Introduction

Heracleum mantegazzianum  Giant Hogweed is an invasive species in North America that is still in the early stages of expanding its range. I first encountered it when I grew up in West Sechelt, BC, Canada. It was about 30 years ago. I had no idea what it was at the time, but it just seemed out of place somehow. Not long after, I started getting interested in plants, and I looked into them and discovered that, yes, in fact, it was out of place in my environment. Fortunately for me, at the time, I had not yet touched the plant to experience the phototoxic effects. I did, many years later, accidentally touch a smaller one that was among many other plants. I quickly realized what it was and stepped back immediately, but it still left some permanent discoloration on my hand. That was just from a quick accidental touch. Many others have not been so fortunate, as it causes massive blisters, scarring, and even potential blindness. Keep your children away from it, as many have suffered from contact with it. Unfortunately, it looks very similar to a native species known as Cow Parsnip; see Similar Species Frequently Confused With below.

Description of Heracleum mantegazzianum

Heracleum mantegazzianum Leaves & Stems

Heracleum mantegazzianum is a huge monocarpic herbaceous perennial from a branched root system that goes 40-60 cm deep and 15 cm across. It reaches heights of 2-4(5) m with hollow stems 3-10 cm in diameter. The stems are covered in blister-like bumps with erect bristly hairs on them. The stems also usually have purplish splotches on them, helping to differentiate them from Cow Parsnip (see similar species below).

It has very large alternate leaves from a basal rosette. As it grows in height, the leaves get smaller up the stem. The lowermost leaves can reach up to 3 m long and 1.7 m wide when fully mature. Leaves are pinnately or ternately lobed, often deeply so, and are coarsely toothed. The above-ground growth dies back and returns after each winter until it flowers, and then it dies.

Heracleum mantegazzianum Flowers

Being monocarpic, Giant Hogweed flowers between 3-5 years of age before it sets seed and then dies. Its inflorescence is a terminal compound umbel 80(100) cm across made of 50-150 rays of bisexual flowers. Surrounding the large main umbel are up to 8 other smaller satellite umbels, which often grow taller than the main one. These satellite umbels may be all male flowers only or could be bisexual. The flowers themselves are on 10-20 mm long pedicels with white to pinkish petals up to 12 mm long each.

Heracleum mantegazzianum Fruit

Heracleum mantegazzianum fruits are flattened elliptical schizocarps 6-18 mm long and 4-10 mm wide. The fruits are narrowly winged and split into 2 mericarps, each with 3-5 elongated oil ducts.

Heracleum mantegazzianum Toxicity

Heracleum mantegazzianum is extremely phototoxic, particularly when in flower. Upon touching the plant, if then exposed to the sun, it can cause severe hives, blistering, and permanent discoloration of the skin. If the sap from the plant gets in the eyes, it can cause temporary or possibly even permanent blindness.

Similar Species Heracleum mantegazzianum is Confused With

There are a few other genera that Heracleum mantegazzianum can be confused with within North America:

• Angelica atropurpurea is native to northeast North America, but it has almost entirely purple stems and grows to 1.8 m tall. Its inflorescence is much more rounded rather than flat-topped and only up to 20 cm wide. Its leaves are also much smaller, softer, and divided into 3 parts, each with its own petiole.
• Daucus carota wild carrot is widely introduced in North America and appears similar superficially, but it also grows to much smaller sizes, has smaller flower heads, and smells like a carrot.
• Poison Hemlock Conium maculatum is sometimes confused with Giant Hogweed. However, it has tripinnate leaves that look fern-like and are very soft to the touch. It also gives off a foul odor not found in Hogweed.
• Pastinaca sativa Parsnip is occasionally confused with Giant Hogweed, but its flowers are yellow instead of white and its large leaves have long petioles, and the plant smells of parsnip.

There are also similar species in the same genus, which may be more difficult to tell apart. They can be differentiated as follows:

• Heracleum maximum (Heracleum lanatum or Heracleum sphondylium subsp. montanum, there is some debate as to its proper taxonomy) is referred to as Cow Parsnip and is native to North America. While it does look strikingly similar, it grows to only 2 m but never to the 4 m that Giant Hogweed can reach. Its leaves, while still lobed, are not as deeply cut as Giant Hogweed, and it lacks the purplish spots on its stems that are commonly found on the latter. Its flower head produces much fewer rays, only 15-50 per umbel, and the main umbel is usually no more than 30 cm across. Furthermore, Giant Hogweed is found only in some states and provinces, while Cow Parsnip is widespread throughout most of North America.
• Heracleum sphondylium German Hogweed has been sporadically introduced throughout the USA and southern Canada. It can be differentiated by its smaller size of 1.2 m tall and its pinnate leaves that are pinnately lobed rather than whole leaves that are deeply divided. The leaves are also not as sharply lobed as that of Giant Hogweed.

Native Distribution of Heracleum mantegazzianum

Heracleum mantegazzianum is native to the southern slopes of the Western Greater Caucasus of southern Russia and Georgia. It has spread much further in Russia and eastern Europe in the last century, and it is believed that at least some of this spread has been of natural origin.

Habitat Types Where Giant Hogweed is Found

In its native environment, Giant Hogweed occupies a wide range of habitats from 50-2000 m in elevation with between 1000-2000 mm of annual rainfall. It thrives in temperate continental climates with hot summers and cold winters. There it is typically found in meadows, forest edges, and clearings.

In areas where it is introduced, it also occupies a wide range of habitats and is often found growing along roadsides, train tracks, rivers, riparian areas, grasslands, meadows, and waste sites.

It grows in a wide range of soil types from heavy to light, seasonally waterlogged to well-drained, and alkaline to neutral, but less often in acidic soils. It does not tolerate wet soils that experience long periods of inundation.

Heracleum mantegazzianum is normally reported in full sun to part shade conditions in North America. However, it has also been found to grow in full shade in some areas.

Human Uses of Heracleum mantegazzianum

The seeds of Heracleum mantegazzianum are used in some Middle Eastern cooking as a spice known as golpar. In its native habitat, beekeepers use the plants for honey production. There were also some reports of people eating the cooked stems, though this is not recommended due to the phototoxicity.

Distribution of Heracleum mantegazzianum in North America

The species was first recorded in New York, USA, in 1917 and had made its way to western North America by the 1930s. It was likely brought as a garden curiosity due to its impressive size. Alternatively, it could have been brought through spice importation since its seeds are sometimes used in Middle Eastern cuisine.

In Canada, Heracleum mantegazzianum has been recorded in British Columbia, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland (excluding Labrador).

In the USA, Giant Hogweed is found in Washington and Oregon in the west and in Illinois, Michigan, Pennsylvania, North Carolina, New York, Connecticut, Massachusetts, and Maine in the eastern States.

So far, in Mexico, Heracleum mantegazzianum has not yet been reported.

Globally Giant Hogweed is also considered an invasive species in much of Europe and New Zealand, and it appears to have recently been introduced to South America.

How Giant Hogweed Spreads

Heracleum mantegazzianum is primarily spread over long-distance by deliberate human introductions as an ornamental as well the importation of the seeds as a condiment. Sometimes the seeds are also found as a contaminant in the international food trade. Its preference for riparian habitats also has enabled its spread by long distances via seeds floating down rivers.

Short-distance dispersal occurs through water, high winds, animals, and sometimes birds. Dispersal can also occur through humans transporting soil and on vehicles or clothing.

Most seeds in the field germinate the following year, but the seeds can remain viable in the soil bank for up to 7 years.

Habitats at Risk of Invasion in North America

Heracleum mantegazzianum spreads mostly through water, humans, and animals and frequently is reported along roadsides as well as riparian areas where it spreads by the movement of water. In multiple locations in North America, where it has spread via waterways along the banks, it has since moved into the adjacent woodlands, grasslands, and fields. Due to its tolerance to a wide range of elevations (50-2000 m), soil types, and sun, most habitats are at risk of invasion.

However, it will not grow in permanently wet soils or arid climates without access to water. It also generally will not penetrate deeply into dense forests, but it will grow readily in open forests and forest edges. Its range is still in the early stages of expansion in North America and will continue if left unchecked.

Impacts of Invasion

In addition to the risks of phototoxicity and potential blindness, there are other impacts of invasion. Due to its large size and phototoxic effects, it can impede access to water, trails, and recreational areas.

Giant Hogweed also negatively impacts soil dynamics because some studies have shown that it decomposes more slowly than native species resulting in a slower turnover of organic matter. The annual litter that it produces can also smother nearby native species preventing their germination or regrowth.

Studies by fisheries in the USA have shown that Heracleum mantegazzianum can shade native species in riparian areas and eventually replace them. This has been shown to increase erosion due to the herbaceous nature of the plant and the timing of flooding, leaving bare soil prone to increased erosion. This can negatively impact flooding as well as habitat for native fish.

Because Giant Hogweed grows to such massive sizes, it can shade out native vegetation anywhere it is allowed to grow. Actual studies on species richness have been conflicting, however, where it has been shown to increase in grassland habitats and a decrease in woodlands where it has invaded. Data on its impacts on species richness in North America are lacking altogether, but it is safe to assume that it will have a negative impact on most habitats.

Potential Benefits of Invasion

Heracleum mantegazzianum provides virtually no wildlife value as most wildlife does not eat the plant, and birds seldom eat the seeds. Bees do visit the flowers, however, including native bees. But as with all invasive species, if they were not present, the native bees would find other native flowers instead.

Methods to Remove Heracleum mantegazzianum

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and sometimes can still be found in your local garden stores. Do not buy or transport any Giant Hogweed, and do not plant it in your yard.

If you see them being sold online or in your local garden stores, please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Physical Control of Giant Hogweed

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming, but it usually causes the least amount of environmental damage.

Safety PPE to Use During Removal

Physical methods to remove Giant Hogweed are challenging due to the phototoxic side effects of physical contact. Proper PPE will help prevent any temporary or permanent damage to your skin or eyes. Do not attempt to handle the plant in any way without proper PPE. Gloves must always be worn while handling the plant at any stage of growth. Also, wear a thick long sleeve shirt and jeans or other suitable pants made of thick fabric to ensure that they cannot make contact with your skin. Wearing a hat with mosquito netting is also highly recommended to prevent it from coming into contact with your face. Safety glasses are also highly recommended to prevent possible temporary or permanent blindness should any sap get into your eyes. See the Affiliates link at the bottom for suitable PPE.

Dealing With Young Plants

The best time to remove Giant Hogweed is when the plants are young. At this time, they can be hand-pulled wearing gloves or using a weed puller. Try to get the root as much as possible.

Dealing With Mature Plants

If the plant is already large, this makes removal much more challenging. It becomes difficult to remove the above-ground plant growth, and the rootstock also gets quite large and deep. In this case, it is more effective to wait until the flowering year and then remove all flower heads. Because it is monocarpic, it will only flower for one season, and then the plant will die. To remove the flower heads, simply cut them off into a garbage bag while wearing gloves. Do not dispose of the flower heads without solarizing them in the garbage bag first, as the heads will continue to ripen even once cut off from the plant.

You will likely need to bring a footstool or a small ladder with you to reach the top of the larger plants. You will need to return to the site a few times that season as it often will produce new flower heads. If it has gone to seed at any time, be extra careful to cut the heads directly into the garbage bag to prevent the seeds from dropping on the ground. If there are any seeds at all, be sure to return to the site the following year to destroy any seedlings while they are still small enough to easily pull out by hand while wearing gloves.

Another method to deal with mature plants is to sever their taproot or destroy the crown by digging 10-15 cm below the soil and severing it. This method can destroy a mature plant before it flowers. This can still be challenging, however, due to the size of the plant. The above-ground growth will, in most cases, need to first be removed so that you can even get close enough to the taproot or crown to destroy it.

Mechanical Removal

Mechanical removal has been shown to be ineffective, as the plants will return from their hearty rootstock after they have been cut or mowed. Repeated mowings also have proven ineffective. Sometimes repeated mowings have been seen to turn the plant into a perennial that will flower multiple times before dying instead of the monocarpic perennial that naturally only flowers once and dies. Mechanical removal is not recommended. Burning is also ineffective for the same reason that it will likely regrow from its hearty taproot.

Disposal of the Shrubs Once Removed

If you have plants that have seeds on them, they must either be burned or solarized. If you are unable to burn in your area, then you must solarize them. To solarize, put the shrubs under a thick black tarp or into thick black garbage bags and leave them in the full sun for a good 8-10 weeks at least to be sure that all seeds are no longer viable. Some sources recommend shorter solarization periods but differential heat in the bags or under the tarps, cloud cover, and other temperature variances make shorter periods less reliable. That is why it is important they are solarized for as long as possible if using that method.

Chemical Control of Giant Hogweed

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used, species riches always significantly increase.

Furthermore, there are no chemical control methods that effectively target only Giant Hogweed. And because it often grows in or near riparian areas, chemical control cannot be used there due to risks to fish and humans.

Chemical control is not recommended.

Biological Control of Heracleum mantegazzianum

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. These control methods are often extremely risky and should only be carried out by professionals after years of rigorous study. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also usually not native to the environment and could behave differently when released there.

The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective.

At this time, the only known biological control method of Heracleum mantegazzianum is the use of intensive grazing by sheep or goats and rooting by pigs. Cattle will selectively avoid eating it, as with most invasive species, so they are not a suitable control method.

Using sheep, goats, and pigs can be challenging, however, because of the phototoxicity issue. Animals with thick, dark pelts are more resistant to toxic side effects. The area of control will need to be penned in, and the sheep or goats be allowed to graze for the first season to destroy the above-ground growth. The following year pigs can be let in to root out the roots and eat them as well as any sprouts as they come up. If pigs are unavailable, then the sheep or goats can be left in there for multiple seasons, and an integrated approach can be used for stubborn plants that keep re-sprouting. In this case, you will need to use physical control by manually destroying the crown or severing the taproot using a shovel.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form, this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts or seedlings otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Crucial

In many cases of removal, the site will need to be replanted because the bare soil will allow the seed bank of this and other potentially invasive species to germinate. Single isolated plants can simply be destroyed or dealt with using physical control, and then native species will regrow in their absence. Occasionally, however, Giant Hogweed will produce a large patch. In those cases, a replanting program should be planned and ready to implement upon removal.

Ongoing Monitoring is Essential

In all cases of invasive Giant Hogweed removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not. Seeds will also continue to germinate for several years if the plant was fully established in the area and not an isolated case. Seedling removal is by far the easiest method of control. The area should be monitored once a year for several years after the plants were removed to ensure they do not become re-established.

References and Resources

CABI on Heracleum mantegazzianum https://www.cabi.org/isc/datasheet/26911

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Fire Effects Information System on Giant Hogweed https://www.fs.fed.us/database/feis/plants/forb/herman/all.html

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Willis, Lyrae (Unpublished).  Plant Families of North America.

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