Crassula helmsii Management – Case Study – Lake Habitat

Crassula helmsii – Lake/Freshwater Habitat– 06.04.20 – 19.06.20

Identification:

We first picked up that the East lake potentially had Crassula helmsii on 27.07.19 and studied the samples further. At the time, green algae was covering the plant throughout most of the lake and it looked in poor form. Some identifying characteristics were not as obvious such as the darker ring 1mm below the leaves meeting the stem. However, it had opposite leaves, elliptical and no notch. Leaves placed alternate up the stem & the upturned arch at leaves meeting stem. It was robust albeit stringy. The 20x magnified photos were studied further and our concerns were confirmed.

It was in December when we were able to see the significant evergreen growth of C. helmsii.

Lake with Crassula helmsii — East of Lake December 2019

Habit and habitat

In May when entering the water, it was noted that the growth was in most cases a mat of approx. 1ft deep. Extensive roots had created its own substrate. The roots can only be explained as fibrous, creating a solid thick carpet. Extremely strong and durable. In the water, although tall & stringy, it was mostly healthy & succulent in appearance. These thick mats were in the East and Southern corner.

Identification and terrestrial Crassula helmsii

The remaining areas, mostly central to the lake were looser and not luxuriant green growth. These areas were less physical to manage, but the concern was how much of this was floating about and sometimes obscure under the surface. These areas may have either been loosened by waterfowl or potentially the use of barley straw breaking it down and causing it to be unhealthy or dying. This growth form also seemed to be among the lily sections. So perhaps a lack of light to keep it healthy. This plant was thin & stringy and although showed some green growth, it had a reddish/brown appearance. Roots were sometimes blackish.

Lastly the margins. This being difficult to see from the banks in Spring, Summer and Autumn. However, in the water we were able to look back at the edges and see through the Iris and bulrushes. We have seen the exact same pattern before. Crassula helmsii has a preference in margins around the roots and bases of these marginal plants. This was succulent, green & very healthy. A little softer in appearance perhaps.

Location

Yellow is the lake border.

East is the largest concentration in Red with orange showing the densest area.

Blue: The southern corner where we were sensitive to the water outlet (marked with blue cross). The southernmost point being denser than around water lilys.

Green triangle: Further small concentration. Grey dots: Exit points. Purple: Landing strip

Also shown are the areas of Water Lilies. These are important areas to revisit and manage in December. Although the smaller patches were not there when we visited. Map also shows location of island with path.

Considerations:

Types of Management. – There were numerous forms of management considered:

Light limitation (black plastic) Large agricultural sheet plastic was strongly considered but the length of time this would be required to remain covering the lake was problematic. Ensuring sinkage in the middle of such a large area would have been difficult to set up. We concluded this was not the least intrusive method and potentially hazardous for wildlife and waterfowl.

Manual handling from water (Boat or Waders?) We initially tried entering with a small boat, however we found this method not possible as large mats of C. helmsii rolled under the water when pushing towards the mats to get close enough. It is also very heavy to lift when waterlogged.

Herbicides: We understand the benefits of spot spraying on marshes which in some cases offers the least disturbance to other flora and fauna, such as when using plant machinery. However, herbicides were not an option here due to the aquatic nature of its growth, too much of the plant was submersed. However, spot spraying is likely the only management of the remaining terrestrial growth around margins.

The use of tractors and pully systems: Using plant machinery and pulleys when considering large areas of invasive species is not a sensitive method without a close & watchful eye on the habitat.

Draining the lake was not an option.

Difficulties

Not to displace small parts of plant. Managing by hand was the least likely to cause unnecessary breakage and spread.

Waterflow from one lake to another. Small mesh netting was placed over outlet to other lake. Prevailing wind pushes surface water towards the East of the lake. The West Lake entrance needs continuous monitoring.

Time of year for young invertebrates. Managing by hand was the least intrusive with utmost care given. Margins and plants where invertebrate eggs might be laid and any birds that could be nesting were undisrupted. 3 well considered entry points were the only areas disturbed. Amphibian spawn was not observed. Crassula not correct habitat for newt eggs. There was an obvious lack of any biodiversity seen in crassula helmsii habitat when inspected.

Urgent need to stop waterfowl spreading crassula. Birds were swimming through sections.

Depth of lake. Mostly known by landowner. Two persons checked by boat and carefully walking with waders.

Understanding that eradication is not possible Management rather than eradication, was the only way of saving the lake from being suffocated from the species. Further management will be required each year including alternative terrestrial management.

Time involved to use a habitat sensitive method. Management in intervals, with 2/3 people seemed the best way to be sensitive to the environment. Pleasant work for anyone who cares about this habitat, albeit physical work.

Difficulties

Not to displace small parts of plant. Managing by hand was the least likely to cause unnecessary breakage and spread.

Urgent need to stop waterfowl spreading crassula. Birds were swimming through sections.

Depth of lake. Mostly known by landowner. Two persons checked by boat and carefully walking with waders.

Objectives

The main objective was to clear C. helmsii to bring the lake back to the freshwater habitat it once was. To do so efficiently to limit the spread by waterfowl. To manage the C. helmsii with the least intrusion to wildlife whilst being as effective as possible.

Crassula helmsii — Pile after several days of management with breaks between*.

Method used

It was decided we would enter the lake with waders and manage the the C. helmsii by hand. This was only possible due to the lake being the right depth to be firmly on two feet.

We decided on 3 landing points to enter and exit the water to minimise disturbance. 2 of them about 5m apart along one strip* of the lake to remove approx. ½ acre of C. helmsii. The other exit point was where a smaller concentration was at the Southern end. However, the plant was to be isolated in a trailer and taken over to the main landing strip.

*The landing strips had to be managed due to the large scale of C. helmsii being brought to edge. Firstly due to being able to lay the mats out without weighing heavily on one another. Allowing any larvae/small amphibians/freshwater invertebrates missed by our watchful eye, to crawl back to water. Secondly the landing strips had to allow for drying out without having to move the species any further away from the site.

Large sections could remain intact and be floated with ease to the exit point. As the waterlogged mass of the plant was extremely heavy, several methods of getting it up to the bank were considered to limit breakage and dispersal. The least damaging was to carefully pull apart or where needed cut with a large hook knife. One of us, kept a large metal sieve to catch any stray stems of crassula surrounding the exit point/working area.

As the Crassula became less dense and unmatted, we needed a vessel to collect & place the crassula on and float it to the edges. It could not be a boat as lifting the crassula out whilst we were chest height in the water was an H&S issue as well as causing further breakages. So we came up with the idea of a foam body board. We were able to scoop the plant out with the large metal sieve and place on board. We spent many days doing this to unsure as much of it was caught and unable to float off to recolonise. It was necessary to keep checking after management as some of this less luxuriant growth temporarily sunk with movement.

Removed Crassula

On the second visit 10 days later and throughout management, the C. hemsii looked mostly dead/dying but growth from underneath was still taking place and coming through to the surface. This was carefully turned over where viable, checked for green growth, dried & treated again. When checked even up to November, Crassula is still surviving.

Freshwater wildlife.

It is worth noting that due to being chest height in the lake, every section removed was carefully looked over for invertebrate larvae/nymphs and amphibians. All placed back in the water away from management. This care cannot be taken managing C. helmsii using other methods.

Freshwater life included (approx. figures): Several species of dragonfly nymph (x 6), Greater Diving beetle (approx. 8), Backswimmers (numerous), 2 x Ramshorn Snails, 5 x young adult Newts (unsure if palmate/smooth) Young frogs/toads around margins.

Observations

Removal of the visible aquatic C. helmsii was we believe highly successful. However, It’s important to note that Crassula helmsii was not smothering the lake in the 2 to 3 years prior, or noticeably destructive. Monitoring will determine how quickly we need to react but its preferable to have a proactive approach to management where viable, or other possible methods discovered.

It is also essential to monitor the West lake a couple of times a week, paying particular attention to the water entry cascade.

Between management days, the prevailing wind was on our side and pushed any small islands or loose Crassula to the eastern end. (Away from water outlet to other lake) The other safety came from the banks of lilies, stopping sections of crassula floating West. We do not have that benefit in December.

It’s a guess but the Eastern bank appears to be where the invasive plant started. Possibly near the waterfowl nesting site and likely where it will start to form aquatic mats again. The lake owner supplied a barley straw netted bale to secure at the very tip where the spring feed enters. This will help determine whether the less luxuriant growth in the centre, was in fact helped by previous bales about 5 to 10 meters in from top of lake. C. h being most abundant here also suggests this was a good place for the landing strip to help containment.

Limitations

It is not possible to manage the marginal terrestrial growth by hand due to the structure of the strong roots in muddy banks. This needs further alternative management in winter. Spot spraying is likely the only option and the least intrusive.
Herbicides on other marsh sites, has not yet proved successful in killing the terrestrial plant within 1^st year or so. Treatment continues on those sites.
For December management (post water lilies dying back), we are expecting that dry suits and thermals may be required. 1) because the lily areas maybe deeper, 2) the water levels higher and 3) the temperatures much lower.
helmsii can be spread when traversing by foot whilst treating. Special attention to cleaning of boots was practiced. We also removed waders/boots and took the long route Southeast of lake to avoid west lake when leaving site.

Notable patterns and habits of Crasulla helmsii observed

1)The habit of C. helmsii’s terrestrial growth around the lake is similar to that at Stover marshes. It particularly likes the conditions directly in and around the Reed mace and Iris borders of the lake. This is not one consistent line by any means but a pattern, nevertheless.

2) I found it coexists well with the moss Calliergonella cuspidata. From a distance it can look similar and be considered as dense patches of C. helmsii, when it is in fact both species growing together or just the moss. I have noted this at both marshland habitat sites as well as lake margins.

Further treatment this year

We are monitoring periodically to check if any urgent treatment will help manage halt its future spread. Dense areas shown coloured on map and margins should be paid particular attention to.
Returning to manage Crassula under lilies (shown on map) in December
Potentially burn landing strip