Plastic weed mat was widely used
to suppress weeds for revegetation projects during the 1970's and
1980's. However it use is now largely viewed as a disaster and
has been replaced by biodegradable weed mat.
Unfortunately the landscaping
industry and the gardening public did not get the message. Woven
plastic weed mat is still used widely by the industry and in
worse case scenarios black builders plastic is used as a cheaper
alternative.
Plastic weed mat will cause many
problems for your soil and your garden:
- It impedes or stops rainfall
soaking into the soil.
- It prevents worms from mixing
organic matter from the mulch through the top soil.
- It impedes or prevents aeration
of the top soil.
- Its slippery surface causes the
mulch to slide off exposing unsightly patches of bare plastic.
- Weeds still grow through cuts
and holes in the plastic or on top of it and then you unavoidably tear large
holes in it when you remove them.
- Contributes to plastic
pollution.
What you inevitably end up with is
a water repellent layer of mixed topsoil and mulch on top of the
plastic and dry, hard and compacted soil underneath. If you use
builder's plastic then, when you water your garden or it rains,
then the water tends to pool above the plastic or runs off your
garden beds.
As long as you lay you mulch to an initial depth of around
10cm, such that it eventually compacts down to 5cm or so, and
particularly if you use fresh tree mulch from the tree loppers,
then that alone is sufficient to suppress weeds for a few years.
The only weeds that you will need to worry about are those
that already had well established tap roots/bulbs/corms/rhizomes
prior to you laying the mulch. But as the mulch ages and breaks
down it becomes easy for weed seeds to germinate and establish.
Hence you need to refresh the tree mulch every few years or so.
The best mulch to use, from a soil ecology and health
perspective, is fresh tree mulch from tree loppers that contains a
generous amount of green leafy material. It encourages worms and
worms are essential to good soil health. They incorporate the
decaying plant matter into the soil, consume weed seeds, aerate the
soil and allow moisture to penetrate easily.
As a result doing this in my garden, and also due to dense ground
coverage, I spend very little time weeding my garden and heavy
black clay has become rich friable topsoil for a significant depth.
Every where you dig is teaming with earth worms.
Above is a soil core from part of my own garden where I have been
laying tree mulch directly over the soil surface for around 10
years.
You will note that:
- The right end of the core consists of unaltered native clay
that is quite plastic in texture.
- The left end of the core consists of broken down mulch which
is black in colour. I scraped away a lot of the loose mulch and
compost before taking the core so there is only very thin layer of
it left.
- The middle left of the core is a mixture of clay and compost
and is quite friable, as you can see from the break, and very
rich.
Please also note that where ever there is abundant organic matter
over or in the soil, it is often teeming with earth worms. The above
incorporation or organic matter into the clay soil has been carried
out by earth worms over the years.
Why don't you take a core of your soil, below your weed mat, and
see how it measures up!
How many earth worms do you generally find beneath your plastic
weed mat?
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Biodegradable weed mat is designed
to suppress weeds while your plants are establishing and then
slowly rot away as the plants take over the role of weed
suppression. It has many advantages over plastic weed mat:
- It does not impede
rainfall soaking into the soil.
- As it starts to rot it
encourages worm activity in the soil.
- It does not impede aeration of
the top soil.
- Its non-slippery surface is less
likely to cause the mulch to slide off.
- Its natural colour blends
in with natural mulches or it can be dyed with coloured oxides
to blend in with coloured mulches.
- It contains natural fibres and
no chemicals so it will not contribute to pollution.
Weed mat of any sort is merely a
short term measure to suppress weeds until dense plant growth
permanently takes over that role.
It is normally used in situations where you
can't use mulch, such as on steep slopes where mulch will slip
down to the bottom in a heap and on the edges of creeks and lakes
where the mulch will wash away if the water level rises.
However you can also use it if you wish to
cover your tree mulch with a decorative layer of pebbles or
tinted mulch etc.
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