Autonomous weeders for Christmas tree plantations - a feasibility study
1 Introduction
To achieve good growth and quality of Christmas trees it is traditionally considered necessary to weed very intensively to compensate for the weak competitiveness of Christmas trees, especially Nordmann’s fir, which is the most grown species. Intensive weed control is in most cases achieved by application of persistent soil herbicides with a broad spectrum of effect.
In resent years many growers have shifted to screened spraying with foliage acting herbicides. The rate of application is usually close to the allowed maximum, and the spraying takes place every year over the entire production period (app. 10-12 year).
The extensive use of herbicides is alarming in itself, especially seen in the light of the latest investigations, which show that it is leaching into the soil (Jacobsen et al. 2000). Leaching of nutrients – especially nitrates – from a soil almost without vegetation is also considerable, as the sparsely placed trees are only able to utilise an insignificant fraction (Rubow et al. 2000). The biodiversity of total cleaned areas is probably very low as a result of the lack of food and hiding places.
This makes Christmas tree plantations a high priority area for alternative weeding methods (Bichel, 1998). Some growers have moved in that direction during the past years and are beginning to use mechanical weeding (Keller, 1997). The weeding implements developed for the purpose are well able to remove weeds between the rows, but mostly not in the rows, where the need is greatest. Most of the implements are rather heavy, of low capacity, and costly to use. The general strategy of weeding using these machines is the same as for spraying: to achieve a total clean area. Therefore, this also leads to problems of leaching and relatively low biodiversity.
The purpose of the present project was to investigate the feasibility of developing a light, autonomous weeding machine being able to perform mechanical weed control more competitively within a relatively short period of time and also to reduce the environmental problems associated with the present methods. The machine should have an acceptable behaviour, be able to operate unattended and safely for longer periods of time.
The project comprises:
 | a description of the present Christmas tree cultivation systems, |
| an investigation of the need for weed control around single Christmas trees, |
| specification of the conditions in which the autonomous systems should work, |
| specification of the stakeholder requirements of an autonomous system, |
| specification of the technical requirements of the system |
| two proposed machine concepts, |
| a proposal for a system architecture, including navigation system, safety system and overall control system, |
| an evaluation of the proposed concepts compared to present methods, including biological, technical, economic, environmental and safety issues as well as the possibility to achieve added values by collection of information on the trees for management decisions. |