Mulch - your questions answered

 

Why should I mulch with clear plastic/black plastic?

Plastic mulches of various colours are widely used in commercial vegetable production.  Black plastic is most commonly used in strawberry production.  Its use raises some environmental concerns (see table below).

One of the most interesting aspects of plastic mulch is the use of coloured plastic to manipulate production.  Tomatoes can yield earlier and more heavily when mulched with red plastic.  The plastic intensifies red light increasing plant photosynthesis.  The effect of colour is still being researched with different plants and colour combinations producing quite different results.

How does the application of mulch affect soil temperature?

Mulch has a complicated effect on soil temperature.  It is very much determined by the type of mulch material, season and climate.  Thick organic mulches have a lot of air pockets and act as insulators.  The result is cooler soils in summer but also mean cooler winter temperatures.

Frost settles on mulch in winter.  The mulch acts as an insulator during the day preventing the soil from warming.  At night it prevents heat loss into the air, meaning colder air temperature and frost potential.

A moist, weed-free, compact soil is far more frost resistant.  Counter to this, black plastic mulch in winter acts as a conductor, both heating during the day and releasing heat at night.

What is the best time of year to apply mulch?

Again there is no one size fits all answer.  In temperate Australia I would recommend mulching in late winter/spring.  The soil is at its maximum moisture content and fear of frost is over.  If you have lots of bulbs in your garden I recommend mulching before they emerge to avoid having to mulch on tiptoes.

Avoid mulching in the dry heat of summer as you will prevent any water from reaching your plants and most of the soil moisture will have already evaporated.  
Mulching works wonders in the aesthetic department so before a garden open day or visit from a dignitary is a good idea.  If this is in the height of summer or deep winter aesthetic mulch should be applied as what I call lip gloss, not too thick.  In areas that receive wet season rain mulch can be applied beforehand to mitigate erosion.

When should I use chip-bark as mulch?  Is it wise to use it around the vegetable garden?

Bark chips make excellent mulch especially for trees and shrubs.  Having fewer fine particles wood chip is very effective at allowing water infiltration.  The particles can be screened to ensure a particle size of not less than 16-20 cm.  Wood chip is relatively slow to decompose, making a longer lasting mulch.

You have to be careful where you source your wood chips from.  The local arborist may seem a good option but they deal with mainly diseased trees.  What disease will you be bringing in to your garden?

Woodchip is low in nitrogen, good to suppress weeds, less useful for gross feeding crops.  It is also slow to breakdown and large particles can interfere with seed bed preparation.  For this reason I would not use it on vegetables.

What is the best mulch for the vegetable garden?

At Heronswood we use organic lucerne mulch on our kitchen garden.  It allows good water infiltration but also insulates the soil.  On our parterre, partially for aesthetic reasons, we use a thin 50mm layer of our own compost. This compost layer is regularly “tickled” with a small rake to suppress weeds.

How does imitation turf as a mulch differ from grass in terms of its effect on the soil?

The biggest problem with synthetic turf, apart from environmental impact of manufacture, is its extreme heat in summer.  How hot?  On a 37°C day in Provo, Utah synthetic turf reached 93°C!  Natural grass rarely gets over 38°C and is usually lower than air temperature.  Need I say more?

Can I use carpet as mulch?

Carpet can function as mulch but has a few pitfalls.  As a recycled manufactured product you cannot be sure what it is made out off or what chemicals it has been treated or glued with.

I have found it can make a snug home for rodents and other pests to tunnel under.  It may prevent moisture penetration.  It looks pretty bad as it breaks down.  It is perhaps best used as a weed barrier on pathways where you want nothing to grow.