Hogweed munching sheep at work

Al Reeve, Project Officer, Scottish Invasive Species Initiative, June 2019

In an era where people are increasingly environmentally aware and there is a move away from the over-use of chemicals we want to investigate a solution to invasive species control that can be done inexpensively, at low intensity, and with minimal environmental impacts….so, enter the humble sheep!

This trial project by the Scottish Invasive Species Initiative is a practical experiment to discover if a land manager could use sheep to control a substantial giant hogweed invasion. A previous sheep grazing trial, by the Deveron, Bogie and Isla Rivers Charitable Trust at Auldtown (near Huntly) which began in 2013, shows that in a controlled environment, sheep can make light work of giant hogweed infesting a field. (Read the Auldtown Grazing Report). However this new trial is not just about measuring the impact of the sheep on the giant hogweed and woodland environment, but of the real life example of managing the site and sheep from the farmers perspective, recording the successes, failures and lessons learned from the outset.

The new trial site is at Kirkside Farm, Macduff, where we are working in partnership with the Gordon family who farm here. It is a 1km stretch of woodland sandwiched between arable fields and the River Deveron. Giant hogweed has begun dominating the woodland, particularly around the sunnier edges, and along the numerous streams that flow through the wood to the river. In the past, attempts to control the giant hogweed has been via pesticide application using a knapsack sprayer, which proved very time consuming and costly. Due to the scale of infestation and the difficult terrain of the site so far little progress has been made in restricting the giant hogweed’s expansion.

After fencing in the site, in April this year we released 25 sheep into the woodland area, asked dog walkers to keep their dogs on leads and waited to see what happened… and for the first month, nothing did! The sheep lazed around the entrance to the site, maybe due to the new environment or maybe because they had never eaten anything other than grass before.

During this time we started mapping the extent of the hogweed through the site both though ground surveys as well as using drone footage (courtesy of TAHR Media) to visualise the infestation. Working in partnership with The University of Aberdeen we will be monitoring the site and have developed a base map of the hogweed which, after an annual survey, we can compare to. We have also identified 40 monitoring spots where we will measure the effect of the sheep on the rest of the woodland vegetation.

As time went by though, we started to pick up more indications of the sheep growing in confidence with clear trails developing through the wood showing where the sheep had been walking as well as a few nibbled hogweed stems.

Fast forward a couple of months and the sheep have well and truly gained a taste for the hogweed. There are now large patches of much-munched hogweed, a huge change from previous years. It’s worth mentioning here that sheep don’t actually kill the hogweed plants just by chewing on them but by restricting the plants from flowering they are slowly removing the amount of energy stored in the hogweed’s tuberous roots. Hogweed take 3 or 4 years to mature before finally sending out a flower spike to release seeds but by continually grazing the plants sheep can stop the older plants from flowering and mow down new seedlings as they emerge from the soil.

The alternative control method of using a broad spectrum herbicide like Glyphosate results in the killing of the target plant but, however carefully applied, also results in some collateral damage of the other species surrounding it. This has the effect of opening up the soil which enables more hogweed seedlings to germinate and fill the newly cleared space. One of the many benefits of grazing as a control method is that the ground flora is not killed off and this acts as a barrier against hogweed seedlings emerging.

Despite the great work of the sheep this year, there are still some patches of hogweed that have grown too big and dense for the sheep to get on top of and so this first year has not been a complete success, but we’ve learnt a valuable lesson – next year we need to get the sheep on site earlier in the year. However, on a positive note we’ve been delighted by the response of the site users and dog walkers in acting responsibly and the local awareness raising and signage has been a success.

The sheep will stay on site for a few more months and hopefully continue their good work and we can get a fair idea of what their impact has been so far. Next year, we can act on our lessons learned and improve our methodology.

This is a multiple year project and one of the outputs will be to develop practical and useful advice to empower other land managers to use sheep – or other livestock – as a means of controlling invasive plants on their land. This will include public open days and site visits etc. as well as written findings.

This project truly is a work of collaboration and the Scottish Invasive Species Initiative would like to thank all the partners for their continuing support and input; the Gordon family of Kirkside Farm, The University of Aberdeen, the Deveron, Bogie and Isla Rivers Charitable Trust, the Macduff Distillery and the local people of Macduff.