Facts about Giant Hogweed ~ Heracleum mantegazzianum

Plant Description

Giant hogweed is a very large, herbaceous biennial or monocarpic perennial forb that is well-known for producing rapid and prodigious growth often to the detriment of native plants. It is about 2 to 5 m (6 ft. 7 in to 16 ft. 5 in) tall and under ideal conditions, a plant can reach a height of 5.5 m (18 ft. 1 in). The plant is found growing in meadows, clearings, forest edges, valley bottoms, forest clearings, abandoned grasslands, riparian areas, open woodlands, abandoned pasture, agricultural lands, roadsides, vacant lots, streams and rivers, creeks, ditches, rights-of-ways and uncultivated or waste lands. The plant prefers deep, moist soils with nearly neutral pH and moderate to high nutrient content. Soil textures in giant hogweed habitats may be gravels, sands, loams, or clays. Moist soils are preferred, and waterlogged and winter-flooded soils are tolerated but extended periods of flood are not preferred.

Root

Plant produces thick, branched taproot and tightly clustered, fibrous roots. Roots extend 18 to 24 inches (45-60 cm) deep. Lateral roots are often finer than vertical roots. Root’s thickness and branching increase with plant age and result in a multi-taproot system. Root crown can grow to 6 inches (15 cm) in diameter and becomes lignified when giant hogweed reaches flowering age. Root crowns are often found up to 5 inches (12 cm) below ground. Root crowns have contraction rings capable of shrinking the area between the crown and the shoot and protecting the hypocotyl from extreme temperatures.

Stem

The hollow stem of giant hogweed is coarse, stout, ridged with protruding white hairs that are noticeable at the node and base of the petiole. The stem is mostly green in color with purple blotches that contrast easily with the white hairs. Stems can grow to a height of 10 to 15 feet and measure between 2 to 4 inches in diameter.

Leaves

The plant has been a popular ornamental plant because of its massive size and eye appeal. Leaves are ternately compound and unfold in the early summer into deeply incised, lobed leaves measuring up to 5 feet in width. The leaflets attached higher on the stem are not as large, are triangular-lanceolate, and deeply cut. Giant hogweed has an alternating leaf arrangement. The plant remains in the rosette stage until it develops sufficient root reserves to initiate flower formation.

Lower leaves are 1-2.5 meters long, compound, irregularly shaped in ternate or pinnate segments, deeply lobed, and irregularly toothed. Upper leaves are smaller and sometimes not divided with longer petioles and more inflated sheaths. Leaves are usually pubescent on the underside when young and glabrous above.

Flower

Whitish or greenish white flowers appear in mid-June and are clustered in umbel shaped heads which can measure up to 1 m across. Umbels are an umbrella-shaped cluster of short-stalked flowers, typical of plants of the carrot family. Each compound umbel can have 50-150 rays (separate stem) which can lead to a single plant producing well over 50,000 flowers. Individual flowers are on pedicels 10-20 mm long and have petals up 12 mm long. Terminal umbels are the largest and are surrounded by satellite umbels and additional umbels may occur on auxiliary stems. Sometimes a plant does not flower until one or two years later. After flowering and fruiting, the plant dies. Unfortunately each large flower umbel has about 20,000 seeds.

Fruits

Fertile flowers are followed by dry schizocarps consisting of two mericarp seeds 6-18 mm long, 4-10 mm wide and about 1 mm thick, which are joined until ripening. Mericarps are elliptical, flattened, and emarginated at the apex with thin low dorsal ridges and broadly winged lateral ridges. It is tan in color with brown lines (so-called oil tubes) extending 3/4 of the length of the seed. The endosperm is oily and mature fruits have a strong resinous smell

Mature seed of the current year is dormant and will not germinate until the following spring at the earliest. Dormancy of these seeds is overcome by cold and wet weather conditions that occur during normal winters in Ohio. Seeds can remain viable in the soil for many years. Water, especially in flood plains, and wind are the primary dispersal mechanisms for giant hogweed seed; however, animals may account for some dispersal. The most efficient seed dispersal is known to be through human activity.

Other Facts

• Giant hogweed has been cultivated for silage in Russia and has been recommended as a forage crop in Poland.
• One giant hogweed plant can produce 20,000 seeds, allowing it to spread quickly when not managed.
• Animals, rain and wind facilitate seed dispersal.
• Seed is able to germinate even after 10 years of dormancy.
• It can live for several years but once it flowers and bears fruit it dies.
• Rock band Genesis, wrote a song “The Return of the Giant Hogweed” in 1971. Song describes negative influence of the plant and inability of people to eradicate it.

Precautions

• Sap of giant hogweed is phototoxic and causes phyto photo dermatitis in humans, resulting in blisters and scars.
• When skin is exposed to sap and the sun, severe skin rashes, blisters, and possibly permanent scarring or staining can occur.
• Giant hogweed sap in the eyes can cause temporary or possibly permanent blindness.
• Burns or blisters often appear within 24 to 48 hours of plant contact.
• Other effects may include welts, rashes, and blistering, followed by pigmented scarring that may persist for as long as six years.

Safety Precautions for Controlling Giant Hogweed

• Do not touch the plant with bare skin.
• Do not touch your bare skin with sap covered gloves.
• Prevent UV sunlight from reaching skin by:
• Wearing long waterproof gloves, long sleeves, pants, boots, and eye protection; synthetic water-resistant materials are best since cotton and linen fibers can soak up the plant sap and be penetrated by plant hairs.
• If controlling plants with multiple people, keep a good distance from one another as the sap can splash three to four feet.
• Apply sun block before beginning work.
• Launder clothes that may have contacted plants.
• Wash equipment with water immediately after use.
• Limit exposure to sunlight after control OR work around giant hogweed plants after sunset.
• Do not use a “weed-whacker” or brush cutter – sap may splatter as stems are cut.
• Keep water, soap, and eye-wash near work area in case of exposure.

What to do if you are exposed to Giant Hogweed

• Wash the affected area thoroughly with soap and Cold water as soon as possible.
• Keep exposed area away from sunlight for 48 hours.
• If a reaction occurs, topical steroids applied early can reduce the severity of the reaction and ease discomfort.
• If sap goes in eyes, rinse them with water and wear sunglasses.
• If a reaction has occurred, the area of skin may be sensitive to sunlight for a few years and you may want to apply sun block or keep the affected area covered from the sun when possible.
• See a physician if you have a reaction or any questions.

Control Methods

The preferred method of control is Integrated Pest Management (IPM). Site specific control methods need flexibility to match local management requirements.

Four to five years of prevention of seed production, with treatments to remove adult plants, and suitable turf re-establishment seem adequate for eradication of a local population

Property managers and cooperators may use these strategies:

Cultural Control

Seed heads should be removed to reduce reproduction. Mowing does not appear to be effective. The Pennsylvania Department of Agriculture (PDA) has used a roto-tiller and planted a lawn seed mix to return previously infested areas to a natural state. The tillage appears to speed up depletion of the seed bank.

Chemical Control

Glyphosate is an effective herbicide, but it should be used with caution around desirable plants, and is more likely to require multiple treatments: it should also be applied before flowering. The herbicides 2, 4-D, TBA, MCPA, and dicamba may burn down foliage, but are not effective in controlling perennial GHW roots. Rodeo (glyphosate) has been recommended for wet areas. Protective clothing should be worn when applying herbicides or removing seed heads. In urban areas, plant materials should be double bagged in plastic, and buried or incinerated in an approved facility. Other chemical recommendations are available.

Biological Control

In Switzerland 12 phyto-phagous insect species found on native hogweed have been identified as possible candidates as biological control. Cattle and pigs are cited as possible bio-control agents.

Environmental impact

GHW has naturalized in many of the places where it was first introduced. The plants thrive in many habitats but do particularly well where the soil has been disturbed, such as on wasteland, on riverbanks, and along railroads. It prefers moist soil and can quickly dominate ravines and stream banks. It can tolerate full shade, but prefers partial shade to full sunlight. Forest clearings and edges are a preferred habitat for GHW.

Benefits of control

Giant Hogweed is both a public health and environmental hazard, since humans, as well as dogs and other animals may be sensitive to its sap. Furocoumarins in GHW sap cause a skin reaction known as photo-dermatitis or photo-sensitivity. Exposure to the sap in sensitive individuals sensitizes the skin to UV light in sunlight and results in long-lasting swelling, blisters and eruptions of affected sites. Hyper-pigmentation in the affected area may remain for a year or more. In the 1970’s, many cases of poisoning were seen in Great Britain where children played with the hollow stems of the plant as pea-shooters or telescopes. In 1998, cyclists complained that contact with “large green plants” left their legs covered in broken and bleeding skin. Contact with the eyes can lead to temporary or permanent blindness.

References:

https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=502954#null
https://www.cabi.org/isc/datasheet/26911
https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=275991&isprofile=0&
https://plants.usda.gov/core/profile?symbol=HEMA17
https://www.fs.fed.us/database/feis/plants/forb/herman/all.html
https://www.invasiveplantatlas.org/subject.html?sub=4536
https://en.wikipedia.org/wiki/Heracleum_mantegazzianum
http://issg.org/database/species/ecology.asp?si=418&fr=1&sts=&lang=EN
http://www.iucngisd.org/gisd/species.php?sc=418
http://gringlobal.iita.org/gringlobal/taxonomydetail.aspx?id=18896
https://gd.eppo.int/taxon/HERMZ

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