General information
Client: commercial.
Location: Manchester.
The area under treatment: over 1000 sq. metres.
Technique: combining mechanical and chemical treatment.
Status: in progress.
Problem: several areas in the park are densely infested with giant hogweed. The sap of the plant contains a photoactive toxic glucoside called furocoumarin and can be found in the leaves, stems and seeds. Contact with the sap on human skin followed by exposure to the UV light of the sun can cause serious burning and can be lethal to children if the exposed area is sufficiently large.
Objective:
Control actions on giant hogweed in easily accessible areas need to be implemented to protect the general public from the risk of injury from the plant.
Survey results
One of the areas infested with giant hogweed was on the hill a short distance from the footpath. The plant was flowering during the survey carried out at the end of June.
The largest infested area was situated around the swamp in the park’s centre. The area is quite close to the footpath and relatively easy to access for the public, e.g., children.
Individual plants of giant hogweed were located in the immediate vicinity of the footpath, which may put people at high risk.
As the plant has started producing seeds, we proposed to undertake actions in the next couple of weeks. These included cutting hogweed stems mechanically and disposing of the seeds. The proposed actions will prevent seeds from falling into the soil and germinating in the next growing season. Each plant may produce up to 12000 seeds, and up to 2000 of them may potentially germinate.
Treatment
Giant hogweed is at the end of June to the first half of July, reaches 10-15 ft tall, and produces flowers. The flowers form large, umbrella-shaped flowers, up to 5 ft in diameter. These comprise many smaller umbrella-like clusters, each containing over 50 stalked flower clusters. In most plants, the central umbrellas have already produced seeds. Further, the seeds have already been formed in the umbrellas around the central one.
In general, our strategy to treat the plant in the first year included the following steps:
- Cutting the plant to the ground level.
- Cutting the umbrellas that have already produced seeds.
- Destroying the seeds.
- (iv) Visit the site a few months after mechanical treatment and apply herbicide where regrowth occurs.
The plants were cut to the ground level with a machete. During the operation, our staff wore protective equipment to avoid contact with the toxic sap of the plants.
The umbrellas with seeds were cut from each plant and carefully collected.
Each umbrella was individually boiled in brine to kill the seeds. The umbrellas with dead seeds were then left on-site for composting.
It was roughly estimated that the total number of destroyed seeds was about 2.7 million.
The site was re-visited at the end of August.
Giant hogweed dies back at this time of the year. The seeds fall to the ground and spread away from the parent plant. They may produce new plants during the following several growing seasons. Large plants were successfully killed mechanically in July. However, due to the mild weather, the tiny plants up to 4 ft tall have regrown sporadically across the infested areas; no plants that have produced seeds since the last treatment were found. Herbicide was sprayed over a new regrowth.
The site was visited regularly in the second year, from April to September. The visits aimed to treat the giant hogweed before it started to flower. To avoid the plant producing seeds. The plant was mainly treated by spraying a weed killer when it reached a height of 0.5 meters.
It was concluded in September of the second year that the area is generally clear of the giant hogweed, thus suggesting that the previous treatments had been successful. Below are several photographs showing the areas that were treated cleared of Giant hogweed.
In the area around the footpath, no giant hogweed is evident.
In the area around the pound, no giant hogweed is evident.
We will visit the site once a month from April to September 2019 to control the hogweed in the area.