Giant Hogweed
Giant Hogweed - Heracleum mantegazzianum - was introduced into Britain
in 1893 as an ornamental plant. It
escaped from domestication and is now
colonising many areas of waste land and river
banks. It can grow to 5m high and has a large umbel of white flowers from which it produces 30 to
50,000 viable seeds per year. When these seeds fall into water they are dispersed downstream
and washed up along the bank, allowing the plant to spread
rapidly along watercourses. In a natural state, the plant is biennial, growing from seed in the first
year and flowering in the second season. However, if the plant is cut down before it produces
seed, it will survive into a third or subsequent
season, attempting to flower each year. It can form
dense colonies which suppress the growth of native
plants and grasses and leave the banks bare of
vegetation in the winter. These are then liable to
erosion or to recolonisation by seeds of Giant
Hogweed washed downstream onto the bare ground.
Control
Mechanical control - Hand cutting should never be undertaken unless the
operator is wearing full protective clothing to prevent
skin ontamination by the sap. Machine operators
should take similar precautions because the sap can
be spread onto machinery and subsequently come
into contact with skin. Cutting before flowering will, at
best, produce only temporary control and ensures
that the plant regrows in the following season. Cutting after flowering has no benefit once the
seeds have been formed, except to clear away the dying vegetation. Small infestations can be
controlled by digging out the whole plant.
Chemical control - The only herbicide which is known to control Giant Hogweed and which is approved for use in or near water, is glyphosate. Long-lance sprayers may assist in accurate application of glyphosate to plants growing in inaccessible sites along river banks. Glyphosate can be applied as a spot treatment to individual plants, using hand-held equipment, or as an overall spray using machine-mounted spray booms. In the latter instance, total weed control of all vegetation will occur and it may be necessary to reseed the treated area with grass and other native plants. Establishing a good sward of grasses soon after treatment of the weed will help to reduce the rate of recolonisation of the area by seeds of Giant Hogweed.
Due to the toxic nature of the sap from this plant, we strongly recommend using our qualified operators to kill this plant. We can use our boat to reach inaccessible places along river banks.
Biological control - None known.
Environmental control - None known.
The seeds of this plant are thought to remain viable for up to 7 years, and pos sibly up to 15. Once a plant has produced seed, it should be assumed that the seeds will be present in the surrounding area for at least this length of time. Control measures will only affect those plants which have already germinated and viable seed may continue to germinate each year until the seed bank is exhausted. Therefore, eradication, as opposed to temporary control for one season only, requires regular annual checks to ensure that any germinating plants are controlled before they can seed.
On river banks, seeds may be washed downstream from colonies along the river bank. Any attempt to eradicate this plant is unlikely to succeed unless control is exercised along the whole river system.