MULCH FOR YOUR LITTLE BABIES

Mulch can be divided into two forms – organic and inorganic

Commonly available organic mulches include sugar cane, lucerne, tea-tree, cypress, hoop pine and various bark and pine chips.

Inorganic mulches include decorative pebble, river rock, scoria, various gravels and the like.

The main advantages of organic mulch:

•  Adds organic matter to the soil as it breaks down, although some take a considerable time to do this

• Helps retain moisture in the soil

• Regulates soil temperature – cooler in summer, warmer in winter (reducing plant stress)

• Suppresses weeds

The main advantages of inorganic mulch

• Low maintenance

• Aesthetics (if you like the look of a particular stone)

What mulch is best for my succulents and cacti?

A mulch suitable for succulents and cacti should not be acting like a sponge and holding on to water.

The ‘fines’ content in certain organic mulches can help retain moisture, but in others it can actually become water repellent as it consolidates in place. Not ideal for growing succulents, or anything else for that matter.

There are both coarse organic mulches and inorganic mulches that fit the bill for use with succulents, but which should be used?

 The typically available options here in South-East Queensland are:

•  Pine bark ‘Nuggets”

• Cypress woodchip (including coloured)

• Hardwood chip

• Decorative pebble/rock (particularly for use in container planting) both imported (bagged) and bulk (Australian sourced/quarried)

• River rock

• Tumbled sandstone

• Blue quarry rock

Lets start with the organic options first but before we do a quick word on the C/N ratio of these because it’s relevant to the whole question of which one to use.

C/N ratio

This is a measure of the carbon/nitrogen ratio of (in this case) organic mulches.

Basically, the higher the ratio of nitrogen (N) the quicker the mulch will decompose.

As an example, lawn clippings have a C/N ratio of 20:1 and woodchip 500 to 600:1

All the chip/’nugget’ options are high C/N.

They take a long time to decompose and many people will say that because of this the microbes involved in decomposition will draw on the N in the soil for this purpose, with a subsequent reduction in N available to the plant (a process called nitrogen draw-down).

Further, the pH of wood chips is acidic and has an acidifying effect on the soil.

Firstly, as far as nitrogen draw-down in woodchip mulches is concerned, that has been reasonably shown to only occur at the interface between the mulch and the soil and many argue any potential pH effect only affects the very top layer of soil.

But:

It just so happens that most succulent and cacti are shallow rooted and are more likely to be potentially affected by these factors than deeper rooting plants.

Any nitrogen(N) draw-down (if it was visibly causing deficiency in a plant) could be offset by fertiliser N applications and if there is some longer-term acidification from the continued use of wood chip mulch, if you are testing your soil you can potentially monitor pH and address that issue if required.

Personally, we have used coloured cypress chip with some succulent oriented design projects and after several years we can say we have not seen anything to suggest that either N draw-down or low pH/acidity are an issue.

But that’s not to say it isn’t a possibility.

That said, and irrespective of any other factors good or bad, we personally prefer stone and pebble for use as mulch with the following provisos:

• Weed suppression – especially if used in outdoor gardens, wind-blown soil will tend to collect in the nooks and cranies between rocks over time and provide a location for weeds to establish

• Weed suppression is also a factor of sunlight (or more correctly the absence of) and as such you might expect a bit more weed issue with this option

• If your garden bed is on any sort of slope, consider angular type stones as opposed to anything rounded. You don’t want your stones ‘slip sliding away…’

• Be aware of the heat generated by stone mulches in hot sun. Avoid mounding it directly up against the plant stem itself.

• As garden beds establish and plants start to ‘fill out’ an area, direct exposure of sun to rock lessens, unless you have a minimalist design for your garden. Just be aware, if you are planting any sort of groundcover, it might be a bit tricky with plant growth hindered by hot rocks in high exposure full sun situations.

• Consider some sort of water permeable weed mat or geofabric as an underlay that your rock/pebble can sit on. Apart from anything else, it will help stop the stones from sinking into the soil over time.

• Small stone/decorative pebble can be very useful in potted succulents. In wet conditions the pebble stops the lower leaves of the plant touching the wet soil, helping to avoid fungal disease and rot.

• Stone or pebble, given the complete lack of combustibility, is the clear option for mulching in bushfire prone areas