Gunnera

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.