Giant Hogweed

Download our updated bulletins for control with Roundup ProVantage and Roundup ProBio.

Control of Giant Hogweed (Heracleum mantegazzianum) with Roundup Pro Biactive


Giant Hogweed, (Heracleum Mantegazzianum), is a native of the Caucasus mountains between Russia and Turkey and was introduced into Britain as a garden plant at the end of the 19th Century. It is now widespread throughout the British Isles where it frequently colonises riverbanks.

Not only is the plant an invasive weed which can displace native plants and reduce wildlife habitats but it is also a serious hazard to health. It is an offence under the Wild life and Countryside Act 1981 to “plant or cause Giant Hogweed to grow in the wild.”

It is a perennial plant which can grow up to 5m tall with dark green, deeply lobed leaves which can grow up to 1m across. The plant forms a rosette of leaves in March/April and may take 4 years to flower after which it dies. The flowering stem can be up to 5m tall and each umbrella-like flower head can be 500mm across. Flowers are white. Each plant can produce more than 50,000 seeds and these remain viable for up to 15 years. With the seeds being easily dispersed by wind, water or carried by footwear or vehicles it is not hard to understand why it has spread so rapidly.

Health hazard

The stems and leaves are covered with small hairs coated with poisonous sap. Even the slightest touch can cause painful blisters and severe skin irritation. The reaction can take up to 48 hours to occur and can result in recurrent dermatitis. Medical help should be sought if blistering occurs. Symptoms are often worse in the presence of direct sunlight, i.e. the reaction is photosensitive.

Control methods

Cutting every 2 weeks below ground level before the plants reach 1.5m will reduce colonies but is very labour intensive and risk of sap contact is high.

Chemical control in aquatic situations needs Environment Agency approval.

Control with Roundup Pro Biactive

Spraying with Roundup Pro Biactive is the preferred method of control and will give excellent results. The formulation is the safest to use near water and carries no Irritant symbol on the can, so is safe to operators, animals and the environment.

Application should be carried out wearing face shield, gloves, overalls and wellingtons and contact with the plant sap avoided. Try not to spray in direct sunlight. Monitor sites annually for 5-10 years and re-treat as necessary to take account of newly emerged seedlings.

Rates, Timings and Water Volumes

MethodDose rate of Roundup Pro BiactiveDose Rate of Roundup ProBiactive450Application Advice
Overall Spray 5l/ha 4l/ha Spray when good foliage has developed to a height of at least 50cm in late spring.  Flowering plants may be sprayed with extending lances. A second treatment will be necessary if more seedlings germinate. Monitor every 4-6 weeks
Weed Wiper 1 part Roundup Pro Biactive in 2 parts water 1 part Roundup ProBiactive450 in 3 parts water  Useful method when working amongst desirable plants and shrubs

Spot Treatment in Knapsack Sprayer

Most knapsack sprayers are supplied with a set of 4 deflector nozzles giving different swath widths but all delivering 200l/ha of water at 1 bar pressure and a walking speed of 1 metre per second.

Area SprayedRate of Roundup Pro BiactiveRate of Roundup ProBiactive450Volume of Water
50 Square Metres 25ml 20ml 1 litre
500 Square Metres 250ml 200ml 10 litres
1,000 Square Metres 500ml 400ml 20 litres