Archive for December, 2010

One of the biggest, largely unrecognised, causes of plants dying is death by strangulation – literally!  Typically people come to me with plants showing signs of an often slow decline, ending in death.  You can often isolate various fungal pathogens from the roots.  Sometimes there are also above ground signs of diseases like stem cankers.  But these are secondary to the real cause which is root binding.

At some stage very early on while the plant was still in a tube, as the roots were growing, they hit and bounced off the side of the plant tube and this deflection causes a distorted root system to develop.  It doesn’t matter that the plants are potted on into larger containers.  It doesn’t matter how much you slice and tear at the roots when you plant them, that initial deformation will still be there.  As the plant grows and the roots become thicker and continue to encircle the main stem, eventually they strangle and kill the plant.  The plant will be stressed due to impaired uptake of water and nutrients prior to death and this is why other diseases jump in and take hold.

If you dig up the plant, this is often what you see.

In many cases, even considering most plants are sold in larger pots, when these dead plants are dug up, many will still have the central part of the root ball compacted and in the shape of the original tube!

The problem is so bad there was actually a whole research project done on it (Maximising root quality of waxflower tubestock suitable for field planting. RIRDC Report 09/006 by Kevin Seaton Department of Agriculture and Food WA).  Waxflower growers were finding their plantations keeling over after 3-4 years, resulting in major loss of production.

But the problem is not confined to commercial growers, it is equally common in home gardens.

The first signs of trouble often happen in summer – after the first spell of hot weather because as you can imagine, this is when any signs of water stress are likely to first be seen.  And for a little while, pouring on more water and fertiliser will help.  Eventually though the plant succumbs.

The work done on this problem showed there was a difference between the type of tube/cell used, the degree of root deformation and its timing.  The other really interesting fact was that plant growth and flower production was impacted hugely by this issue.  Three years after planting out, flower production was DOUBLE in one particular type of tube (Premium Plastic 64) compared with all others.

Small round tubes actually showed the least amount of root binding but this translates to a very short time that can be spent in the tube and for nurserymen this can be hard to manage – both from a scheduling point of view and also the increased care that needs to be taken at potting on to avoid plant losses.

The recommendations from this work are that the time from propagation to planting should only be 4-8 weeks.  The preferred design of propagation tubes or cells should include:

  • vertical ribs running down the container wall
  • splayed corners
  • air slots on the side walls
  • a relatively open cell base
  • tapered walls (at least 3 to 7°)
  • a more open cell arrangement (in trays) to allow good ventilation

Of course for the home gardener this isn’t a lot of help – unless you buy your plants in tubes.  And even if you buy your plants in large pots you can hardly dissect the root ball to have a look inside!  But what you can do is buy your plants from a reputable nursery.  Don’t buy plants simply because they are cheap – in most cases you get what you pay for.  If they are going out cheap there is a reason!

And take some heart in the fact that perhaps it wasn’t all your fault your plant died!

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My plant died!

Prompted by a comment made to me last night at a family get-to-gether.  Yes if you plant something at this time of year and water it once – or even twice a week it will probably die.  It will run out of water and die.

Now there are a lot of variables at play here.  Remember what I said about the size of the root ball.  Most plants bought in nurseries have quite a small root ball – maybe 8-10 cm across and 10-20 cm deep.  And even in a good potting mix the water holding capacity isn’t that great.  Not great enough to last for 2-3 days in summer.

A little digression about soils/potting mixes and water holding capacity.  To put it in a nutshell just because a soil/potting mix holds more water doesn’t mean that more water is available to the plant. If you have a look at the link I’ve attached (Nichols water relations potting media ) you’ll see what I’m getting at.

Marigolds were grown in a range of different mixes. You can see that in Table 1, although peat moss held a lot more water than pinebark (50.2 vs 38.5%) the time to wilting in peat moss was a lot less (5.7 vs 10.1 days). The right hand column shows how much water was still in the mix that the plant wasn’t able to easily extract.

Clay in soils is similar, more water holding capacity but not necessarily more water available to the plant. This is why simply sticking a finger in a mix to see how wet it is doesn’t work!

OK so now we’ve sorted that out where were we? Ah yes, the potting mix and the root ball of the plant.

Now you plant this plant and depending what you plant it into, some funny things can happen. Depending on the qualities of the surrounding soil/sand you may end up with a situation where the potting mix dries out and becomes non-wetting – very common in some poorer quality mixes (yes as often happens you get what you pay for). When watered it sheds all the water and the surrounding soil/sand wets up at the expense of the plant. And because the plant roots aren’t out there yet, the plant dies.

Sometimes the mix may wet up but the surrounding soil just wets up better. Differences between water holding capacity in the potting mix and the surrounding soil are often the cause of a tussle for soil water at the expense of your new plant. Always a good idea to have some kind of middle ground to transition the roots from potting mix to its new environment.

Commonly I have been asked to suss out why plants have died and when I dig down the reason is obvious. 10-15 cm of damp soil over a dry subsoil. Now you might think this is a good reason for infrequent deep waterings – but its not! It really takes very little time for water to penetrate quite deeply in sand. Depending of course on how the water is being applied. Some lawn sprinklers, even drippers, apply water at quite a low rate, others hurl it out. We have tracked the passage of water in sands and it can be past the root zone in only 10-20 minutes with high output systems. There is almost never any advantage in having water pass the rootzone. I say almost because if salt (fertiliser) levels in soil build up you need to leach them out periodically.

So knowing your irrigation system outputs and matching them to your plants requirements is the best way to water efficiently.

There is also no substitute for a spade. Digging down and having a look at what’s happening. I know that in my own garden, even after this last winter, I had dry soil only 10-15 cm down in my garden in some places. Always remember the plant factor. Plants that grow will pull water from the soil and dry it out and the zone from which they can pull water may be limited.

And do think where your retic is before you dig. One reason I like mine where I can see it! Subsurface is fine for lawns if installed properly but in garden beds if you’re going to be planting and replanting think about the potential for damage in the future.

This blog has only dealt with issues of dead plants in relation to water. Some other time we will cover a few more possibilities.

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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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