Invasion of the Giant Hogweed

What seems like a scene from a science-fiction movie is an unfortunate reality. The species Heracleum mantegazzianum, commonly known as giant hogweed, is an invasive species spreading throughout much of the western world. However, by understanding this species in its native context, the history of its invasion, and its invasive context, the complete story of the species emerges, which will enable better prevention and management practices of its invasion.

What is Giant Hogweed?

Giant hogweed, Heracleum mantegazzianum, is an herbaceous perennial native to the Caucasus region of Eastern Europe (Cuddington et al. 2022). Here it thrives in species-rich meadows, clearings, and edges of forests (Shackleton et al. 2020), where it can grow two to five meters in height with a crown of white flowers eighty centimeters in diameter at the apex (Mullerova 2014).

The life of this plant does not start as grand as the final product; it starts as a tiny seed. The seed will need consistently cold and moist soil to start germinating. From this point, the young giant hogweed will spend the next three to five years of its life growing to its normal height (Cuddington et al. 2022). In those few years, it serves as a refuge for hundreds of species of insects and is besieged by various fungal pathogens, but none of the identified insects nor fungi have evolved to specifically rely on this plant (Pysek et al 2007).

Towards the end of its life, giant hogweed focuses on one, large reproductive event, making it a monocarpic plant (Pysek et al 2007), and through a combination of general insect pollination and self-pollination, each hogweed can produce upwards of 50,000 seeds (Cuddington et al. 2022).

With this overview of the basic life history of giant hogweed, one can begin to see the traits which make this plant such a notorious invader.

How the Invasion Began

Although the nature and invasion of this organism make it seem like a creature out of modern science fiction, it has been spreading outside of its native range for over two centuries.

The earliest documented invasion of giant hogweed dates to 1828 in the United Kingdom, where it had escaped a botanical garden and established a wild population; subsequent invasions in other European countries, Canada, and the United States followed a similar pattern: multiple horticultural introductions lead to major invasions (Cuddington et al. 2022; Henry et al. 2009).

At each point of invasion, giant hogweed tends to form dense, monoculture stands, crowding out native species (Shackleton et al. 2020). The invasion, however, is slow-moving (Cuddington et al. 2022), due to the seeds of giant hogweed being dispersed by wind and water or occasional human-assisted seed transfer- such as accidental vehicular hitchhiking, or beekeepers intentionally planting the seeds in hopes they would produce copious amounts of nectar (Henry et al. 2009). Such means of distribution explain why this species colonizes stream banks, abandoned fields, and roadsides (Cuddington et al. 2022).

The evidence presented here tells an all-too-common tale of how this plant started as a garden curiosity that slowly spread across the western world; despite its slow spread, giant hogweed is causing various problems to abiotic and biotic systems as well as humans.

Abiotic Impacts

The abiotic changes to an ecosystem caused by an invasive species are hard to observe directly, but the impacts are still important.

The most dramatic change occurs in stream ecosystems. For invaded stream habitats, giant hogweed led to erosion of stream banks by killing the native plants that were holding the bank intact (Shackleton et al. 2020; Pysek et al 2007). Although this invader is shaping the terrain it finds itself in on a macroscopic level, it has an even greater impact on the unseen but ever-present chemistry of the soil.

A study from the early 2000s found substantial increases of potassium and manganese concentration in stands of giant hogweed (Vanderhoeven et al. 2005). Furthermore, a more recent study looking at the long-term changes in soil chemistry over many decades found that soil became more alkaline with an increase in conductivity and available phosphorous (Jandová et al. 2014). These changes in soil chemistry wreak havoc on the native flora, as they had evolved to cope with a specific balance of chemicals.

Together the physical and chemical changes caused by giant hogweed, although near invisible to the passer-by, severely harm native ecosystem processes leading to impacts on the native residents.

Biotic Impacts

The biotic impacts of giant hogweed are the most noticeable to the average person, but by understanding them, one sees the danger this invader poses to the natural world.

The most obvious biotic impact of the invasion is the direct competition for sunlight with native plants, as the invasive stands create a dense canopy blocking most sunlight from reaching the ground (Shackleton et al. 2020; Pysek et al 2007). The main consequence of this localized interspecific competition is a decrease in species richness (Pysek et al 2007; Dostál et al 2013).

Some have speculated that giant hogweed not only competes for sunlight but actively secretes chemicals to kill other plant species, a process known as allelopathy (Pysek et al. 2007), but a recent study- focused on European herbs- has shown that there is little evidence to support these claims (Wille et al. 2013). Unfortunately, the impacts are not limited to plants, as recent studies have explored the impacts on the soil microbiome.

One study looked at the change in fungi to bacteria ratio in the soil of giant hogweed stands; they determined that over many decades the fungi became more prevalent, but only to a certain point before it returned to a normal ratio (Jandová et al. 2014). Additionally, a 2021 study looked at the effects of the invasion on nematode communities; although it is lacking in statistical power, it shows a tantalizing glimpse that the nematode communities are becoming simpler to cope with the changes to the microbial community (Renčo et al. 2021).

These two studies shine light on the fact that giant hogweed shapes the subterranean world as much as it does the surface; these changes in the plant and soil biomes show that the invasions are tugging at the threads of the delicate tapestry of life.

Impacts on Human Welfare

The mere presence of giant hogweed is not just terrible for the ecosystem, as previously outlined, but also a serious threat to human wellbeing.

Giant hogweed and other members of its taxonomic family, the Apiaceae, produce secondary compounds, known as Furanocoumarins, in their sap to protect themselves from harm- whether it be anything from the size of a microbe to humans (Pysek et al 2007). The main mechanism of toxicity of these chemicals is their ability to damage DNA (Pysek et al 2007). Once the furanocoumarins are absorbed into the cells, they can migrate toward the nucleus; if the compound is hit by ultraviolet light, it becomes energized and can create cross-linkages in DNA (Baker et al. 2017). The kinks in the double helix created by this process will lead to various mutations and eventually cell death (Pysek et al 2007); the cells most likely to suffer this fate are skin cells- as they are capable of being exposed to both the compound and ultraviolet light.

With the loss of these cells, the body is more susceptible to sunburns, which can vary in intensity from minor inconveniences to full-thickness burns (Baker et al. 2017). Although the burns are the most conspicuous aspect of the furanocoumarins, there are other less noticeable impacts it has on human health such as the generation of oxygen free radicals and the inhibition of unsaturation enzymes (Pysek et al 2007) and even the rare case of causing blindness (Cuddington et al. 2022).

The medical community is not the only group that these conditions impact, as it colonizes areas- meadows, trails, roadsides, stream banks- that tourists are drawn to (Cuddington et al. 2022). Although little research has been done on the tourism impacts, giant hogweed is certainly a concern for many countries. The chemical mechanism of furanocoumarins from giant hogweed and the cellular havoc they can cause are serious harm that impact overall well-being, the medical sector, and even the tourism industry.

Controling the Invasion

From attacking the biosphere, human welfare, and major economic systems, it is abundantly clear that invasions of giant hogweed are a significant threat, which needs to be prevented or managed.

Although this giant of a plant seems a daunting foe to challenge, the understanding and communicating of its biology is its Achilles’ heel. Due to the seeds’ need for consistently cold and moist conditions to start germination, the range of giant hogweed is limited by temperature and precipitation (Shackleton et al. 2020). Fortunately, most governments-the United States, United Kingdom, and European Union- are aware of this and have introduced legislation to limit the spread of the weed (Mullerova 2014), and many are rapidly adopting programs to detect and educate the public about the invader (Shackleton et al. 2020).

The management of the invasion is also proving successful in many parts of the world. A common and low-cost solution to controlling the invasion is the use of grazing animals- namely sheep, goats, and pigs- to eat large stands of giant hogweed; sheep and goats are best for eating above-ground foliage while pigs prefer the roots, disrupting the seed germination process through rooting (Pysek et al 2007). Such practices are most successful over many years by grazing animals, not destined for human consumption, often eliminating the stands (Pysek et al 2007).

More active methods of management such as physically cutting back plant growth and treating with chemicals- treatment with glyphosate is common- are quite effective but can be demanding and suffer from chemophobia in the public view (Pysek et al 2007). A 2020 study found that the best management strategy is an integrated approach of both physical removal, chemical treatments, and public awareness (Shackleton et al. 2020).

There may also be evidence that the human struggle to manage giant hogweed is being aided by mother nature. A few long-term studies have shown that the biotic components-the native plants and the microbes- of invaded ecosystems can adapt to and combat the invasion over many decades (Dostál et al. 2013; Jandová et al. 2014).

This brief look at the many tactics to combat giant hogweed gives one hope that the invasion can be halted and even reversed.

Conclusion

Giant hogweed is a fascinating organism with an even more intriguing invasion. By understanding its biological context in its natural range, the history of its invasion, and the impacts it has on both the natural and human landscape, one gains the knowledge that despite its imposing nature, giant hogweed can be prevented and managed.

References

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Renčo M, Jurová J, Gömöryová E, Čerevková A, (2021) Long-Term Giant Hogweed Invasion Contributes to the Structural Changes of Soil Nematofauna. Plants 10: 2103. https://doi.org/10.3390/plants10102103.

Shackleton R, Petitpierre B, Pajkovic M, Dessimoz F, Brönnimann O, Cattin L, Čejková Š, Kull C, Pergl J, Pyšek P, Yoccoz N, Guisan A. (2020). Integrated methods for monitoring the invasive potential and management of Heracleum mantegazzianum (giant hogweed) in Switzerland. Environmental Management 65: 829-842. https://dx.doi.org/10.1007/s00267-020-01282-9.

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