Archive for the ‘Mulches’ Category

Just a quick one – firstly, observations in my own garden last weekend.  Wherever there was a layer of organic matter over the ground there was bone dry soil underneath nice wet organic matter.  Not much point in that being wet though when the  soil which is where the roots are! Now, I don’t mulch as a general rule, so I’m talking whatever lands there from trees around the place, and breaks down over time.  Fine textured organic matter.   Conversely, wherever there was soil only and no layer on top, the rain of the past week or so had wetted the soil up to a depth of a few inches. 

I also received some progress reports on the mulch work done at Murdoch TAFE over the last summer.  They quite definitely show that coarse mulch is the way to go and no more than 50 mm thick if you still want soil to be wetted in the root zone (bear in mind that work was in summer with supplemental irrigation).

We are doing some work outside of Perth (north by a few hours actually) in some quite clayey soils with a range of soil moisture monitoring gear.  We’re getting quite high moisture levels in the soil but the shape of the graphs and the way the probes react to irrigations are telling us that much of that water is not plant available, the bulk of it simply sits  there making the soil feel moist but not helping the plant much at all!

And in case you haven’t been tracking rainfall and you have just been blindly following the watering days regime, perhaps you wouldn’t like to know that in May I had 210 mm rain at home compared to 21 mm in June.  So if you didn’t water in June  as per instructions, you may be in some degree of trouble especially if you’re growing things with shallower root systems eg veges or if you have new plantings in the garden.


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I’ve been away for a while.  Mostly because work is busy but also because I haven’t had much new to report.  But in the last couple of weeks I managed to get some more data on some recent mulch trials and have written it up and presented it to a group of people at a meeting.  The most interesting thing to come out of it was that you CANNOT rely on winter rains to wet up the soil profile.  Even after 600 mm of rain over winter, at 20-30 cm depth the soil was still dry!  And of course the thicker the mulch layer the worse the problem.

The other things that’s apparent is soils are highly variable.  Lots of preferred pathways exist in soil so, in these trial for example, there was a high degree of variability between the three replicate treatments in all cases.  When a significant rain event occured (33mm) the response under the unmulched soil was less than under the other treatments – probably because of runoff due to non-wetting.

While some of the mulches maintained the moisture content in the soil below over summer most were so dry that it really didn’t matter!  They were drought stricken.  This raises an interesting point.  What is the point of amending soil to hold more moisture when there isn’t any!  Certainly, in discussions with someone that conducted trial on a range of soil amendments recently, under the Water Corporation watering regime there was absolutely no difference between treatments. Drought is drought!  If plants are water stressed it doesn’t matter a damn about the improved water holding capacity of the soil if there’s no water in it they can’t access it anyway!  And in fact the situation could be made worse because clay, while holding more water, holds much of it a higher soil moisture tension (ie its harder for plants to extract it).

So this whole issue is complex.  But the main thing to think about is whether or not the soil profile is wet up throughout the root zone over winter.  And due to preferred pathways that exist in soil, you will need to dig down in more than one place to find out!  If the soil largely remains dry in the root zone (20-30 cm) at the end of winter then you need to think about how you apply mulch, especially if you are a) overhead watering and b) only watering 2-3 times a week.

I would dearly love to do some research on this topic but alas funding for “home garden” type issues doesn’t exist so we are reliant on the bits and pieces done here and there, often by institutions like TAFE.  And extrapolations from the commercial stuff done by eg DAFWA.

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Don’t do it!

Just a bit of a sneak preview here. We’re running some trials at the moment and in only two weeks we can see that the soil covered with fine mulch is dry – 5% moisture in sands is dry and stressful for your plants. All this rain we’ve had in the last week or two is staying in the mulch. Where we have coarse materials the rain is going through and the ground underneath is nice and moist and running at about 10% moisture at a comparable depth.. And we’re not even really into summer yet. Imagine how much worse its going to get in summer when you’ve got two-three waterings a week! This trial also doesn’t have the added factor of plants in the ground, its just mulch over sandy soil. With plants it would be so much worse.

And just a reminder to use composted materials as well – to minimise the problems of disease, non wetting and nitrogen drawdown.

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Gosh, nothing provokes more discussion than the topic of mulching. We had a big round table discussion last week and we reached a consensus on a few things. But firstly lets define what we mean by a mulch and what we expect to get out of it.

Some degree of temperation moderation in the soil and with it some degree of water conservation. Weed suppression.

So where does that take us?

Mulches MUST be coarse. In excess of 25 mm particles. Big and chunky. Because if they aren’t, they use more water than they save. They don’t allow water to go past them in into the soil where the water needs to be which is around the plant roots.

Mulches that permit roots to grow in them are bad. In the average home garden, those roots will be exposed to wild fluctuations in moisture and that’s bad.

You don’t need to reapply mulches year after year. You might need to top them up with a few mm of extra but to apply 25-50 mm of mulch year after year is stupid. All you do is raise the level of your garden beds to ridiculous heights. The height of a mulch layer must be relative to the plants you’re putting it round. 25 mm is fine for most things and only big trees will take 50-75 mm.

Composts are NOT mulches. Composts are finer. They need to be incorporated into the soil. They won’t discourage weed growth for very long when placed on top of soils and they may become non-wetting if they dry out. And with Perth’s current silly watering regime or 2-3 days a week this is highly likely. They supply nutrients which must be taken into account when you apply them. But they seldom contain enough nitrogen in relation to phosphorus and for many plants eg veges the rate of supply of nutrients is too slow to promote good plant growth without supplementation. If it is you can bet a good slice of the phosphorus in it is leaching through into the aquifer below and polluting the waterways.

A few other things that you need to take note of. That free council supply of green waste is the best way of spreading weeds we know of. If you have a pocket handkerchief size garden that probably isn’t so important but if you have ANYTHING to do with bush regeneration eg as a member of a Friends group then it does matter, and it is important because you are likely going to end up with all manner of weeds in your prized piece of bush. Some real life examples cited recently were Cocos palms and Cape lilac trees coming up in bushland due to the use of green waste.

All green waste and indeed all mulch and compost should be composted or at least pasteurised to reduce the risk of spreading weeds and diseases.

I think its fairly well recognised that cypress canker was spread around Perth in the early stages by mulch. Cypress canker is that disease that decimates pencil pines and many other conifers. It’s a foliar (leaf) disease that makes whole patches of leaves go brown, it kills them branch by branch. I’m sure you will have seen it around.

There endeth the lesson for today. Short and sweet.

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(This is a long post – mainly because I think you need all this at once.  So bear with me).

Mulching your garden is widely publicised as the right thing to do for your garden.  But is it?

What do you need to think about before putting down mulch?  In fact do you need to mulch?  Is there a case for NOT mulching?

Mulches are widely associated with reducing moisture loss, moderating soil temperatures and  suppressing weeds.  But mulches behave very differently on sands compared to other soils.  Things that make them good in some situations can be a problem in others.

Things to think about when considering choosing mulch

How fine or coarse the material is.

Coarse mulches are better because they don’t break down as fast.  They suppress weed growth for longer and you can get away with applying them less often. Coarse mulches trap air and help provide an insulating layer over the soil.  Because they are coarse, water penetrates more easily and is not taken up by the mulch at the expense of the soil.

Fine mulches break down quickly (in a matter of months) and often form a discrete layer of organic matter on the soil surface.  Over time if you reapply these sorts of mulches without being able to mix previous applications into the soil. a separate layer develops above the usual soil profile and this can be a problem.  That surface layer can be non wetting and so prevent rain or irrigation from penetrating.  Other mulches break down and become sludge like, holding water at the expense of the soil below.  Depending on the situation, roots may end up in the mulch layer instead of down in the soil.  This means they are exposed to high temperatures in summer and also to much greater fluctuations in soil moisture than if they were at depth (eg 15-30 cm).  Since fertiliser is applied to the soil surface they are also more easily burnt from high salt levels in the fertiliser.  Sludge-like layers of degraded mulch also promote weed growth and if near the base of plants – collar rots.  When the plants in question are grafted or budded (for example roses), the layer of mulch may enable the plant to root from the scion (top bit).  For roses which are usually on a nematode tolerant rootstock, this means the plant then becomes susceptible to their attack, allowing them to breed and resulting in disease.

Fine mulches are perhaps better considered as composts or soil amendments.  They can contribute significantly to plant nutrition and are better worked into the soil prior to planting if they are to be used at all.

Irrigation and soil moisture

The situation in WA on the coastal sands is quite different to that in other parts of the country.  The advantages that exist for mulches over soils are much less and some may not exist at all, over sands.

Because sands hold so little plant available water, the fact that  mulch may reduce evaporation from the soil and enable a soil to hold onto a little more water for a little longer may not be of PRACTICAL benefit to the plant.

A more common scenario is that plants, when mulched, are unable to gain benefit from light showers of rain because all the moisture is taken up by the mulch and doesn’t reach the plant roots.  This can happen even with coarse mulches.  Shredded pine based mulches have been shown to store 20 mm of water (20 L/square metre of soil which is significant) in an 80 mm layer of mulch (Geoff Connellan, Burnley, Uni Melb).  This means that only relatively heavy  and repeated showers of rain will penetrate and wet soil.

When mulch is applied think about your irrigation system.  If irrigation is installed above the mulch, use outlets with high precipitation rates (>30 mm/hour) or bubblers so that water has the best chance of draining through the mulch and into the soil.  If a drip system is installed on top of the mulch bear in mind it has to wet the mulch before the soil.  For that reason use drippers with outputs of over 4L/hour so water drains through the mulch.

If drip is installed below the mulch, whilst the water is able to reach plant roots quicker, it is harder to see problems such as leaks or blocked drippers – the first sign may be a dead plant!  Extra caution will also be needed when digging around plants.  This is about where I promise you a post all on sub-surface irrigation.

Of course there is never any substitute for checking.  Dig around under the mulch and in the soil to check that water is getting through.  Don’t assume that wet mulch means wet soil in the plant root zone.  Rapidly growing plants will suck water out at depth so soil may be damp at 10 cm and bone dry at 30 cm.  In sand, on a hot day, that can mean instant death.

Plant nutrition

Greenwaste and other organic materials that are not composted may also reduce the amount of nutrient available to the plants in your garden.  Organic mulches (and composts) are living materials, constantly changing.  As they break down they use nitrogen which can be at the expanse of your garden plants.  In sandy soils, nitrogen is the nutrient which tends to restrict plant growth most of all so you can not afford to lose any!

The finer mulches in particular are dynamic materials.  As they break down their degree of acidity or alkalinity can change dramatically.  Typically acidic to start which, as composting proceeds, they may become quite alkaline (tying up iron and phosphorus) and then returning to a more neutral pH.  So if your plants start going yellow after applying mulch there are two possible reasons –  nitrogen drawdown or pH change.

Mulches containing a lot of raw or partially uncomposted plant material may also contain chemicals that are toxic to your plants.  Bark and the leaves and roots of some species contain chemicals that will suppress their growth.  These substances helped them compete and survive when they were growing but are antagonistic to other plant species (ie your garden plants).  Composting breaks these chemicals down.


How well a mulch will work at suppressing weeds will in part depend on what sort of weeds are your problem.  Most annual weeds such as dandelions, portulaca etc will be suppressed but if you have creeping perennials such as couch or bulbous weeds such as oxalis or Guildford grass these will still grow.

Fine mulches or those that break down quickly are not effective as weed suppressing materials for any length of time.  In fact as previously discussed since they can compact and become sludgy they may encourage the growth of weeds, especially sedges.

Uncomposted greenwaste may carry both weeds and weed seeds in it as a well as pests and diseases so you may end up with a whole suite of new problems.

While we’re on the subject of pests and diseases….…….

Many mulching materials are popular because they are free.  Many councils give out greenwaste (also called enviromulch) which is the shredded product of land clearing, street tree pruning and local government kerbside pickups of prunings etc. from the home garden.  When fresh, this material is highly likely to introduce pests, diseases and weeds that may not have existed previously.  Any greenwaste that is contaminated with soil is a particular risk to native gardens as it may carry dieback.  For these reasons, organic based mulches should be at least pasteurised and preferably composted, before use.

The first rule of applying mulch is not to heap it up around the collar of the plant as this will encourage collar rot.  This is less of a concern with woody trees and vines, providing the mulch is coarse and the depth around the trunk/stem is less than 6 to 7 cm.

Water repellency

As mulches are organic they can become water repellent over time.  This is especially true for the finer materials.  Application of soil wetting agents will fix the problem however this is a problem which creeps up on you and before you know it you may have dead plants.  Potentially, it can happen anytime after a mulch dries out – another good reason to check regularly to ensure the soil under your mulch is moist.


The darker the mulch, the more heat it will absorb.  While this can be a good thing in winter, in Western Australian summers this is the last thing you need!  Some of the fine (lovely black) organic materials sold are dyed and contain chemicals which you may not want in your garden.

So what is the bottom line here?

How to use mulch

Vegetable gardens and annual plants such as bedding plants

Mulches are very useful in vegetable gardens.  Even fine mulches (composts) can be used since most vegetables are annuals and the ground is regularly cultivated.  This allows the mulch to be mixed in, providing useful organic matter, nutrients, and over time, increased water holding capacity.  Be aware of the effect on soil level over time.

Flower gardens and fruit trees

The benefits of mulching a flower garden may depend on what you have planted.  If you have mostly perennials such as roses or azaleas then think about coarse mulches applied at planting to a depth of about 50 mm.  Keep them away from the main stem of the plant and don’t apply year after year raising the level of the bed.  The value of mulch around fruit trees is debatable.  Effective irrigation is paramount, especially for citrus.  Consider your irrigation system and how it will work with mulch in place.  Citrus trees are surface rooting so mulch cannot be incorporated without damaging roots.

Do not mulch over seed or newly planted seedlings.

Native gardens

As above.  Fine mulches or composts are best avoided for reasons already given.  They may also contain significant amounts of phosphorus which can be harmful to some species such as banksias.


  • Is my mulch from an accredited supplier?  This will guard against nitrogen drawdown and the importing pests, weeds and diseases.  It should also ensure it is not too alkaline or acidic.
  • Is my mulch sufficiently coarse to allow easy penetration of water and ensure it lasts a long time without allowing weeds to germinate in it.
  • Is the soil I am applying this mulch onto, already wet throughout the root zone of my plants?  If not consider what will be required to achieve this once the mulch is down.
  • Have I kept the mulch away from the collar of all plants?
  • Is my mulch non wetting? Does water penetrate the mulch and reach the soil below?

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