The giant hogweed is dangerous for two reasons: as an invasive species it displaces native plants and its sap causes skin irritation. Here are information about the problematic perennial.
Profile of giant hogweed:
Scientific name: Heracleum mantegazzianum
Plant family: umbellifer family (Apiaceae)
Other names: cartwheel-flower, giant cow parsley, giant cow parsnip, hogsbane, wild parsnip, wild rhubarb
Planting time: do not plant anywhere
Flowering period: June to July
Location: sunny to partially shady
Soil quality: sandy to loamy, nutrient rich, humus rich
These information are for temperate climate!
Use in: underplanting, water garden
Winter hardiness: hardy, USDA Plant Hardiness Zones: 3 (-37 °C / -35 °F)
Bee and insect friendly: Yes
Plant characteristics and classification of giant hogweed
Plant order, origin and occurrence
The giant hogweed (Heracleum mantegazzianum), also called cartwheel-flower, giant cow parsley, giant cow parsnip, hogsbane, wild parsnip or wild rhubarb, is one of the invasive neophytes. It is a so-called newcomer, which was introduced to Central Europe from the Caucasus as a garden plant only around 1900. It likes it here so much that it has been spreading rapidly in the wild ever since, displacing native plants. The reasons are obvious: the perennial shows enormous growth and can reach a height of up to 4 meters (12 ft) under good conditions. Added to this are its large white umbel flowers, which are also highly prized by beekeepers as bee food.
Characteristics of giant hogweed
The giant hogweed is a short-lived perennial and rarely grows older than two years. After germination of the seeds, it remains relatively compact in the first year and produces only leaves. Only in the second season does it grow rapidly upwards and within a few weeks it reaches a growth height of over 2 meters (6 ft). Its hollow, tubular stem is finely hairy and covered with irregular purple dots. Depending on the size of the plant, the stem can become lignified near the ground, become very stable and can have a diameter of up to ten centimeters.
Starting from the stem, the giant hogweed usually forms jagged, multi-fingered leaves up to one meter long. Depending on the size of the plant, the individual leaf blades fall out in three to nine parts and are fiddle-cut.
One of the reasons why the plant was imported is its striking white inflorescences. Depending on the size of the plant, the double umbels reach a diameter of 30 to 50 centimeters (12 to 20 in) and consist of 30 to 150 flower stems. It is noticeable that the flowers are larger at the edge of the umbels than inside the umbels. Despite its bad reputation, the perennial is interesting for beekeepers because it forms around 80,000 individual flowers per plant and thus provides the bees with a rich supply of food.
When the flowering season is over between June and July, a single plant produces on average about 20,000 seeds.
Toxic plant sap of giant hogweed
What makes the giant hogweed particularly dangerous is its plant sap. The liquid contains phototoxic substances that cause severe burns on the skin when exposed to sunlight. These burns are also called grass dermatitis, are very painful and often leave pigment changes after healing. Because of its potential danger and the rapid spread of the plant, the giant hogweed was awarded the questionable “Poison Plant of the Year” award in 2008 in Germany.
Especially for children and animals, contact with giant hogweed can be a painful experience. Burnt legs, arms and hands with children and burnt noses with dogs are the most common injuries. The contact is favored by the locations that the giant hogweed prefers: The perennial plant loves nitrogenous, moist soils and therefore often grows on water banks and forest edges in clearings and along paths. Especially the last location is of course predestined to come into contact with the giant hogweed. If you have come into contact with the plant sap, you should immediately protect the skin from sunlight at this point and then wash the sap off thoroughly with warm water and soap.
For native plants, the giant hogweed is an overpowering competitor because of its strong growth in most locations. It may not be long-lived, but its enormously high seed production ensures that it can spread and the large leaves cast so much shadow that weaker competing species are reliably suppressed. After sowing, the mother plant dies in most cases, but several new perennials soon emerge in its vicinity.
Fighting giant hogweed
If the giant hogweed is spreading in the garden, it should definitely be removed before it can seed itself. Protect all bare skin with waterproof clothing and put on rubber gloves to prevent contact with the plant sap. It is not enough to simply knock the plants off at ground level, as they will then usually sprout again from the tap root. Therefore, remove them with a spade or a daisy grubber afterwards. Many local environmental associations also organize occasional campaigns to drive back the giant hogweed in the wild. Volunteers are very welcome here as well.
No fighting without protective clothing
However, before you get to work, you should first put on protective clothing to protect your eyes, skin and mucous membranes from the nettle hairs and plant juices. It is important here
- Covering the whole body with firm clothing and closed shoes
- This also applies to the face
- Wear strong protective gloves
- Safety glasses made of Plexiglas and with lateral protection are also mandatory
Furthermore, all measures should take place on a day with cloudy skies: Injuries caused by giant hogweed mainly occur under the influence of sunlight.
Successful control of giant hogweed – Methods
Giant hogweed can be effectively controlled by mechanical methods, whereas the use of herbicides and other chemical substances is generally not allowed – especially not in recreational areas and near water bodies.
Timely removal of the flowers / seeds
The giant flower umbels of the giant hogweed develop up to 50,000 seeds, which are widely spread by wind, water, animals or vehicles. For this reason, the flowers must not even reach the state of seed formation. Therefore, cut off the flower umbels in June at the latest.
Excavation with rootstock
However, it is better and more effective to dig up the whole plant at once. A sharp spade should be used to dig at least 15 centimeters deep into the ground and, in addition to the above-ground parts of the plant, the rootstock, which is not dissimilar to a beet, should be separated or dug out completely. Afterwards, the giant hogweed cannot sprout again due to the lack of a root. If you want to be on the safe side, remove the soil layer, dispose of it and put on new soil.
Mowing / Tilling
However, digging up the plant is only useful for single specimens. Sometimes, however, larger areas are infested, which then have to be regularly mowed down close to the ground and, if possible, tilled up to 15 centimeters deep. Start with this in May and repeat the procedure every ten days – and all summer long. Let sheep or goats graze, if available. The animals don’t mind the poisonous plant, and they eat it in time. On grazed areas the giant hogweed is therefore almost never found.
Safe disposal of the plant remains
To dispose of the plant remains safely, you would have to burn them. The giant hogweed does not belong on the compost, not in the organic waste and has no place in the residual waste.