Rosa rugosa removal trials for the RSPB

Introduction

Rosa rugosa is now an established invasive of sand dunes in much or Europe and is considered one of the main issues for coastal sand dunes. It’s root system and ability to replicate from root fragments means that control is complicated and costly. In some dune systems in the UK, it has been recorded that it can spread by 22% per annum meaning that if eradication is not achieved, continual management is needed to reduce negative impacts on native species.

GroundLord Environmental were contracted by the RSPB to help remove R.rugosa across a rare area of sand dunes in Chichester Harbour. Rhte site was attached to a military base and highly designates, SAC, SPA and SSSI, meaning we had to come up with a very detailed methodology to ensure that there was minimal impacts on the designated features.

Sections of removal were trialed at Pilsey Island, where an approximate 2000 m2 area has established R.rugosa coverage.  We could not follow standard methodologies for the removal due to the depth of the sand. We therefore created a range of trial plots that could then be monitored.

Trial sections

Scraped area

The initial prescription was to remove an initial layer of the rugosa and then burn it. Due to the vegetation layer on the dunes, combined with the rugosa, this layer does not pull away without a significant amount of sand. The very wet conditions further complicated this and burning was therefore not a viable solution. The mix of roots from the marram grass and other species penetrate up to 50cm, meaning that it effectively held the sand together. Despite shaking the mats, not enough sand could be removed.

In a very prolonged period of dry weather, it may be possible to leave it to dry then shake more sand and burn it. It is unlikely to remove all the root fragments though and there will be a high risk of the fire spreading during that period. 

This method would also not affect the rugosa as a lot of the roots were still left in. It would create bare areas of sand, which are useful as part of wider dune management and regrowth could be easily treated with a herbicide.

The scraped area could be removed off site with a tracked dumper.

Figure 1 showing the scraped section and the resulting arisings

Subsoil burial

Following the standard methodology for subsoil burial of the was also not viable. The method involved removing the layer with rugosa roots in, then extracting a layer of clean sand from underneath and burying the first layer with this. The root system went down about 1 metre. At 1.5 metres the water table made extraction very difficult, and the composition of the sediment changed. It was mainly composed of shingle/sand mix and pockets of anaerobic mud as this was then level with the base of the harbour. Removing this to the surface would change the composition of the sand dune.

Figure 2 showing the sub-soiled section and area of surrounding cover

With the equipment to hand, digging the trenches to the depth needed was complicated, especially due to the depth of the water table, causing the trenches to collapse very quickly. A stepped approach would be needed with storage of the clean sand in a separate area. This would be a much more significant operation and realistically, the sand dunes are not deep enough for this approach. This would also mean that the majority of this section of dune would be completely stripped of vegetation, meaning that the stability may become compromised.

The first section therefore had a long middle section of approximately 15m x 2m that was sub-soiled, with the rest of the area with a covering of 30cm of clean sand.

Figure 3 showing the initial depth needed and the water table

being reached. The change in sediment can also be seen.

The combination method

The third method tried was more efficient. A trench was dug parallel to the area to be cleared, in a section with no rugosa. The turf from this trench was carefully removed and laid to one side. All the material extracted therefore had no rugosa roots in and was laid on a tarpaulin, to keep it from being contaminated. The trench was dug until the water table was hit, approximately 1.5m. The surrounding area was then scraped using the rake and the top 20-30cm was removed and compacted into the trench. This and the scraped area were then covered to an approximate depth of 30cm, with the clean sand. The turf from the original trench was then replaced to the same location.

This method was the most efficient but was still quite slow when considering the area needed to be removed. Regrowth through the layer of clean sand will then need to be followed up with a treatment, either cutting or herbicide treatment.

The work allowed us to produce a detailed report for the RSPB that describes our findings as well as outlining future management of this issue. We believed that, given the substrate depth and root structure, continued management of the site would be the best method to allow native flora to reestablish. Methods such as regular mowing and possible herbicide usage should be explored.

Figure 4 showing the trench being cut with clean material read for spreading