will also just rob plants of nitrogen, making sure the landscape always looks sick, and newly planted trees die from diseases found in the mulch, which were present in diseased on site trees.  I see it all the time in Queensland, and still too often in other states.

So what can be done with all these mulched trees from land clearing, when a new project is started? Simple, grind it smaller, sit it in piles for at least 6 months, turning it regularly, and composting it very well. Then use it like it should be used, as a soil conditioner. The composting will kill diseases, and making it into fines and a soil conditioner is easier than trying to make a true commercial ground cover mulch to landscape specifications. In fact I have never seen on site made mulch meet landscape groundcover RTA specifications. If it cannot be made into a soil conditioner mulch on site, then send it to a recycling centre, but never never use it as a ground cover mulch.
There are now many recycling centre’s that use green waste to professionally make organic soil conditioners and soils. When touring them you can smell the goodness in the mix as they compost away.

My preferred method of using organic soil conditioners on a commercial landscape site is to firstly engage a soils scientist to test the site soil. He may recommend other additives other than just organics, but it will be organics that will usually have the biggest benefit. The key is to then blend the organics and other additives to the site soil. This can be done in stockpiles, or by ripping and rotary hoeing the soil conditioner into the soil directly in situ. The essential aspect is to mix the soil conditioner well into the soil, and not create laying effects where possible. New Australian research shows that clay soils that have organic soil conditioners incorporated to these depths in agriculture could boost crop yields by up to 60%. These results were achieved by incorporating 20 tonnes per hectare. (Higher rates are usually used in the Landscape).  The research showed that the treated soils were much more able to capture water and keep it for drier times. This has to be good news for Landscape projects.

The benefits of using soil conditioners or even chunky ground cover mulch extends past the landscape benefits. Much of the carbon used in this process is locked up by the soil, and soil is such an amazing environmental carbon sink. With the current debate on taxing carbon so emotional, we need to consider real tangible ways to quickly lock up carbon, thus urban landscapes locking up carbon through recycled composted green waste in the form of soil conditioners are a valid and necessary option.

8 Tips for using ground cover mulch

  1. Use border plants to keep mulch into gardens. Small Dianella (such as Little Jess, Lucia and Aranda), Liriope (such as Isabella), Mondo and Rhoea in Queensland.

  2. Use at a depth of between 50mm and 75mm. If using 100mm depth, the plants need to be planted in larger sizes.  

  3. Only use chunky mulch without fines. This specification is often used by Roads Departments because they need mulch that will last a long time. For general gardens or cooler areas well-made chunky bark products will also work. Only use mulch that is comprised of hardwood chips or other chunky wood material with no more than 5% fines by volume (preferably zero fines). The material must not contain any bark. The average size of the woodchip must be approximately 30 mm x 20 mm x 5 mm and the maximum length of chip is not to exceed 50 mm. It must be free of soil, weeds, stones, vermin, insects or other foreign material. Do not use raw tub ground mulch, or composted mulch with more than 5% fines.  Whatever specification is used, make it chunky without fines.

  4. Try very hard not to bury the plant’s crown.

  5. Do not try to spread the mulch on a wet day when the soil is soft, especially if using machinery.

  6. When mass planting small plants like tubes or 90mm pots, it is often much quicker to spread the mulch first, then use special drill planter that has a mulch wiper attached. You can buy these from Plant Planters. www.plantplanters.com.au

  7. Free mulch is rarely free of diseases and other problems.  Only mulch that meets the Australian Standard, AS4454, should be used.

  8. On slopes long pieces of ground cover mulch is less likely to wash off. Try to find one with no or very few fines. Larger shredded or chipped pieces are best, or long pieces of bark.  

Six tips for using organic soil conditioners

Suggested by Simon Leake, who is the founding Director and Principle Soil Scientist at Sydney Environmental and Soil Laboratory.

  1. Analysis is key. To ensure value and to determine there are no dangerous contaminants, ask for the product analysis, and if they cannot give it be suspicious. For example, high Zinc is a possible problem, and that is very damaging for plants and soils.

  2. Knowing the correct rate. This is where an analysis of the site soil and the product is needed. This analysis will determine the rate. Usually between 3 litres and 10 litres per metre squared, incorporated well into the soil at a depth of 200mm.

  3. For Phosphorus sensitive native plants make sure it is low in Phosphorous.

  4. Beware of high salt levels, especially in dryer areas such as Western NSW and South Australia.

  5. Make sure it meets the Australian Standard, AS4454. If it does not, you as the contractor could be liable for plants that are lost.

  6. Do not use the soil conditioner in the bottom of planting holes, as it will reduce available oxygen for the plant, instead mix it well with the site soil and use it as back fill around the plant.

Ground cover mulch with lots of fines is a haven for disease, and worse traps moisture from moderate rain preventing it from reaching the subsoil.

Tub ground raw mulch with fines is much better at growing weeds than landscape plants. Disease in this type of mulch will kill the plants, then opportunistic weeds germinate easily, as the fine particles act as a potting mix.

A chip type mulch that meets roads department specifications.

This drill bit with attached mulch wiper is for planting tubes.

Composting is key to making a good organic soil conditioner.