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Autonomous weeders for Christmas tree plantations - a feasibility study

6 Specification of technical requirements for an autonomous Christmas tree weeder

The Royal Veterinary and Agricultural University, Section for AgroTechnology

The Danish Forest and Landscape Institute, Department of Forestry

On basis of the described cultivation methods, minimum weed control requirements and stakeholder requirements the following full and reduced operation scenarios and requirement specifications have been specified as goals for the first generation autonomous Christmas tree weeder (ACW).

6.1 Overview of cultivation methodology

Christmas trees are mostly grown on farmland in rows 90 to 125 cm apart and with similar intra row spacing. Another pattern is a 60° triangular pattern (referred to as rhombic), which corresponds to closest packing of equal size circles. Using this latter pattern about 20% more trees can be grown on the same area. Different tree species are chosen depending on the soil types.

The production cycle starts when typically two year old trees (about 20 cm high) are planted on prepared land, and it ends after 6 to 10 years when the trees are cut selectively. In some cases new, small trees are planted in between trees remaining from this selective harvest.

Mechanical weeding during the first two years after planting is normally performed with special spring tine harrow (langfingerharve) developed for weeding in agriculture (see chapter 4), which is relatively cheap, and yields a satisfactory result. At the later growth stages this method is not suitable. This is when the ACW is to be employed.

Weeding is needed until the trees are about 1,5 m high. It must at least cover a circular area of 40 cm radius from the stem to avoid competition and stiff shoots of perennial weeds (including trees) to cause mechanical damage to the branches.

The weed control may be performed either as shallow soil tillage or as weed cutting near the ground. The latter provide sufficient reduction of weed competition, but in some areas it is an advantage to clear the soil around the trees during springtime, as it reduces the risk of frost damage of emerging shoots. A shallow form of soil tillage is considered suitable for this purpose. Weeding should be done 4 – 8 times a year (chapter 4). Recently emerged shoots are brittle and easily damaged mechanically.

About 6 years from planting, when the trees are about 1 m high, some of the lower branches of the trees are removed to reduce the top shoot growth. This operation is in some cases done twice. Further growth regulation is some times made by removing part of the bark around the lower part of the stem.

Late in the growing period tracks may be cleared in the plantations for transport depending on the harvest strategy (chapter 3).

6.2 Definition of operation scenarios for the ACW development

It is clear that the full range of plantation scenarios occurring in practical conditions is wide, and that it would be difficult and costly to develop an autonomous machine that could cope with all situations. Therefore a somewhat narrower range of scenarios is chosen for the analyses.

Table 6.1.
Christmas plantation parameters and requirements chosen for the ACW development.

In addition the mechanical damage of trees between planting and harvest should be less than 5%. Also any contact between trees and the ACW should be gentle or avoided during 3 weeks from medium May when the bud are very sensitive to damage.

6.3 Definition of work tasks

During the workshop and further analyses (chapter 5) the following occurring as well as new work tasks were identified as suitable for one or more autonomous machines. However it was decided to limit this initial work to weeding while keeping the other potential tasks in mind (Table 6.2).

Table 6.2.
Work tasks considered in the present analysis.

6.4 Sub conclusion

In conclusion it is recommended to focus on development of an autonomous vehicle for weeding with rotor cutter near the ground in a slightly reduced range of occurring scenarios. Further development of the vehicle for some of the other tasks appears feasible, but should be postponed to a later stage.

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