Ukonputket – Heracleum

Plants of the genus Heracleum are umbelliferous (Apiaceae) and very large. They can grow tall: more than three metres and even up to five metres in a lush location. In Finland, the most common species is giant hogweed (Heracleum mantegazzianum), and Persian hogweed (Heracleum persicum) has also been found here. There are also some observations of Sosnowsky’s hogweed (Heracleum sosnowskyi), but the findings have not been verified. Alien Heracleum group plants have been imported as garden plants, and they are especially found in built-up areas, as well as on roadsides, wastelands and courtyards in sparsely populated areas, but also in nature, including beach groves.

Giant hogweed and Sosnowsky’s hogweed are biennial, or they die after flowering. Persian hogweed is perennial and can flower several times, but it is able to reproduce not earlier than the third year. Heracleum group species can only reproduce through seeds and they can spread to new habitats with garden waste or soil contaminated with seeds. Their seeds generally germinate well and remain incubated in the soil for several years, which is why new populations may come as a surprise.

Separating different Heracleum group species from each other may be difficult, even for specialists. Their stem can be up to 10 cm in diameter and hairy. Giant and Sosnowsky’s hogweeds have a single stem with red or red-and-brown spots. Persian hogweed is perennial, and its stem is normally violet or red and brown, especially at the base. Its large leaflet-like leaves can be up to 3 metres in length, including the petiole. Giant hogweed leaves are single-paired and pointed, whereas the leaves of the other species normally consist of 2–3 pairs of leaflets. Blooming takes place in June–August. Flowers consist of a compound umbel, which can be up to half a metre wide. Flowers are white or slightly pink, and a single plant can consist of up to 80,000 flowers. The flowers of giant and Sosnowsky’s hogweeds are flat on top and consist of 60–120 smaller umbels, while Persian hogweed has fewer umbels (35–85), and its flowers are often slightly convex on top. After flowering, giant and Sosnowsky’s hogweeds usually die. Heracleum group species can also be identified based on their scent, as they have a distinct, strong and anise-like scent – especially Persian hogweed. 

To control hogweeds, it is unnecessary to identify each species. However, it is important to distinguish Heracleum group hogweeds from large natural umbellifers. Natural hogweeds (Heracleum sphondylium, H. sibiricum, Angelica sylvestris, and A. archangelica) are smaller than the species in the Heracleum group regarding all their dimensions. For example, natural hogweed stems are at most 2–3 cm thick.

In Finland, the plants were initially introduced as garden plants due to their large size and spectacular inflorescence. They are very resistant to northern conditions and have spread throughout the country, except for northernmost Lapland. Persian hogweed is less common in nature than giant hogweed. Observations of Sosnowsky’s hogweed have also been reported on the Finnish side. However, the findings have not been verified, and its distribution in Finland is poorly known.

Some of the existing populations are remnants from cultivation. They have spread from gardens to nearby forests, roadsides, wastelands and uninhabited plots along with garden waste and transferred land masses. They are also found in nature reserves and in the vicinity of endangered species.

Alien plants in the Heracleum group are native to the Caucasus and Southwest Asia. The name of each species hints at its origin: giant hogweed originates from Western Caucasus, Persian hogweed from the territory of Iran, Iraq and Turkey, and Sosnowsky’s hogweed from the Caucasus. Giant hogweed was introduced to England as an ornamental plant in 1817, and Persian hogweed a few years later. Only about ten years later, giant hogweed was first reported as a fugitive from gardens.

The history of Sosnowsky’s hogweed differs. It was apparently brought to the Kola Peninsula Botanical Gardens in the 1930s. It has been used as a fodder plant in Eastern Europe and especially in the territory of the former Soviet Union, e.g. in the Baltics and the Karelian Isthmus. It occurs on the Russian side in Karelia, very close to the Finnish border.

Sosnowsky’s hogweed (Heracleum sosnowskyi), giant hogweed (Heracleum mantegazzianum) and Persian hogweed (Heracleum persicum) are classified as invasive alien species throughout the EU. Invasive alien species may not be released into the wild, imported into the EU, kept, bred, transported, placed on the market, transferred, sold or otherwise handed over.

This information sheet applies to all three Heracleum species. All observations of these plants should be reported under the Heracleum group if you are unsure of the species.

Alien plants in the Heracleum group are very competitive, and they have spread to a broad range of habitats such as fields, roadsides, forests and beaches. In the best habitats, they form wide dense stands – “Heracleum forests” – which stifle all the other vegetation. They also affect other biota. For example, Sosnowsky’s hogweed affects communities of nematode in the ground and reduces their abundance. Tall Heracleum populations simplify the landscape and reduce biodiversity. Their extensive occurrence can also completely prevent outdoor activities in their vicinity, as well as recreational use of riversides, including fishing and walking. A property or land conquered by alien Heracleum plants may lose its value, and in old cultural landscapes, they can also be considered a scenic nuisance.

Plants in the Heracleum group may also damage people’s health. When the plant fluid reacts on the skin with sunlight, severe burn-like, slowly healing, or even persistent skin symptoms may arise. Sensitive people can suffer from shortness of breath or have allergic symptoms simply by staying near the plants.

Eradicating plants in the Heracleum group is laborious but possible based on the knowledge and skills accumulated in various control projects. There are many ways to control the plants mechanically. Control measures should already be started in early May, when the plants are still small. Individual plants can be controlled mechanically by cutting the main stem at a depth of 10–20 cm using a pointed shovel or by removing the stem completely by digging. First-year seedlings with delicate roots can be removed by hand. Frequent mowing stops photosynthesis, prevents the plants from collecting nutrition, and slowly reduces their vitality. Mowing is safest and easiest when the plants are still short. Because plants in the Heracleum group only grow from seeds, simply removing the flowers can already be effective. Cutting the flower prevents new seeds from developing and falling to the ground.

Small plants can be covered with a thick, black and opaque plastic sheet held in place by weights. This will cause the plants to wither, and seeds cannot grow in the soil. Covering can best be done after mowing. 

Any plant waste and soil contaminated by plants in the Heracleum group should be disposed of thoroughly. Any plant parts unable to reproduce, including leaves, can be composted. Seeds are not destroyed in compost, and they and any seed-containing flowers should be discarded as combustible mixed waste. To confirm your area’s instructions, contact your municipality or the local waste management company.

Control agents can be used with care, especially in large groups of plants. Control measures should be carried out in the early summer when the plants are still small. Large plants are difficult and dangerous to handle later in the summer, and larger amounts of control agents are needed. Control agents should not be used close to waterbodies or children’s play areas. An ecological and biodegradable product should be selected. More ecological options for chemical control agents are needed, and the impact of a pyrolysis agent made from birch (birch distillate) on giant hogweed seedlings has been studied in Finland.

Special care must be observed regarding plants close to waterbodies, as seeds spread to new habitats through water. The plants should be eradicated with neighbours if they have spread to several plots. If the neighbouring plot is unused, and you cannot request the landowner to eradicate the plants, contact your municipality. Control measures must often be continued for several years to eradicate the plants. The area should be monitored every year after eradication, as a single plant can produce a large number of seeds (20,000 on average) that may remain vital for up to seven years in the seed stock.

Because the species in the Heracleum group are included in the national list of invasive alien species, they cannot be imported, bred, sold or otherwise held, or released into the wild.

Do not buy or grow these invasive alien species. Ordering plants in the Heracleum group online is also prohibited. Plants in the Heracleum group cannot be handed over. Eradicate any invasive alien species in your garden and handle garden waste so that no seeds or plant parts able to germinate can remain vital. Make sure that the species cannot spread through soil material. Prefer domestic plants or plants that do not spread easily in your garden.

Professionals are obligated to prevent the spread of the species in the Heracleum group and their seeds and other plant parts able to reproduce outside their area through dirt, soil or other plants.

Any observations of the plants should be reported under the Heracleum group unless you are absolutely sure of the species.

Particular care must be observed when eradicating the plants due to the plant fluid they contain. Due to the risk of skin injuries, the plants should be eradicated in cloudy weather.  Protective clothing, a respirator and goggles are required when handling plants in the Heracleum group. If any plant fluid comes into contact with your skin, wash your skin immediately with water and soap. If large skin areas come into contact with the plant fluid, seek medical attention.