Skip to content

The incorrect use of mulch in landscape plantings is causing many mass planting landscapes to fail, and is inflicting major environmental damage, by spreading disease and weed problems. Unknown to many, there are two major categories of mulch. Ground cover mulch, used to cover the ground generally at 50mm to 100mm thick, and soil conditioner mulch, which is mixed with soil to improve and condition the soil. Unfortunately, there is a recent trend to use soil conditioning mulch as ground cover mulch. This is causing the deaths of many plants around Australia, as well as removing the benefits usually gained from using ground cover mulch. Not only are many projects using the wrong type of mulch, but too many are using a new and very dangerous form of mulch; non composted tub ground mulch (Raw Mulch) which is often disease ridden, and is produced by land cleared site wood and leaf material, that is not even composted correctly. This type of mulch is usually too poor to fit into either category of ground cover or soil conditioner mulch, and really should not be used at all. Do not confuse this with well processed recycled organics, which when produced correctly, and used in the right way, can be very advantageous. Used correctly mulch has many benefits, but used incorrectly mulch can become deadly to plants, and can cause maintenance nightmares.

Ground Cover Mulch – Should always be coarse grade with no fines.

Myth one: All mulches save water by keeping the ground moist.

Only ground cover mulch with no fines will do this, as mulches with fine grade material generally become water repellant.

Ground cover mulch is used to keep moisture in the ground, inhibit weeds until the plants cover the ground, and even to slowly release nutrients into the soil below. This type of mulch used correctly can also make a landscape project look great. So why is it that all too often, soil conditioner mulch is used as a ground cover mulch, or worse still, poorly produced non composted tub ground land cleared mulch (Raw Mulch) with fines. (Fines are the smaller particles, often with soil like texture). These wrongly used mulches provide none of the benefits that people expect from ground cover mulch. Ground cover mulch must have NO FINES. Most standards allow for 5% fines, which is the absolute maximum that should ever be allowed. Zero percent fines are better for the plants, however until standards improve and for the purposes of this article, 5% fines or less will be classed as No Fines. Ground cover mulch should be chunks of wood or bark. Wood chunks are preferable to bark. It can be pieces that are 15mm, 20mm or 30mm, depended upon what the client wants, but should never have any material less than 15mm. It is airflow that makes good ground cover mulch. Mulch with fine grade material has very poor airflow, and will remain wet for long periods, causing many problems, including crown rot. It is bad to have low airflow and constant moist conditions in the top 50mm to 100mm around the crown of plants. Ground cover mulch with fines create ideal conditions for disease to live and prosper around the crown of the plant, often causing, or enhancing root rot diseases in many plant varieties.

Some may say that mulch with fines helps keep water in the soil below, but the reality is that mulch with fines and little airflow often become hydrophobic, actually repelling water, and not even letting it into the soil below, unless the rain fall is very substantial.
So, mulch with fines used as ground cover mulch actually wastes water, whilst good airflow chunky ground cover mulch saves water. Chunky ground cover mulch with NO FINES (No more than 5% of fines), lets the water freely flow through to the ground below, but still helps keep the soil below cool, reducing the amount of water evaporation from the soil into the air. This point has recently been backed up by research conducted by Yates. We will bring you this research in the next issue of Landscape Manager.

Myth two: All mulch inhibits weed growth.

Only Ground cover mulch with good airflow and NO FINES does this.

Many believe that by spreading mulch over the top of the ground, weed invasion is reduced. This is true when you use mulch with NO FINES, as weed seeds find it very hard to germinate. Open mulches with lots of airflow allow weed seeds that blow in to either fall to the bottom of the mulch, thus smothering their growth. Or after much rain they can still germinate in the top of the mulch, but the open air regions of the mulch make it very hard for the roots to reach the soil below, generally causing the weed to die soon after germination. Mulches with lots of fine grade material that are used for ground cover actually act as a potting mix, or seed raising mix. The weeds’ seeds blow in, receive water, and then germinate. The mulch with fines and very little airflow, act as a soil, providing ideal conditions for the weeds to germinate and prosper. By placing mulch with fines at 75mm to 100mm thick over the ground will only inhibit weed seed germination for those weed seeds already in the soil, but will not stop weed seeds that blow in.

Myth three: All mulch releases nutrients into the soil.

Only well composted mulch or non composted coarse grade mulch with no fines does this.

Many believe mulch spread on top of the ground will provide nutrients into the soil. In fact poorly composted mulch with a high degree of fines, typical of most non composted tub ground land cleared raw mulch actually causes nitrogen to draw down, locking up nutrients needed for plant growth. Well composted mulches with fines do not cause this problem, but they do cause the other problems as discussed. Course grade chunky mulch with no fines, whether composted or not, does not cause this problem to any significant level, most likely because of its good air flow and its larger size, it degrades slowly releasing nutrients, rather than composting on top of the ground.

Myth four: All mulches spread over the ground result in healthier better looking plants.

Again only coarse grade mulches with NO FINES do this.

Mulch with fines spread over the ground becomes a haven for disease. Many plants have a growing point that is called the ‘crown’ of the plant. If this crown is covered by mulch with fines, it is often kept moist, particularly in wet humid times of the year. The result is a great environment for disease and bad fungus, particularly Phytophthora. Phytophthora is devastating to many plant types, especially in NSW and Queensland. Recently, an observation was made that showed mulch with too many fines can cause disease problems. Two types of mulch were used in a planting, one was course grade hardwood chip with no fines, which did not show any major outbreaks of Phytophthora, and the other was composted tub ground mulch which, unfortunately, contained considerable amounts of fine grade material. This mulch with fines had many major instances of Phytophthora occurring, which lead to many plants dying, particularly, but not limited to any areas near paths where water ran off the path and caused water logged conditions. Mulch and plant samples were sent to the NSW agricultural departments Plant Health Services, where both the mulch and plants were found to have both the Phytophthora and Fusarium fungi growing. The main recommendation from the Agricultural department was; ‘Mulches with better air-filled porosity would be better alternatives as they suppress some diseases.’

Unless poor mulching practices are stamped out, humid climate states such as Queensland and NSW, will find themselves with a very small range of plants that can survive, if any. For decades plants such as Lomandra, Agapanthus, Dieties, Liriope etc have been used with great success, but the recent use of these poor, land clearing tub ground mulches is threatening the success of these very tough plants. The constant disease pressures from using the wrong mulch, is causing even tough plants like these to fail. If you want plants to do well in the landscape use the correct mulch, namely coarse grade mulch with no fines.

Well composted fine grade mulch reduces the disease problem, but only slightly. Well composted fine grade mulch at least has any disease composted out of it with heat from the constant turning and composting. The fine grade material in the mulch still however greatly reduces the air flow around the crown off many plants or the collar of trees and shrubs, so it is much better to use coarse grade ground cover mulch with no fines. Sure, many will say just plant the crown of the plant above the mulch, or push the mulch back from the tree, but this is simply unpractical. Contractors planting thousands of plants find this hard. Even if the plants are planted in such a manner, the first heavy rain sees the mulch pushed up against the plant anyway. Manufactured mulches such as coarse grade hardwood chip, or bark nuggets or other chunky mulches with no fines are ideal ground cover mulches, as are newly available recycled mulches meeting the appropriate standards, and containing less than 5% fines. In Queensland and other areas, where humidity is extra high, even 5% fines can have bad effects, so zero fines are always safer. In fact, the recycled organic industry should be trying to produce chunky mulch with zero fines, so that customers have a choice. They should give the customers what they want, and not force customers to use what they produce.

On the other hand, non composted raw mulch that is produced by tub grinding trees and shrubs from land clearing is almost bordering on pollution. The crazy aspect is that the people organizing this mulch, claim they do it for the environment, recycling rather than burning it. All they are doing is polluting the environment. If they really cared about the environment, they would instigate correct mulch production and composting techniques, as is conducted by professionals in the recycled organics industry.

Recently, there has been a move to clearing land, pushing the trees and shrubs into piles, and then using large tub grinders to mulch everything into a ‘bits and pieces’ mulch. This mulch is then usually left in piles, to supposedly ‘compost’, which it will not do unless it is turned regularly. I have never seen onsite tube ground mulch turned and composted correctly. This often wrongly called recycled mulch, which should really be called non composted land cleared tub ground mulch or Raw Mulch, or better still, ‘pollution mulch’, is usually produced using every bad practice possible. Diseased trees and shrubs are included in the mulch, often resulting in the spread of fatal tree and shrub diseases, such as Armillaria, Phytophthora and other bad fungi are also left to prosper. Generally, contractors with little professional mulch making experience are producing this mulch. Mulch at this stage should be graded into separate mulch products and then treated correctly. This of course is rarely if ever done. Once this mulch is ‘ready’, it is usually spread by the landscaper on site, and then planted with plants; often resulting in major plant loses due to disease. This type of mulch also is well known to spread weed, cause nitrogen draw down, and repel water, causing drought stress in plants. This so called cheap or free mulch causes major problems; in fact it would be better to use no mulch at all, than use this ‘Pollution Mulch’. Please do not confuse professionally recycled tub ground mulch with, with this non composted land cleared tub ground mulch.

Another recent dubious practice is to put mulch on top of jute mat. Whether it is hard wood chip or any type of mulch, placing it on top of jut mat can lead to prolonged damp conditions, greatly increasing the incidence of disease, particularly root rot. Plants considered tough and resistant to disease have died in these conditions. Humid summer areas such as Sydney and Brisbane should never entertain such practices, and should always strive to have good airflow around the crown or collar of a plant.

Fire is another reason to only use coarse grade mulch without fines.

Fire can be one more result of using this tub ground non composted mulch with fines. Recently, one roadside project had its poor grade mulch catch fire, killing many of the newly planted plants. When it is dry, all it takes is one careless person to through a cigarette butt on the unprocessed tub ground mulch, and a major fire can be born. There have been many instances on the side of the road or even in the back of trucks where piles of composting mulch left for a week or more, particularly when palm plants are included, and these piles are not turned, that the mulch has got hot and caught fire. So unless you know what you are doing, producing recycled mulch can even be dangerous. Even well composted mulch, with fines catches fire more easily than the chunky mulch without fines, although even chunky mulch can catch fire. Think about starting a fire with two sticks, it is the fine pieces of wood that ignite easily, not the big pieces of stick.

Basic soil science suggests only coarse grade mulch with no fines should be used as a ground cover mulch.

Soil scientists will all tell you that layering soil types is bad. Mulch with large amounts of fines behaves like soil, so layering a soil like substance on the top of soil is soil layering, and by definition, bad. Layering soils often causes drainage problems, increases in soil type diseases, and erosion problems. Coarse grade mulch with no fines has good airflow, and as such does not act as a soil. Layering this over the top does not create soil laying problems. Another benefit of using chunky mulch with no fines, is that it is less prone to wind and water erosion. Quite simply the bigger and heavier the particle, the less likely it is to move.

What needs to happen?

What needs to happen is either we go back to the way the ground cover mulch used to be made from land clearing operations, which was to remove all leaf material and sticks etc, and chip the trunk parts of the tree, making sure to reject any diseased trees. This all has to be done by qualified people that understand trees and mulch. However with new work cover laws, large wood chippers are often thought to be dangerous, so in practice this is hard to do on site. Off course it is easier to do in specialized mulch making facilities.

Another alternative is to have the tub ground mulch graded. So far I have been told this is difficult, but achievable, and it would have to be done by professionals. Both Mulch Makers and Australian Native Landscapes have told me they are looking into ways to do this, and are currently producing sample products, with the hope of having these products available by the time this article is read. To achieve a usable product from this process, this raw tub ground mulch would have to be graded and treated. A simplistic description of the processes involved is as follows; fines and pieces too small to be used in ground cover mulch would be screened into one pile (called the soil conditioner mulch), whilst chunky pieces at least 20 mm or larger would go into another pile (called the ground cover mulch). The ground cover mulch would not need to be composted if diseased trees were taken out first. Perhaps random tests for pathogens could be conducted. The soil conditioner mulch would need to be tub ground again, or regarded, and well composted, ensuring any pathogens were killed, as the lack of airflow in this type of mulch would create an ideal environment for pathogens.

In reality, on site processed land cleared mulch needs to be used as soil conditioner, as I doubt it could ever be graded correctly to form ground cover mulch with no fines, and no diseases. This is hard to do on site. It would be better to simply tub grind it repeatably until it is fine enough, than composted well and professionally, and used as soil conditioner. This will help everyone. It will help get rid of the land cleared waste, it will improve the soil, and it will allow the mass planted areas to live. However, major green waste recyclers should be able to take land cleared waste, and in professional facilities, produce products off site, that meet standards which allow it to be used as a ground cover mulch.

Soil Conditioner mulch is generally made up of well composted fines.

Soil conditioner is where fine grade mulches should be used, not in ground cover mulch. To produce soil conditioners from mulch and other waste products, a thorough composting procedure must be adhered to, with solid quality control inspections. Screenings from land cleared mulch or any recycled mulch that has not gone through rigorous processing should never be used. Again, whether on site or in a mulch manufacturing business, this needs to be done by specialists.

Soil conditioners are used essentially to add organic matter to sandy soils and to improve water holding in the soil. When added to clay soils, they help break it up and improve drainage. In general, soil conditioners should preferably be applied to a depth of 50mm, and then incorporated to a depth of 200mm using implements such as a rotary hoe, or power harrow. This results in a minimum of 25% by volume being organic matter. The maximum rate used for sandy soils should be 50% by volume. (It is on top of this conditioned soil that the specialist ground cover coarse grade mulch with no fines can be used.) Basic soil science says never to create layers in soil, so incorporation of the conditioner is important.

Whilst humus products are generally suited to most plants, native plants, (and in particular plants belonging to the Proteaceae family such as Grevilleas), cannot tolerate excessive phosphorus. For these native plants, products such as Greenlife® `Mulch & Compost™’ available from Australian Native Landscapes are recommended.

Soil conditioners are highly recommended as a way to develop better soils from poor soils. One of the greatest aspects of using these products is too increase the presence of beneficial fungi and bacteria, which in turn help displace bad micro-organisms. This can really help improve the health of plants, and reduce the incidence of disease. Obviously poorly composted soil conditioner could have the opposite effect, consequently only buy professionally made product.

Other benefits of Humus & Soil Conditioner

  • Increases water holding in sandy soils
  • Breaks down clay soils
  • Provides slow releasing organic nutrients
  • Retains fertiliser in the soil
  • Reduces the watering requirement of gardens

To achieve the above benefits, only professional, well composted soil conditioner mulch derived material should be used.

Whether ground cover mulch or soil conditioner mulch, special expertise is required to produce a relatively problem free mulch. Most roadside sites or land development sites however, would not have the patience or expertise to undertake such an extensive process. So unless land developers are willing to undertake professional standards, the reality is that soil conditioners should be made by specialists off site. Maybe in the future, this is an opportunity for enterprising mulch making companies to get involved in on site mulch making.


I recently visited Florida in the USA, and asked a meeting of 40 Landscape Architects (for which I was guest speaker), if they had problems using cheap land cleared tub ground mulch that was not correctly processed. They laughed, and all told me they would not use such trash. I was told that about 5 years ago there was a trend to using land cleared trees, branches and leaf material, put through tub grinders and spread as mulch. After a couple of years of plants dying, weed invasion etc, everyone stopped using it. Now they use professionally produced mulch, and only the uninformed use what they now call ‘FREE MULCH’.

Some basic commercial mulch products.

Ground cover type mulches with NO FINES.

Hardwood Chip: Mulch favored by the NSW Roads and traffic authority. It is generally a chip with sizes of 15mm plus. It is long lasting, excellent at stopping weed seed from germinating, allows water to easily flow through to the soil below, and best of all, it has excellent air-filled porosity, which helps suppress diseases in plants. It’s only down side is that it is virgin material coming from forestry. The challenge is for recycled mulch companies, to come up with something that looks and works similar to hardwood chip. Currently there is nothing the same, but professional mulch companies are now offering good recycled products, with less than 5% fines, which hopefully can be 0% soon.

15-40mm Forest Blend (Currently the best recycled product): This is a product available from Australian Native Landscapes in Sydney (Ph: 02 9450 1444), but if you live elsewhere, I am sure other mulch makers could create something that fits the specification. It has good Air Filled porosity compared to most other recycled mulches, but not quite as good as hardwood chip. I believe future improvements could make it even better. Eg: Make 20 to 40mm, or make the pieces chunkier.

An uncomposted 100% recycled wood waist, with a particle size of 15mm to 40mm, with no fines, and good air filled porosity. It should have an organic matter (%) of 98 to 100 with a bulk density (Kg/l) <0.35. It should have following chemical analysis; pH 5.0-6.0, EC (dS/m) <0.35, Total nitrogen <0.3, Calcium (%) <2.0, Magnesium <0.5.

Pine Bark mini nuggets: Medium (25mm) graded pine bark. A recycled by-product from plantation timber. Australian Native Landscapes, Sydney.

Redwood chip: A medium (20-25mm) Red/Brown colour enhanced wood chip. Australian Native Landscapes, Sydney.

Wood Chip (Cypress pine): A wood chip created from the waste wood when the cypress pine tree is milled for its attractive timber. (It is not from the export wood chip industry) It is also reported to deter white ants and termites from your garden. It is a very hardy and long lasting ground cover that breaks down very slowly during the life of the landscape. The chip is generally 30mm in size. (Photo for this mulch supplied by Centenary Landscaping Supplies in Queensland. (Phone: 07 3373 4900).

Chunky Pine Bark: This attractive tan bark consists of 50mm curly chunks of clean bark. A vacuum process is used to free this product of all the wasteful fine wood dust and small pieces that often makes up 30% of normal pine bark. This unique process saves you money, as well as providing a mulch with no fines. The strong texture makes it look great in your garden as it lasts a long time. (Photo for this mulch supplied by Centenary Landscaping Supplies in Queensland. (Phone: 07 3373 4900).

Soil Conditioner mulch types

UltraGrow Humus: A richly coloured, finely textured organic material that is pH neutral and is high in readily available nutrients. It is a premium quality soil conditioner mulch and will provide nutrient rich organic material to hard, dry soil which will help create an ideal environment for earthworm and microbial activity. The humus will help retain valuable moisture in the landscape. Incorporate into the soil.

The Humus can also be dug into heavy clay soils with the addition of Gypsum and possibly a medium sand. The Gypsum will help break the clay down over time while the humus will add organic matter. The sand will open the texture to help replicate a good natural soil. Centenary Landscaping Supplies in Queensland. (Phone; (07) 3373 4900)

Botany Humus Mix: This humus product is ideal for conditioning nutrient depleted soils. Composted sawdust, composted pine bark, spent coffee grounds, composted organics. Australian Native Landscapes. (Ph: 02 9450 1444)

Greenlife mulch and compost: 100% recycled green waste. Ideal soil conditioner for mulch, suitable for natives and exotics. Fine, black composted. Australian Native Landscapes, Sydney.


This article has listed many reasons why mulches with fines should not be used for covering the ground. Hopefully information like this will reduce the practice of using the wrong type of mulch, and encourage people to use chunky mulch with no fines as ground cover mulch. Even of higher importance, is to stamp out the practice of using incorrectly composted and poorly graded ‘Cheap, Free or Raw’ mulch, which is produced by tub grinding garden and land clearing waste (Not to be confused with composted recycled organics). The disease spreading implications of not banning this practice are immense, and I am surprised the various state quarantine, agricultural and land and water conservation departments have not stepped in to stop the practice. The dangers of incorrect use of mulch or poor quality mulch must be heeded, otherwise mass planted landscape projects are in risk of failing all over the country.

Key Benefits of Mulch Types

Benefit Coarse Ground Cover Mulch with no fines Composted Mulch with fines wrongly used as ground cover mulch Composted Soil Conditioner Mulch used as soil conditioner Non composted Land Cleared Tub Ground Mulch
Saves Water
No, often Hydrophobic
No, often Hydrophobic
Stop Weeds
No, weed seeds germinate easily N/A No, weed seeds germinate easily
Saves Time
Keeps Soil Cool
Reduces Plant Stress
No, often creates stress
No, often creates stress
Helps build good soil microbe population
No, generally creates bad microbe population
Adds organic matter to the soil
Helps retain fertiliser in the soil
No, can cause Nitrogen drawdown

Problems Caused by Mulch

Problem Coarse Ground Cover Mulch with no fines Composted Mulch with fines wrongly used as ground cover mulch Composted Soil Conditioner Mulch used as soil conditioner Non composted Land Cleared Tub Ground Mulch
Disease Problems develop onsite Yes, regularly Almost always
Brings disease in with mulch Yes, generally
Spreads weeds with mulch Yes
Allows weed seeds to germinate Yes Not much difference Yes
Mulch repels water and is causing plants drought stress Yes Yes
Can easily cause fire problems Not generally Sometimes Yes, often

<< Back to        Articles Home Page       Maintenance Articles       Full List of Articles

Back To Top