Manual interventions have existed since it became necessary to launch control operations against aquatic plants, probably around the beginning of the 1900s. Changes in the cost of labour, in the available techniques and notably the use of herbicides since WWII, as well as the rather negative perceptions of manual labour (tiring and often dirty) have resulted in the progressive halt to manual interventions on many sites.
In a report on mowing plants in rivers, Isambert (1989) discussed the various techniques used in the Seine-Normandie basin to control aquatic plants. Manual maintenance work was carried out on some 15 rivers with a cumulative length of approximately 250 kilometres. Virtually all the work was organised by river boards.
The tool used for this work was a “châtelaine”, a cutting bar drawn manually across the river bed by two workers on the river banks. This type of tool is suitable for rivers less than 12 metres wide and where the banks are easily accessible. Two experienced workers could mow approximately one kilometre of river per day. The use of part-time labour and volunteers was also mentioned. Unfortunately, no information is available on recent updates to these traditional techniques.
Concerning alien plants, the management plan for aquatic plants in the lakes and ponds of the Landes department, prepared in 1989 for Géolandes (Dutartre et al., 1989), recommended manual uprooting for
“sections invaded to a limited degree” by curly waterweed, water primrose and parrot-feather watermilfoil.
At that time, the proposal for manual interventions elicited a number of negative reactions from both elected officials and the concerned technical services. Some people perceived the proposal as a “return to the Dark Ages” given that an array of available machines and herbicides were seen as effective in meeting the management needs of the plants. Other, rather excessive, reactions spoke of prison camps and the use of inmates to do the work.
165 (a nature park) and another nearby pond produced five cubic metres of water primrose the first year, but just 0.05
cubic metres the second, a reduction by a factor of 100 that demonstrated the effectiveness of the work (see Figure 69).
At the same time, efforts to explain the rationale, noting for example the risks of cuttings being dispersed by the machines used and the lack of selectivity by herbicides, resulting in the disappearance of not only the targeted plants, but of all plants on the site, produced a change in opinions concerning manual interventions.
This change was fairly rapid because in just 20 years, manual interventions were regularly organised for amphibious plants, water primrose on a large number of sites and parrot-feather watermilfoil on a smaller number sites, on lakes in the Landes department, the Marais Poitevin marshes, the Brière marshes and many other sites, primarily in western France.
An analysis of the change revealed the main reasons (Menozzi and Dutartre, 2007), namely the precision and effectiveness of manual work, leaving the “non targeted” plants in place, something that mechanised techniques cannot achieve. The authors even wrote about “manual uprooting of water primrose, an age-old technique that is truly an innovation” (Menozzi and Dutartre, 2008). It is, however, clear that this type of intervention is justified only under certain conditions, e.g. at the beginning of a colonisation, for collection of plant fragments remaining on site after a mechanised intervention or in areas that machines have difficulty in reaching, and on the further condition that the quantities of biomass to be extracted are not excessive.
The physical difficulties involved in manual interventions should not be exaggerated. The work is physically demanding, occasionally under difficult outdoor conditions, but a considerable number of the negative opinions mentioned above concerned poorly organised work sites involving persons having received little or no training.
Similar to any other type of intervention, the objective is to correctly organise the work and consequently to reduce its difficulty and any harsh conditions.
a, b © A.Dutartre, Irstea
Figure 69
Manual uprooting of large-flower water primrose (Ludwigia grandiflora) by volunteers in the Noir Pond (Landes department).
The work conditions for manual interventions were improved by providing the workers with a boat for their meals and to store their tools.
© Nicolas Pipet, IIBSN
Figure 70
The easing of the physical conditions may concern, for example, the movement of personnel if the sediment is highly liquid or the water is excessively deep. Boats may be necessary, if possible with flat bottoms to improve stability. Similarly, transportation of the extracted plants from the site may be a complex and tiring task. Suitable bags, sufficiently large boats and equipment on the banks to discharge the plants from the boats are examples of means that can be employed to facilitate the task. Considerable efforts have been made by IIBSN to improve the operational conditions for the team working on water primrose in the Marais Poitevin marshes. A boat was purchased to provide the workers with a place to eat out of the weather and to store their tools, etc.
(see Figure 70).
Other efforts to improve hygiene and safety include training sessions on first-aid and ergonomics, vaccinations and a safety manager to implement a Special plan for work safety and health (PPSPS) (Pipet and Dutartre, 2014) (see the management projects in volume 2, pages 34 and 67).
As already noted, these interventions concern almost exclusively amphibious species. Manual uprooting of submergent plants is possible, but more complex and less effective due to the fragile stalks and the deep water conditions in some cases. However, when manual interventions are carried out rigorously and taking the necessary precautions, in particular when a maximum quantity of plants and stalk fragments, even very small pieces, are removed from the site, the work is very effective and sharply reduces regrowth of the targeted species. Because of their targeted nature and the very limited use of equipment, their impact on environments and habitats is very slight and even inexistent because the non-targeted species are not affected, which means they can continue to develop because the targeted alien species no longer exerts any competitive pressure within the living community. For example, the regular management work on water primrose in the Marais
167 On banks where invasive plants have appeared, they can be uprooted by hand as long as they have not
developed too much, otherwise tools (spade, pickaxe, hoe) must be used. It is very important to remove the entire root system, particularly for suckering plants, to avoid regrowth and even multiplication of the plants if removal is only partial. The uprooted plants must be completely removed from the site to avoid leaving any plants capable of putting down new roots. Manual cutting of these species is also possible if the installation process has just begun or if the site is only sparsely colonised, or in those cases where, given the ecological value of the habitats or of the native species, mechanised interventions are not possible (see the management project in volume 2, page 99). Depending on the diameter of the stalks, the plants can be cut using billhooks, sickles, secateurs, saws, chainsaws, etc. (see Figure 71).
This work can be done by volunteers under management, temporary or permanent staff of local governments,
"make-work" companies for the unemployed or private companies. Whatever the case may be, minimum training is required concerning identification of plant species, the precautions required during plant uprooting and transport, and safety conditions for work in areas that are generally difficult to access. The existence of technical specifications for each type of intervention facilitates the work and improves its effectiveness.
For a number of years, several private firms have specialised in manual techniques and offer their services to local governments.
In a number of other cases, very small firms offer manual uprooting by scuba divers. Generally speaking, the treated sites are fairly limited in size, e.g. ports where the presence of docks, chains and other factors hinder or make impossible the use of machines to harvest the plants. This technique can be used in deep waters, however its cost even greater than other manual techniques, the specialisation of the personnel employed and the difficulties involved in subaquatic work mean that this technique is restricted to limited operations on sites having high added value.