American skunk cabbage control on the Bladnoch catchment

Funding

The Scottish Solway INNS control and knowledge programme (Phase 1) was funded by the Nature Restoration Fund (NRF), a fund launched in 2022 encouraging projects that restore wildlife and habitats addressing biodiversity loss and climate change.

The project is now in Phase 2 now funded by The Heritage Lottery Fund and match funded by Killgalioch Community Funded (administered by Foundation Scotland).

American skunk cabbage;

American skunk cabbage (ASC) is a tall herb with basal rosette stemmed leaves, which can grow up to 70 cm wide and 1.5 m high. Yellow flowers are produced between late March to May and a strong odour (like that of a skunk) is emitted while green berries are produced in the summer. Seeds disperse via waterways as well as by birds and other animals. American skunk cabbage historically was widely planted in ornamental gardens and was still available in garden centres as recently as 2009, however as it gradually escaped from gardens and became established in the wild the results of its invasive nature became very clear and control was considered important to protect native species.

Spread is via seed production and underground rhizome roots. Seed banks can build up in the soil and can remain viable for up to nine years, however plants grow slowly and only seed when they are three years or older. Negative environmental impacts caused by the spread of ASC is extensive damage to native flora including vascular plants, mosses and smaller plants which are out competed and shaded out. Being very resilient it has the ability to grow in a wide range of environments and even thrives in disturbed areas too. Control programmes may be lengthy but significant reduction in numbers and full eradication can be achieved.

The River Bladnoch Catchment SAC;

The River Bladnoch is designated a Special Area of Conservation for Atlantic salmon. GFT are not aware of American skunk cabbage on the main River Bladnoch however we have identified three populations on tributaries of the Bladnoch;

• Glassoch Burn - GR 233678 569518
• Grange Burn - GR 237063 557104
• Torhouse Burn - GR 239014 556125

Methodology:

Landowner permissions to survey and for control to take place is required by GFT. Walk-over surveys are conducted to identify and record American skunk cabbage at sites to be controlled. Once the source location is identified surveys continue for a further 2 km upstream to ensure that this is indeed the most upstream location. The data gathered is then used as baseline data for designing a long-term control plan. This is then followed up by control using a solution of Roundup (the active chemical being Glyphosate) and water at a ratio of 1:0.025. This is applied to each plant via backpack sprayers by qualified operators. Control only takes place in dry, cool weather to ensure the best possible outcome from the treatment as well as minimising possible contamination of watercourses and non-target plants. Efficacy of the first treatment is assessed during a second site visits and any missed/late emerging plants are controlled at this time. Baseline data will be used to create a long-term strategic control plan to control ASC on the Bladnoch over the coming years.

The Glassoch Burn (SAC designated):

Control on this burn started in 2022. A survey in September recorded five large plants and approximately 100 smaller plants in the wetland at Glassoch Farm House - but it was difficult to find all of the plants due to the dense undergrowth on the site. A small plant was also found further downstream at GR: 233551 569374 highlighting that the ASC were spreading downstream from the wetland at Glassoch via a drainage ditch to the banks of the Glassoch Burn.

A second survey took place on the 21st September 2022, which was 21 days after the initial control took place, by the project officer. The survey showed around 50% efficacy from the first control. As the surrounding shrubs had started to die back the project officer noticed an additional 20 missed/late emerging plants. Hence, a second site visit and control was consequently planned to ensure all plants on the site had at least one control spraying in 2022.

Year 2 control:

In the first week of April 2023 the project officer surveyed the area again. This time the undergrowth was cleared back to ensure all plants were found. Approximately 200 non flowering plants and 30 flowering plants were found on the farm ground. Eight non flowering plants were found in the ditch (LB) and a further seven plants found along the banks of the Glassoch Burn. Two flowering plant were found in a nearby wooded area. At this stage all flowerheads were removed to ensure plants do not seed. The increased number of plants found in 2023 is due to the earlier survey being more effective as the surrounding undergrowth was still low.

The first control took place on the 11th of May 2023, using a knapsack sprayer with a Glyphosate solution. At this stage a further 20 new plants were found of which 3 were flowering. All plants were treated. A second site visit took place on the first of June during which any late/missed plants were controlled.

Year 3 (2024):

Approximately 80 small non flowering plants were found during the first survey on the 19th March 2024. Plants were too small hence no control was carried out at this time. A second survey took place on the 13th May 2024, were approximately 200 plants were recorded of which 8 flowering plants were further downstream from the source. Control using backpack sprayers was carried out at this point. A significant decline in the number of flowering plants at the source was recorded showing control is now targeting the seed bank.

The Grange Burn (SAC designated):

First site visit took place on the 6th of April 2023. Initial thoughts were that only a small number of plants were present on an un-burn leading down to the Grange Burn. However, after surveying the site it became apparent that the extent of the problem was greater than initially thought. Approximately 400 plants were found at the top of the burn (figures 9 & 10), a further 80 plants were found further downstream close to the Grange Burn and about 50 plants present in the adjacent field on the right bank (Figure 11 - 13). At this time all flowerheads were removed by the project officer with assistance from landowners.

The first control took place on the 12th of May 2023 with the PO and one other GFT staff member over one working day. At this stage a further 100 plants were found further downstream with one plant present on the banks of the Grange Burn itself. The latter was dug out making sure all the root was taken out and safely composted. The Grange Burn is considered particularly sensitive as it is included under the Bladnoch Special Area of Conservation designation.

A second site visit took place on the 2nd June 2023 over half a day by the project officer. This visit highlighted the effectiveness of the first control as well as the importance of carrying out two site visits when undertaking control. Due to the large amount of plants some plants were missed and some late emerging ones were also noticed on this visit. Hence, a second control took place to ensure all plants had at least one treatment.

Year 2:

First survey took place on the 2nd April 2024 with only approximately 30 plants recorded. With a significant decline recorded at the source, management protocol was discussed with landowners who opted to dig the plants and safely compost the plant matter throughout the season. Approximately 200 plants were recorded by PO at the time of control (17/05/24) with the last plant recorded at GR: 237121 556557.

Torhouse Burn:

Year 1:

While undertaking surveys the project officer discovered a new population of ASC at the bottom of Torhouse Burn. She surveyed upstream to find the source of this population. One plant was found at Torhousemuir House and a small population found at Meadow Bank. After discussions with the landowners it seems that these two sites are the source however both landowners have knowledge of the invasive nature of the plant and have been controlling it for a number of years. The project officer therefore moved her attention downstream. A further site visit took place on 20th July 2023 during which the project officer, assisted by another GFT staff member. Approximately 100 plants were found on this section of the burn. As this site was found late in the season only one control visit was possible.

Year 2:

First site visit took place on the 19th March 2024; at this stage plants were too small for control. Approximately 90 plants were recorded along Torhouse Burn on the 17th June 2024 during which PO carried out survey and control using a backpack spray and a solution of Glyphosate.

Conclusion

American skunk cabbage control on these 3 sites has been proceeding well and PO has observed decline in the number of plants in all sites. The Glassoch Burn source site has now reached a manageable level and control has been handed over to the landowners under the advice of PO. The Grange Burn and Torhouse Burn populations are in Year 2 of control. PO recorded a decline in the numbers of flowering plants. PO plans to continue control programme working closely with landowners in the coming years. It is expected that plants will continue to decline resulting in less man-power and chemical for control and eventually the seed bank to be depleted.