Autonomous weeders for Christmas tree plantations - a feasibility study
3 Current cultivation methods
Bent Keller
The Danish Forest and Landscape Institute, Department of Forestry
This chapter provide an introduction to the most common methods of Christmas tree production in Denmark with focus on mechanical weeding. Other control methods are described briefly.
3.1 Establishment
At present Christmas tree plantations are established on considerable areas. The majority of the plantations are established on previously cultivated land, and only a small part on forestland. When establishing plantations the area is in most cases prepared with thorough brushwood clearing and stumping, sometimes followed with several haulages with a spring tine harrow to remove irregularities. The area is on the whole ready to be treated like previously cultivated land.
Today three cultivation methods are used:
- Clear cutting (planting - harvesting of all trees – planting). This is the most
prevalent cultivation system. It is highly systematic and is easy to manage regarding
planning. The planting is generally done with planters. The planting distance is typically
1,2 x 1,2 m, equal to app. 6.000 plants per hectare (Figure 3.1).
- Current planting (planting every time a tree is cut). Planning within this cultivation
system is much more difficult because the trees in each area are of different ages and
thus have different demands regarding treatment. However this system gives a more even
division of expenses and income. Here planting is always carried out manually. The method
has some biological as well as environmental advantages.
- Regeneration (new trees develop from the stumps of the cut trees). This method is only used to some extent because it is technically difficult, hard to manage, and often gives a poorer quality. The advantages are that there are no planting expenses, and as the plant already is established there will be no period of stagnation of establishment.
Figure 3.1.
Normannīs firs planted in rows.
3.2 Weed control methods
There are advantages as well as disadvantages to the presence of weeds in the cultivation area.
Advantages:
 | Leaching of nutrients is diminished |
| Microclimate is improved |
| Risk of wind- and water erosion is diminished |
| The biodiversity is improved |
Disadvantages:
| Increased competition for water, light and nutrients |
| Physical damage such as wear on the trees |
| Grass vegetation increases the risk of spring night frost damage |
| Complicates trafficking |
Normally the disadvantages are rated to carry great weight and this is why a very intensive weeding is started. This weeding can be chemical, mechanical or using animals.
3.2.1 Chemical
Chemical weeding is by far the most used weeding method. Traditionally slowly degradable broad-spectrum soil weed control was used. The rate of application was always close to or the maximum allowed dose, and the application was repeated each year throughout the rotation, which was 10-12 years. Several of the traditionally used persistent soil weed controls are now either forbidden or are being re-evaluated, and many of the producers have now started using screened spraying with leaf spray. The purpose of weeding is still the same – to keep the cultivation totally or almost free weeds.
3.2.2 Mechanical
Mechanical weeding of Christmas trees is the method that at present is regarded to be the best alternative to chemical weeding. Mechanical weeding in plantations on previous forestland demands careful brush wood clearing and either stump removal or stumping, as the available tools have been produced for cultivated land without obstacles in the soil.
Mechanical weed control can be done by mowing plants above the ground or by soil tillage. Soil tillage is superior and the safest method. Mowing does not reduce the risk of spring night frost like soil tillage, and the competition from the weeds for water will not be completely reduced. A high occurrence of natural vegetation has a beneficial effect however, during winter as it reduces the risk of winter frost damage, and a certain competition from the vegetation can probably have a beneficial restrictive effect on leader shoot growth. Therefore mowing can sometimes be justified, for example combined with other weed control methods (sheep grazing) and at certain times in the rotation or growth season. The following descriptions focus on soil tillage methods only.
The weed controlling effect of soil tillage consists of detaching and covering of the weed. The effect of this depends on a number of factors: among others the weather, the soil structure and moisture, the structure of the weed and the stage of development of the weed, travelling speed and depth of tillage. The effect is very dependent on the weather and can be considerably reduced in moist weather and soil. The effect is highest when the weed control is carried out in a dry period, as the detached weed will dry more rapidly. In moist weather the weed will often strike roots and keep on growing. Most tools operate better in sandy soil than in heavy clay soil, where the effect can be very poor. Furthermore moist clay soil is restricted to drier periods. Root weeds are difficult to control mechanically, whereas seed weed is easy to control. The majority of the tools known today have not sufficient effect on strong and well-developed weeds, but are effective against smaller weeds. For many tools the effect is better speeds of 8-12 km/h than at 4-6 km/h, however, it can be difficult to drive at high speed without damaging the trees if the trees have not been planted correctly at equal distances. The depth of soil tillage has great influence on the effect on root weeds and larger seed weeds; deep soil tillage can damage tree roots.
To obtain a satisfactory result with mechanical weeding it is therefore important to weed frequently, while the weeds are still small. One should not clean but keep clean. Blind harrowing – before the weeds starts to grow – often gives extremely satisfactory results. However, early actions can be hindered by moist soil, which is not suitable for traffic and tillage. The weeds start to grow in April and grow tremendously from May and onwards. Since the risk of tools damaging the newly busted buds is large, it is advisable not to drive in the plantations for 2-3 weeks in the period of bud breaking. It is therefore essential to weed before bud breaking. Depending on the circumstances the treatments have to be repeated 4-8 times per growing season.
Only very few tools have the capacity to clean in the actual rows (spring tine harrow, hydraulic Dutch hoe, rotor harrow on flexible arm), most tools are only able to weed in the inter-row area. In that case weeds are left around the trees, where competition for water for the first 2-3 years probably is most intensive. When the tree crown has grown bigger and denser the competition for water will be most intensive in the periphery of the crown. The trees are by now shading away the majority of weeds near the stem, nevertheless there will still be competition for light, and wear on the branches by tall weeds present in the row.
3.2.2.1 Conditions for mechanical weeding
The planning is one of the most important conditions for a successful mechanical weeding. The whole sequence of the cultivation has to be thoroughly considered and carefully planned. Firstly to make sure that the weeding practically can be carried out with the tools and machines at disposal, and secondly to ensure as low weeding costs as possible
Row length. It is crucial for the achievement and with this the economy of mechanical weeding, that the rows are as long as possible to minimise the number of turnings.
Appropriate headland. It is important that the rows do not continue all the way out to any fence around the plantation. A headland sufficiently broad for the later operating machines and tools is necessary. If the headland is too narrow the operator of the machine is forced to go forward and backwards to bring the machine in position.
Irregular areas. Irregular areas should as far as possible be avoided or be adjusted, as wedge-shaped areas with the direction of row tapering proportional to the headland are time-consuming and space demanding to weed. It is comparatively time-consuming to turn and bring tractor and tool in position. The headland, which is an unproductive but indispensable area, must also be considerably wider for the tractor and tool to make turns.
Distance between the rows. The distance between the rows must match the tools and machines to be used. It almost seems commonplace to mention this, but it is very often seen that the plan of cultivation had to be rearranged because the distance between the rows appeared not to fit to the machines at disposal.
Straight parallel rows. To carry out mechanical weeding at a sufficiently high speed the rows should be straight and parallel, at least in sets corresponding to the width of the weeding implement. Sufficiently accurate planting can be obtained either by a planting machine with a number of rows which fit the subsequent tools, or by drilling the area with a suitable number of holes before the plants are manually planted. Manual planting after two or more sticks does not give a satisfactory result.
The size of the weed. In general only a small number of the tools on the market right now can manage taller weed. The tools normally operate satisfactory on weed sizes up to 5 cm. If the weeds get much higher they may cause implement clogging or get entangled.
Soil conditions. Mechanical weeding is most successful and easy on light soil. The heavier the soil the harder mechanical weeding is to perform. On very stiff clay soils the traffic conditions in wet or moist periods can be so difficult, as to exclude the possibility of mechanical weeding.
The weather. The ideal weather for mechanical weeding is warm, dry and a little windy. The detached weed dries quickly and does not strike roots again. In practise it is not always possible to wait until the weather is ideal because of the risk tat the weeds will grow too high. Therefore weeding is often carried out on wet soil and in weather conditions. This means that a lot of weeds strikes roots and keep on growing. Even though the weeds are not controlled they are impeded and will not get so high that the tools can no longer manage.
3.2.2.2 Tools for mechanical weeding
Several types of soil tillage tools are used in Christmas trees. These are described below.
Spring tine harrow. This is one of the few tools able to weed in the rows. However, as it worksng in full width and thus running over the trees, the use of it is limited to the second and in rare cases to the third year after planting. Then the trees have become too high and will be damaged. In some cases where the spring tine harrow has been used, damage as found to on the underside of the branch on the lowest branch whorl from the second or third growth year. However the survival and the growth of the trees does not seem to be influenced considerably neither in the short nor the long term. There is a particularly great risk of damage when using the spring tine harrow during and just after bud breaking. The tool works best on sandy to medium heavy sand, whereas the effect on heavy clay soil is poor. On sandy soil the tool works best on dry soil. On clay soil the effect is normally best on moist soil. By regulation the weight of the harrow e.g. by means of hydraulics, the effect can be increased even on dry clay soil. The harrowing depth can also be adjusted to fit to soil, weed and so on.
The effectiveness of the tool is due to covering of the weed plants. This means that the impact is highest on little seed weeds from the stage of seed leaf and up to 5 cm height. The spring tine harrow has poor influence on larger seed and root weeds. If the spring tine harrow is the only tool used for weeding in the first 2-3 years, there is risk of extensive couch grass development.
As the spring tine harrow works independently of rows, there are no demands as to accuracy when planting. However as the tool has a highly reduced effect on weeds higher than 5 cm it is necessary to carry out 5-8 haulages per season.
The spring tine harrow has many advantages on lighter soils:
| it works in the full width and is thus weeding in the rows close to the trees, |
| it has a wide working width and therefore a high field capacity, |
| it is easy to manage and suitable for fast driving, because it is not necessary to drive accurately in relation to the rows, |
| it does not demand specific distances between the rows. |
However the spring tine harrow also has some distinct limitations:
| the suitability on heavy soil is limited, |
| it can only be used in quite young cultivation, as explained above. |
Disc harrow or spade-roller harrow. The Lindenborg harrow, which is a disc harrow, is usual in forestry for soil tillage. It has a good effect on seed as well as root weeds. The tool leaves comparatively broad unprepared streaks around the trees, and sometimes it causes problems with banks or soil thrown at the planted trees.
The Loft spade-roller harrow runs over every second row and weeds in the two row spacing. High weed can get entangled in the spade rollers, which then cannot work freely. To avoid one-sided moving of soil and banking the spade rollers are placed on two axles working towards each other. The effect of the harrow depends on the travelling speed. It takes quite some speed – 12 km/h or more – to obtain a satisfactory effect.
Hoe. A hoe is a tool that follows the rows. The working organs can have different shape, but the most common is tines with one- or two-winged shares. If mounting half shares towards the row the roots of the trees will be spared damages. The tool undercuts and covers the weed; the most important effect is the undercutting. The tool controls weed efficiently; the hoe normally even has an effect on root weeds – e.g. couch grass . Due to the aggressive undercutting it has a good effect on higher weed, and is much less dependent on the size of the weed than the spring tine harrow. At any rate this also concerns the hoe the less weed the easier the control. The hoe can be used throughout the rotation. Only the height of trees compared to the tool bearer’s free height reduces its utility. The decisive disadvantage of hoe, is that it can only weed in the spacing between the rows and not in the actual rows.
The conditions of success with this type of tool:
| that the rigidity of the tines fits the type of soil, |
| that the winged shares give full cutting of the soil and have a good overlap, |
| that the tines, to give the highest possible material flow, are placed with large distance |
| that the tines do not go deeper than app. 5 cm, often even higher, |
| that the travelling speed is at least 5 to 6 km/h, so that the tines are vibrating, keeping themselves cleaned, and bringing the cut off or detached weed to the ground surface. |
Hydraulic Dutch hoe. The hydraulic Dutch hoe can be used for weeding in the actual rows. A hydraulic run knife of 50-65 cm undercuts the weed at the depth of 2-5 cm. The tool is mounted with a mechanically or ultrasonic sensor that disconnect the hydraulic system when it senses some resistance – e.g. the stem of a tree. As the tool is driven forward due to the resistance from the soil, the knife is pushed back and turns out into the space between the rows. After the tree has been passed the knife is then again hydraulically activated into the row. Hydraulic Dutch hoes are used in ecological as well as conventional fruit plantations. In the fruit plantations 7-8 haulages per season are needed to achieve sufficient effect. The travelling speed is 7-10 km/h.
Rotary cultivator. The rotary cultivator is also an option. The knives are mounted on a horizontal-rotating axle. The rotary cultivator works by tearing up, cutting up and covering. The effect on weeds is good, and the rotary cultivator can handle even large amounts of weeds and thick grass. Root weeds, however, can only be controlled with repeated treatments. The rotary cultivator can be problematic to use for repeated haulages in the cultivation due to the risk of damaging the soil structure. The rotary cultivator is only weeding in the space between the rows. The PTO run rotary cultivator has a low performance, whereas the friction driven rotary cultivator has a high performance.
Under the category of rotary cultivators also belongs the hydraulic driven cultivator from the firm PolyTrac Inc. – the so-called "Mulcher". The Mulcher can tear up and break even high weed without getting entangled.
Rotary harrow. The rotary harrow has vertical stiff tine rotors that has a cycloidal movement through the soil. This levels the soil fairly well across the direction of travel. The firm of Silvatec produces a PTO driven rotary harrow intended for weed control in Christmas tree plantations. The rotary harrow is also available mounted on a flexible arm, which makes it possible to weed in the actual rows between trees big enough to reject the tool. The rotary harrow is suitable for weeding in overgrown cultivations.
3.2.3 Animals
Together with mechanical weeding grazing is among one of the most promising methods of weed control in Christmas tree cultivations at the moment. In principle grazing can be carried out with many different species of grazing animals; however, sheep of improved fattening breeds – especially Shropshire – are the most used.
3.3 Harvesting
The harvest of Christmas Trees starts in the early autumn with classification and marking every tree. In the middle of November the actual harvesting starts. Felling is accomplished using handsaws, brush cutters or chain saws. During the past years there has been a development within mechanic felling machines. After felling the trees are dragged or carried out manually to the tractor tracks in the cultivation, where the trees are wrapped to protect them and facilitate further transportation to a central loading place easier. The trees are then loaded on lorries. Over the past years there has been a development of more mechanized transport systems, where the trees after wrapping are placed on pallets, which makes the further transport considerably easier.
3.4 Contractors and contractor costs
The use of contractors has become more common within Christmas tree cultivations. This is partly because of the general reduction of the permanent staff within the trade, and partly because the individual Christmas tree producer gets the work done by experts having special machines at their disposal; machines that would not be economical for the individual Christmas tree producers to own. The contractor can today carry out all the tasks within Christmas tree cultivation, from little isolated jobs to turnkey contracts.
It is difficult to get an exact overview of the contractor costs, as these are strongly dependent on the conditions. Some contractors operate with a fixed rate per hour and a special road rate. Others have fixed prices per hectare with extra charges for small or difficult jobs/areas. Others again are not paid per hour or hectare but work out an offer for each contract
On good areas without road transport the contractor costs for mechanical weeding with a tractor mounted harrow are estimated to be between 2.200 to 3.000 DKK/ hectare/year at a price per hour of about 400 DKK weeding with rotary cultivator/mulcher mounted at a special tool carriers will amount from 1.250 to 2.850 DKK/hectare/year at a price per hour of about 475 DKK. depending on the planting and weeding system.
3.5 Costs of the various operations
In table 3.1 is shown a typical cultivation model for 1 hectare of Nordmann’s fir Christmas tree cultivation on former farmland. Planting is assumed to be carried out manually, and spraying with leaf and soil herbicides is used for weed control.
Table 3.1.
Estimated cultivation costs (DKK/hectare).