Giant Hogweed Can Be A Giant Problem

Giant Hogweed in flower

By Kelly Reid
Special to WHT

Along with the lush growth during spring weather comes the rapid growth of an ever-increasing problematic weed. Introduced as an ornamental plant in the 1900’s, Giant Hogweed is considered among the worst of invasive plant species.

This giant pest, which can grow to 15 feet, has been located in the Brockton area. Over the past two years volunteers have been eliminating it from along the banks of the Saugeen and along the river trails by digging it out. There is quite an infestation in Carlsruhe that runs along the creek for a mile or more. That creek runs into the Saugeen bringing the seeds of the Giant Hogweed plant with it.

The seeds germinate in early spring. From July to August a large flower head (2.5 feet across) is formed which produces more than 10,000 seeds that can remain viable in the soil for up to 10 years. Giant hogweed is a highly competitive plant that can form large stands within a short time, thus the need to maintain a steady vigil in eradicating it. Several methods may be used effectively. Note that the caustic sap from the plant can cause blistering of the skin, burns and dermatitis however, with basic knowledge and a few precautions, this plant can be eradicated safely.

Control Methods

Seeds are mainly dispersed by water so if an up-stream seed source is ignored, control programs down-stream will prove unsuccessful in the long term. During removal procedures, protective water-resistant clothing must be worn and any kind of direct contact with the plant avoided. If contact occurs, wash the affected area with water and soap and avoid exposure to sunlight to minimize any effects. Regardless of the method chosen, total eradication requires persistent control for a minimum of 7-8 years. Periodically, combinations of controls may be necessary.

Digging:The entire plant can be dug out. This effective yet, time-consuming method is only feasible for smaller populations.

Root cutting: To deter regrowth, the buds on the upper parts of the root must be removed; the root should be cut at least 10 cm below soil level using a spade. The cut plants are pulled up and either destroyed or left to dry out. Repeat twice a year until the germination of the seeds ceases.

Mowing/cutting: Cut plants above ground leaving the roots intact. Mechanical mowing techniques, e.g. a flail mower, are useful for large areas. Mow 2-3 times during the season to prevent re-sprouting plants from setting seeds. Plants can be cut manually using a scythe or a brush cutter along rivers or on slopes. The plants are ultimately starved through depletion of their nutrient reserves and will eventually die if the method is conducted consistently during 7- 10 years. A very serious problem with the cutting technique is the risk of skin contact with splatters of the toxic sap.

Seed Head Removal: Timing is critical: the umbels should be removed when the seeds are formed but not quite mature, usually in August. If the umbels are cut off when flowering, the plant simply forms new umbels. The umbels must be removed from the site and burned to impede seed maturation (avoid inhaling the smoke from the burning plant). Control may need to be conducted several times during the flowering season in order to be effective.

Grazing: Grazing by sheep and cattle is very efficient if initiated early in the season when plants are small. Negative effects are less likely from the hogweed if the diet is mixed. Skin eruptions around the ears and the mouth indicate poisoning and affected animals must temporarily be removed from the field. Grazing is a cheap method when large fenced areas can be established. The livestock requires daily inspection and access to water; nutritional supplements may be necessary.

Chemical applications: Brocton’s Integrated Pest Management Policy dictates that this be used as a last resort when all other methods have failed however, chemicals should always be avoided in riparian areas; therefore this is not a suitable alternative when removing the plants from the area around the Saugeen River. Herbicides containing the active ingredient glyphosate are considered most effective but should be used cautiously since they are non-selective. Where Giant hogweed grows intermixed with other plant species, spot spraying directly on the plant’s leaves is recommended. Spraying must be conducted at least twice in a growing season. Initially when the plants are approx. 10 cm tall and follow up with a second spraying in July or August.

All landowners are encouraged to eliminate populations growing on their property to ensure that control programs implemented on public lands will be successful. A Giant hogweed population grows very quickly, so it is important to eradicate the population when it is still small and easy to eliminate. For further information or to have a plant identified, call the Brockton Community Centre at 881-0625.