Have you have ever come across a mature gunnera plant?
If
you have I’m sure it’s something you’ll never forget. From a distance they just
look like a clump of rhubarb leaves, but get closer and this herbaceous
perennial just seem to grow and grow. Reports of 3 metre high leaves with a 2m
leaf span are regularly documented.
It is one of those architectural plants, like the giant
hogweed, whose scale astounded Europeans and made it the must-have ornamental/
edible plant for 19th century horticultural fashionistas and collectors. In Ireland,
Kylemore Abbey (Co Galway) and Ilnacullin (Co Cork) planted them up. They were
more recently introduced into the Irish gardens but then this triffid-like
plant has escaped and naturalised along swathes of Irish coastline as well as
along riverbeds, coastal cliffs and forest margins.
The biggest problems are now in particular on Achill Island
and in Counties Mayo and Galway where they are classed as an invasive weed
There are two main types of large gunnera. There’s the manicata
variety that isn’t invasive and sold in garden centres then there’s the
tinctoria which gives us the problems. There are loads of others too down to
tiny 10cm high groundcover called monoica but are overshadowed (groan) by their
larger cousins.
Gunnera tinctoria produces a large cone-shaped flower head
from spring to early summer, with small flowers. The fruit is orange. It is a
deciduous plant whose leaves die back in autumn (October) commonly leaving the
large brown rhizomes exposed. The flower clusters on the stems occurs early in
the spring, with the fruits maturing in late summer/early autumn. Large numbers
(up to 250 000 seeds per mature plant) of drupe like, red or orange seeds are
produced. Small fragments of the rhizome have the potential to establish new
plants.
Growth starts in early spring (March) It can reproduce by
both sexual (seed) and asexual (vegetative) means.
Problems caused by
Gunnera tinctoria
Due to Gunnera’s wide leaf span, large dense cover can have
a dramatic impact on the local biodiversity by light exclusion. On rivers it
causes erosion to banks, exposing them to fast running water after die-back in
winter. When the plant dies back its dead leaves fall into watercourses and can
block drains and watercourses, resulting in flooding. It can also alter the
visual aspect of once beautiful landscape vistas, by rapidly colonising large
tracts of fallow ground.
While Gunnera has become a major problem in the west of
Ireland, INVAS Biosecurity believes in its mission statement of providing the
“first line of defence”, and that preventative eradication programs should be
put in place now by those stakeholders whose responsibility it is to protect
our environment. This would be extremely cost effective both in the short-term,
and save millions in the long-term.
INVAS
Invas Biosecurity, a private company in Dublin are among a
growing number of companies that want to control invasive species in the
countryside. They can carry out baseline distribution surveys, risk assessments
and develop comprehensive management plans, which include spraying, Rhizome
injection, Cut and paint herbicide injections in much the same way Japanese
Knotweed is controlled. Biodiversity Ireland would also like you to report any sightings
of the plant if you see them in the countryside (not in your neighbours garden
I don’t think).
Care for gunnera in
your garden
Larger gardens can showcase the gunnera manicata and keep it
under control without issues. One thing that stops the plant in its tracks are
temperatures lower than -8°C, so a good hard frost will kill them off unless
they are well wrapped up with dead leaves. If you have a smaller variety in the
garden you will probably cover the crowns with dry mulch. The leaves of larger
varieties could be cut off after the first hard frost. Inverted, the leaves
provide excellent coverage for the resting crowns.
Another method to help keep out moisture is to remove leaves
after the frost, cover the crown with 60cm of straw, cover with a burlap tarp, or
large plastic container, such as a tub, and then add another 60cm of straw.
Seasoned wood chips or sawdust will work even better. After all danger of a
hard freeze is gone, in late March or early April, protection can be removed.
Just to say again, it’s reckoned that G. manicata – the
Chilean one – doesn't share its Brazilian cousin's invasive nature. So perhaps
this should be your preferred Gunnera should you have the time and space to
grow one.
