“Where a weed can
grow, a plant can grow,” I was told once, years ago, by a senior CSIRO scientist.
I thought he meant that a weed prevents a desirable (‘good’) plant growing in
the same spot, or that if a weed can grow ‘there’ then that soil must have
qualities that good plants could exploit too, if their seed had been there first.
But perhaps he was referring to the paradox below which occurred to me after Alison
Elvin’s workshop on 10 November.
Alison, who is a
rural educator and local farmer, gave a comprehensive overview of weeds
management to an attentive and responsive group of southern NSW landholders. The whole-of-a-Sunday session was the first in the series called ‘Action on Weeds near Wallaroo’, run by the Ginninderra Catchment Group. It was held just
outside the ACT at the Wallaroo Fire Station, on a day that became very wet, with the fire
engines quietly asleep in their adjacent shed after the previous spell of hot dry weather
and destructive bushfires.
The landholders first outlined weed issues that
were worrying them (which Alison noted on a board: photo below), and several brought examples to be identified. In
exchange they heard about possible reasons for the infestations, and a number
of ways of managing land and livestock to deal with weeds and avoid
reinfestation.
As Alison pointed
out, in our brief time outside, if there is bare ground in a paddock (or garden or anywhere else) then a
weed is the most likely type of plant to move in. Paddock weeds have trouble
growing where healthy ‘good’ paddock plants are offering strong competition, so
if there are bare patches on your land you can expect paddock weeds to grow
there!
Although we heard
about weeds’ competitive features, Alison also told us some of their ‘benefits’,
such as … fat tap roots which open up the soil (e.g. Paterson’s Curse Echium plantagineum); ability to
mobilise cations out of the soil matrix, such as copper (Paterson’s
Curse/Salvation Jane) and calcium (Capeweed Arctotheca
calendula); and the way some weeds reveal soil character by their presence,
with thistles for example apparently showing that the soil is well structured,
and Serrated Tussock (Nassella trichotoma)
protecting the ground so well that the ‘best worms for fishing-bait’ may be
found beneath it!
As competitors, weeds excel
— as we know! — and this is because, Alison said, weeds include plant species
that can flourish in low-nutrient, acidic (comparatively rich in aluminium ions)
compacted soils (therefore few pore spaces and little oxygen, water or microfauna). Weeds succeed also because they often have a strong and fast growth pattern, cover the ground with rosettes of leaves, flower vigorously, have lots of
pollinators and few predators and are able to send plentiful seed out to new ground. On that basis, Serrated
Tussock seems to be an ideal example weed, judging from the NSW Dept of Primary
Industries webpage at http://www.dpi.nsw.gov.au/agriculture/pests-weeds/weeds/profiles/serrated-tussock:
“Serrated tussock is a perennial,
drought-resistant, highly invasive tussock-forming grass which is a serious
weed in Australia, New Zealand and South Africa. It is highly adapted to a
range of environments, seeds prolifically and is difficult and costly to
control. Large volumes of seed are spread long distances by wind; allowing new
populations to establish over large areas.”
Here is my paradox.
If you have bare ground which offers too little amenity for pasture species to
survive, then the weeds that move in may improve it for you. If the soil is
compacted, those invading weeds will force their roots through it, creating
pore space. If it is deficient in minerals, weed roots may draw them up from
deeper horizons; the minerals will then become available in the topsoil when
the plant parts die and fall. Weed plants supply organic matter to that previously
bare ground, and protect it from wind and water erosion. Once you remove the
weed, the ground could well be in better condition for other plant species. (Note: be
sure to spread desirable seed there immediately, and/or mulch.)
All that makes me
wonder why we define weeds as ‘plants that grow where they are not wanted’.
Perhaps we just don’t know what is good for us! Is this what my CSIRO colleague
meant, years ago?
Another odd
situation is that while paddock weeds in many cases are non-native to
Australia, so too are many of our favourite pasture and crop species! Paddock
weeds, then, are mainly non-native species that are not valued for grazing or
harvesting here? (This is probably also the case in England, for example, where
many similar mixtures of species are likely to be similarly classed as pasture,
crop and weed.)
If our
pastures comprised mainly native species, like our bushland, then non-native
invaders would be more understandably weeds. ... Actually, though, Alison
pointed out that native grasses such as Kangaroo Grass (Themeda triandra) and Weeping Grass (Microlaena stipoides) make good pasture grasses …
Alison’s whole-day
overview was based on her own and others’ experience and reading across a large
literature ranging from official ag-department information to traditional
northern hemisphere knowledge about species. Her workshop introduced
thought-provokingly creative ways of managing weeds; and although she did touch
on the use of herbicides, we were told that considerable weed-control has been
achieved via non-herbicidal whole-land management on her own farm.
Whole(some)
land management, we heard, can include annually mapping the weeds (including
their density) on one’s land in relation to topography, watercourses,
neighbours (weeds on high land are more likely to blow next door), other
vegetation and grazing. Timed management of livestock, as well as type of
animal (donkeys and goats eat weeds other stock reject), and cultivation, slashing,
burning, chipping out, mulching, and scattering desirable seed to change the
balance in the seed pool, all have their place in Alison’s portfolio of weed
control methods.
I recommend
attending a workshop by Alison Elvin if you are interested in weed ecology or
economic botany or land management, even if (like me) you are not a landholder.
Her rural education and native seed business is Natural Capital Pty Ltd, based at Gundaroo, NSW. Alison
will be leading a paddock walk, also in the Wallaroo area, on 30 November to
show us how to identify (and perhaps map) weeds in the paddock. I aim to be
there, if at all possible.
