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Management of poplar, aspen and willow

Soil Preparation

Arable land

For planting of poplar, hybrid aspen and willow

There are usually two different conditions at planting on agricultural land:

1) The land has been used for grain-growing or similar until the planting occasion, and grass competition is a moderate problem. Then it is possible to plant directly into the stubble. It may be relevant to do one or two herbicide treatments against weeds before planting.

2) The land has not been cultivated for the last few years. Then it is strongly recommended to plough the land the year before planting. In spring harrowing is done, and then herbicides are used before planting. If the grass occurrence is high both ploughing and harrowing is recommended, followed by a herbicide treatment, the year before planting. If necessary a new herbicide spraying is carried out in the next spring.

After herbicide treatment a period of about 14 days shall pass before planting is carried out and no soil treatment should be performed after the spraying. In cases where the pH is low, i.e. below about 5, liming should be considered. There are now also opportunities to use mulch to prevent weed problems. UV-sensitive plastic is often applied as protection from weed competition. Since weeds will most probably reestablish, they should be controlled for about another two years after planting. It is therefore important to plan for the plant distances so that weed removing tools can pass between the plant rows during the coming years.

Forest land

For planting of poplar and hybrid aspen

On forest land, a common soil preparation may work where there is limited grass growth on poorer sites. Otherwise the inverse soil preparation technique is recommended, where the upper soil layer is placed upside down by using an excavator or the new unit Karl-Oskar, which can be mounted on various machinery. Soil preparation may be regarded as expensive on fertile forest land, but it is necessary to use a powerful measure that prevents excessive grass growth. Since poplar and hybrid aspen are not utilizing water very effectively, they are sensitive to competition in the establishment phase. It is even more important to have knowledge about the pH situation on forest land than on agriculture land and to lime if the value is low. The experience is that especially poplars are difficult to establish successfully on the acid forest soils in Sweden. Polars need a pH-level of about 5 or above.

åtgärd ogräs

One way to manage weed competition is by mounding with an excavator. Photo: Bo Nilsson.

Planting

Arable land

Planting with poplar and hybrid aspen is usually made with 1,100 to 1,500 plants per hectare. The number depends on whether thinning is planned, or if the aim is to move directly towards final cut. Poplar can be planted as cuttings or plants (rooted cuttings), where cuttings are cheaper but plants are a safer alternative. The cutting length can also be varied where longer cuttings will be more expensive but safer.

Willows are normally planted with about 20 cm long cuttings in double rows of alternating 75 cm and 150 cm between rows, and 60–65 cm between cuttings in the row, providing about 13,000 seedlings per hectare. The planting layout is designed to match today's technological conditions. Willow plantations are rarely fenced against browsing animals while it is a standard procedure for Populus species, especially for the game attractive aspen.

Forest land

In general, 1,100 to 2,000 plants per hectare are used at planting on forest land since the conditions are "tougher" and there should be room for a minor plant loss. Willow is regarded exclusively as an agricultural crop and establishment on forest land is hardly an option. Since game damage is often more common on forest land the need to use fences is higher than on agriculture land.

ogräsbekämpning

Weed control between plant-rows means a faster establishment of the plants when competition for water and light decreases. Photo: Bo Nilsson.

Management after planting

Grass removal around the plants

Weeds must always be controlled on arable land. A common way is to drive between the rows of plants with a mower or similar 2–3 times during the growing season during the first 2–3 years. One can also remove grass manually around the plants. The need for grass control depends on grass and plant growth. Weed removal is generally an important measure as poplar, aspen and willow species are sensitive to competition for water and light. In addition, a profuse grass occurrence also means that vole infestation can become a big problem.

Thinning

Thinning of poplar and hybrid aspen is usually a planned action where planting density and cultivation purposes are known. Generally, a strong thinning is recommended where the stem number is halved, e.g. from about 1,500 stems to 600–800 stems per hectare at 10–12 years of age. Alternatively, 1,100 plants per hectare or less are planted and then thinning is not necessary if the rotation period is short. In practice, poplar stands are often managed without thinning because the main assortment is fuel wood, while hybrid aspen stands in many cases are thinned and using a longer rotation time when the aim is to produce logs and pulpwood.

Willow is usually managed with 3–5 year cutting cycles and the biomass is almost exclusively used for energy purposes.

The Next Generation

Hybrid aspen is regenerated by a strong sucker sprouting from which it is possible to choose different management options for the future. Poplars are mostly regenerated with stump shoots but there is a difference between clones which means that natural regeneration of poplar needs to be investigated better. In other countries it is also common practice to remove the poplar stumps and restart with a new planting. Willow plantations are regenerated with stump shoots and it is estimated that the stools can be used for 20–25 years before it is time to renew the culture.