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The share of mechanized weeding of trees in Finland is currently only a couple of percent, but is increasing. When combined with boron fertilizing, both the effectivity of labor and the growth of forest will increase.
As for the traditional forest improvement works, mechanical weeding substitutes clearing. But in reality, it is much more.
Clearing of broadleaves is not very effective, because every stump shoots 2–3 sprouts. In mechanical weeding the plant is removed by an uprooter with its roots, which means that the number of sprouts is much smaller: 70 percent of the weeded compartments do not need any additional weeding.
And as for the rest, the number of sprouts is small and weeding is easy. New sprouts of broadleaves occur only after 3–4 years after mechanical weeding.
Mechanical weeding is often combined with boron fertilization, which is important especially for spruce stands in central and eastern Finland – up to half of them suffer of lack of boron. This in turn, can make the plant to change its treetop even several times. This may even suspend the plant’s growth totally.
When combined with mechanical weeding a liquid of boron and water – 15 units of boron to 35 units of water – is sprayed over the terrain, one centiliter per each lift of weeds. Given that some 5,000 lifts take place on every hectare, a total of 50 litres liquid is sprayed per hectare.
”The effect of boron fertilizing can be seen already during the same growing season and it lasts for the whole rotation period. After four growing seasons the tree is 20 percent thicker and six percent longer than without fertilizing,” says Ms. Merja Nieminen, forest services specialist at forest industry company UPM.
Good result brings productivity
At a presentation of mechanical weeding organized by UPM in Hirvensalmi in southern Finland, the work was carried out by Naarva uprooter designed by Pentin Paja company. The machine has four rubber presses, which are pressed around the weeds, then lifted, after which the weeded plants are dropped back on the ground.
“The surroundings of one spruce plant is cleared by three lifts. During one shift some 1.5 hectares will be cleared,” says Mr. Topi Seppälä, the driver of the machine from Aatsin kone forest services company.
By clearing with a brush saw the result would be under one hectare. The costs of the weeding, a little below EUR 600 per hectare, do not differ much of that of clearing.
Productivity is increased because of achieving a better result with the work. The costs of manual clearing have, on the other hand, increased a lot. Despite of that, there is lack of skillful labor force.
In addition to this, mechanized weeding moderates working seasons. Because of the shooting of new sprouts, manual clearing can be done only after June. Mechanized weeding can be carried out during the whole growing season, from May to October.
“Normal fertilizing of forest aside, there is no more productive method to improve the forest than mechanical weeding,” says Nieminen.
Not for all soils
A weeding machine costs some EUR 20,000–25,000. It is driven by harvester or excavator – but using an excavator the boron fertilization has so far been impossible to organize.
The machine can be used in two turns per day, and there is space for growth in the forest services market. Clearing is carried out on 150,000 hectares per year. Of this, the share of mechanical weeding is currently only some two percent, although steadily growing.
Mechanical weeding does not suit all soils, however. The roots of broadleaves spread on organic soils so effectively that when they are lifted, they may lift the cultivated spruce plants with them.
Weeding is usually carried out when the plants are 4–6 years old. The weeded bushes may be as high as 2.5 metres high, while the cultivated plants usually are somewhat below one metre in height at the time of weeding.