The following is an account of my attempts to control the invasive alien weed Giant Hogweed in a small nature reserve I manage in Norfolk – using a scythe as a key component of an integrated management strategy
One, two! One, two! And through and through
The vorpal blade went scnicker-snack*!
He left it dead, and with its head
He went galumphing back.
From “Jabberwocky” by Lewis Carroll
Scnick verb : to slice or sever with a scythe
* I have added the letter ‘c’ here to snicker-snack to create an onomatopoeic verb “to scnick” : to slice or sever with a scythe (as opposed to with a ‘snickersnee’ = a large sword-like knife, especially one used as a weapon). Pronounce as Schnitz (cut, slice – as in Schnitzel), and as the sound of a scythe blade slicing through vegetation.
Giant hogweed (Heracleum mantegazzianum) is a truly magnificent plant that can reach 10 feet (3 metres) or more in height with individual leaves that can be over 3ft (1 metre long).
It has massive white flower heads in flat topped clusters arranged like an umbrella (called ‘umbels’). These are a magnet to pollinating insects such as bees and hoverflies. It is however, not a British native plant but an ‘alien’ invader. It was introduced to Britain from Russia via Kew gardens in 1817, and quickly became popular with large estate gardens as a monumental curiosity.
Unfortunately, Giant Hogweed turned out to be a bit of a monster, a triffid if you like! Within ten years of its introduction to gardens it had escaped into the wild. It produces seed prolifically which can travel long distances floating along rivers and streams. It has since spread as an alien weed all around Britain and is a problem weed in many other parts of Europe and America too.
For more on its story read or listen to the lyrics of the 1971 Genesis song “The return of the Giant Hogweed”
The problems with Giant Hogweed are twofold. Firstly, it can form dense patches that take over and disrupt and damage our native plants and vegetation. Secondly its sap and glandular hairs contain a toxin which can cause unpleasant blistering of skin. Whilst it is not deadly poisonous, it has most definitely outstayed its welcome. As a result, there is a countrywide effort to see this alien invader repulsed, including an ongoing programme of containment and eventual eradication at the Saltings: the site I manage owned by Heacham Parish Council.
As any gardener will testify getting rid of a determined weed is never easy.
Efforts to eliminate Giant Hogweed at the Saltings began with by employing a contractor to undertake the standard recommended herbicidal blitz on the plants. This was successful in killing the treated plants, but, as the hogweed plants were quite closely spaced, the treatment also took out most of the grasses and other plants in between. The resulting exposed bare soil then provided a perfect opening for a flush of new hogweed seedlings from seed lying in the soil from previous years. Having started with a few dozen of mature plants separated by grass, we ended up with hundreds of new baby monsters!!!
A new strategy was needed, and hopefully one that was not wholly reliant on nasty chemicals, but one based on scientific advice. (I have a 300 page reference manual “The Ecology & Management of Giant Hogweeed” summarising extensive research effort from all around Europe).
The new control strategy is an integrated programme of two parts: containment and eradication.
The first priority objective is to prevent the alien colony from spreading further out across the reserve by seed. This is comparatively simple involving beheading the plants before the flowers can set any fresh seed in July.
The second objective is to restrict future increases in plant numbers arising from residual seed lying dormant in the ground. This is being achieved by reinstating the damaged grassland so as to close down bare soil openings through which new hogweed seedlings can germinate. Exhausting the pre-existing reserves of Hogweed seed will take years of vigilance – as the old adage goes “One year’s seeding makes seven years’ weeding”
The temptation is to go in early and cut down the hogweed plants as they emerge in spring before the leaves get to a size which will shade out plants beneath. I have tried this as a strategy of regular scnicking aiming to supress hogweed, and to encourage grass growth at ground level. However, even after three years of mowing the hogweed plants were not significantly reduced in number.
As with another bit of country lore relating to thistles:
cut in May is back next day!
cut in June will come back too soon,
cut in July will surely die!
This rhyme is actually more reliable for Giant Hogweed as it is what is known as ‘monocarpic’ – this means that once a plant has finished flowering and runs to seed, its job is done, and it dies (as do annuals an biennials in the garden). However, if you go in early and cut before the hogweed plants has fully committed to completing their life cycle, they will simply regrow to flower at a future date (and can keep doing this for some years).
Early cutting will delay flowering and contain Giant Hogweed – but not eradicate it.
So now my main control strategy is to cut down mature plants when they are as fully committed to full flowering as possible but not yet set any viable seed. I let them come on and hold my fire until we can see the whites of their eyes (flower heads!)
Fully grown giant hogweed plants are significantly taller than myself: slicing the scythe blade through fleshy hollow stems up to 100mm in diameter and watching the monsters crash to the ground is really quite satisfying! The bigger plants however do not always succumb quietly: these plants have got me twice with their skin blistering revenge (the sap contains a phytotoxin furocoumarin) when plants have toppled back against me, or scattered fragments have got to my skin. I now wear a full protective coverall (with no gaps for bits to fall inside) and have perfected a scythe swing that imparts momentum so the plant falls away from me – one of the few occasions when a golf-swing swipe is OK!