Emergence of Giant hogweed seedlings in the presence of simulated grazing

The Scottish Invasive Species Initiative has been running a trial since 2019 to investigate the feasibility of controlling giant hogweed through sheep grazing.  We are discovering that a critical factor in establishing a successful grazing regime is the timing of sheep introduction to the site and then how long they remain on site in relation to when giant hogweed seedlings emerge from the soil seed bank.  The giant hogweed emergency study will look more closely at giant hogweed seedling emergence over the growing period and its results will directly inform the sheep grazing project. Read about the sheep grazing trial.

It appears that the emergence periods of giant hogweed seedlings vary between and across river catchments and geographic areas; yet there is currently no information available that looks at the extent or scale of difference across areas in Scotland.  By recording seedling emergence in a number of areas, an emergence curve for giant hogweed seedlings can be produced and we can better understand how much variation there is between sites across Scotland. 

This information is important – it will help us write guidance for farmers who might want to use sheep grazing to control giant hogweed.  The guidance will provide advice on grazing pressure required (informed by our giant hogweed grazing trials) and the optimal timing of grazing (informed by these emergence observations and resultant emergence curve) to enable delivery of effective control of giant hogweed through sheep grazing.

To do that, reliable records taken from a range of giant hogweed sites across a growing season are needed.  The records will be part of a wider study – initially and principally within the Scottish Invasive Species Initiative area – but which is hoped may extend in future to other areas to give a more refined assessment of emergence periods across Scotland and, potentially, beyond.

The Study

During 2021 there were 15 sites monitored for the full 8 months of the study (March - October). Each site contains at least three 1m² plots and is known to have giant hogweed present. 

The sites are visited on the first week of each month and various photographs and observations recorded. The number of giant hogweed seedlings are counted and removed from the plot.