Autumn/Winter 2011/12 - Harcourt Arboretum - University of Oxford

2 Botanic Garden News | No. 79
A few words...
Timothy Walker is Director of the Botanic Garden and
Harcourt Arboretum; part of his job involves lecturing to
undergraduates not only on plant lifecycles but also on
control of invasive species.
by
Timothy
Walker
Is biology a science or was Lord Rutherford
correct when he claimed “that which is not
physics is stamp collecting” It seems harsh
on the chemists but the boundary between
physics and chemistry has always struck
me as very fuzzy. The problem with biology
is that it is rarely black and white, and
particularly ecology and plant conservation.
Whilst punting up the River Cherwell at the
beginning of August the plants that I thought
were duckweed (Lemna gibba) were actually
a mixed community of Lemna and Azolla,
known variously as mosquito fern, fairy fern,
water fern or duckweed fern. Now you might
think that a mat of this non-native species is a
Bad Thing. But the truth is complicated.
“Duckweed fern” is a good common name
for use in countries that have duckweed.
Elsewhere in the world Azolla is known as
“Mosquito fern” derived from a belief, perhaps
misplaced, that if the surface of a pond is
covered with Azolla then the mozzies cannot
lay their eggs in the water. The weed may
indeed help to reduce the amount of oxygen
available to the larvae and so help to control
mosquitoes. A thick mat, however, will shade
out any other plants in the pond and may
result in the death of fish and invertebrates.
“Water fern” is a good name because
Azolla is a fern that floats on the surface of
the water, but it is not the only floating fern.
Another is Salvinia which is closely related
and which has occasionally been placed
in the same family as Azolla. The former,
however, is not hardy at all whereas the latter
will survive the British winter even though it
is originally from tropical to warm temperate
regions of America.
The country of origin is perhaps irrelevant
as Azolla has now spread throughout the
world. It is not considered to be a true UK
native species having been introduced as
an ornamental in the 1840s. Its habitat is
ponds or larger areas of standing water and
it is not very salt tolerant. This summer may
have been the coldest for twenty years but
it has not been very wet and for much of it
the Cherwell and its tributaries have been
flowing very slowly. This, coupled with the
absence of serious frosts in 2011, has meant
that the Azolla has been very happy.
Azolla has a deserved reputation as an
invasive thug because it can grow very fast.
It has been shown to double its weight in
three days. Part of the reason for this is that
inside its leaves are chambers containing
colonies of a cyanobacteria, Anabaena
azollae, which can fix atmospheric nitrogen.
This puts the Azolla at a selective advantage
over its neighbours because its growth is
not limited by the amount of nitrogen in the
water. The real limiting factor for this plant
is phosphorous but not in English rivers into
which fertiliser leaches.
The ability of Azolla to fix nitrogen has
been recognised and exploited by farmers in
China for over a millennium. Azolla is grown
in rice paddies around the plants and this
helps to suppress the weeds in the fields.
A mixed community of Azolla and Lemna
After the crop is harvested and the water has
drained away, the mat of nitrogen-rich Azolla
can also be ploughed into the soil before the
next crop.
It has been calculated that Azolla can
provide up to a tonne of nitrogen per acre
per year. If you have Azolla growing on a
pond in your garden it can be scooped out
and used as a very good compost accelerator.
Azolla is also thought to be a nutritious
food for stock including chickens. If you
have neither a compost heap nor chickens
you may want to control the plant. A
recently developed option is to exploit a
herbivore that was probably introduced