Floating Pennywort Project Progress

Floating Pennywort Treatment 2020 – 2021

By Lincolnshire Rivers Trust and Environment Agency

Treatment Method Data: Hand Pulling V Spraying

The methods being used tell us a lot about the situation of Floating Pennywort in the Witham catchment. Spraying is used for large patches (unable to hand pull) and patches in complicated locations (difficult to reach). Hand pulling is used for small accessible patches. In 2020, 92% of patches were sprayed, 8% were hand pulled, but in 2021, 28% of patches were sprayed, and 72% were hand pulled (Figure 1), indicating patches are smaller overall.

Abundance of Floating Pennywort

In 2020, 4429 patches of Floating Pennywort were treated, whereas in 2021, up to November, 2402 patches have been treated so far (Figure 2). When the December treatment round is complete, these final figures will be added.

Unfortunately, accurate data was not recorded during treatment on the Catchwater Drain throughout lockdown. However, using data dating back to January 2020 – November 2021, it was estimated that 313 patches would be treated per month. When this is factored into the results, this shows a 25% decrease in the number of Floating Pennywort patches treated in 2021, compared to 2020.

Distribution of Floating Pennywort

Floating Pennywort is widespread in the catchment, found as far up as Torksey Lock, along the Fossdyke channel, including the river Till, Catchwater Drain, and Brayford Pool, the upper Witham and lower Witham, all the way to Anton’s Gowt.

A staggering 4,429 records were logged by the contractors during 2020. Each one represents an individual treatment, either spraying herbicide or hand pulling. The map below shows all of these records combined (Figure 3) . It’s important to note that multiple records will be recorded for some locations as often multiple treatments are needed to eradicate the plant.

Move forward to 2021 and again a large number of records were logged. In total 2402 separate sites were recorded. This is encouraging as it shows a decrease in the overall number of separate plants given the effort was broadly similar to 2020. However, the plant is still firmly established over a large area (Figure 4). The only area showing a meaningful reduction is the river downstream from approx. Woodhall Spa. Here the records suggest that there has been at least a successful partial eradication. Again note that there is likely to be multiple records of the same plants as often more than 1 treatment is required.

During 2021 the contractors not only logged each individual record but also recorded the size of each corresponding patch. These were recorded in 3 size categories, including small (up to 1m²), medium (1m² – 2m²) and large (>2m²). The below map shows all the data for the latest round of treatment carried out in 2021 (Figure 5). Crucially the data shows that the overall biomass has been reduced with 93% of the records being for small patches. Please find a breakdown of the latest data recorded per patch size in Figure 6, 7 and 8.

Conclusion

Although we have an extensive dataset to draw on it’s difficult to come to any definitive conclusion about the success of the project. Although the overall biomass has almost certainly reduced compared to the 2020 data, it is hard to quantify what impact the treatment is having. One thing that is clear, the plant is still widespread throughout the catchment. However the size of individual patches at each site has reduced. This suggests that treatment has restricted growth but has not eradicated the plant in the majority of areas – Environment Agency.

Looking forward at 2022