Self-sown trees usually have little value. This is because they are variably spaced and aged, often of poor form, difficult to access, and hard to harvest.

Each tree can produce vast quantities of seeds, which can blow considerable distances, and quickly become established.

Key points for forest developers and landowners to be aware of are:

• Prevention is easier than cure
• Spread is predictable

Wildings are very obvious before they reach cone-bearing age.

Wilding tree spread in the Mackenzie Basin.

Techniques to prevent tree spread

Wilding tree spread depends on 3 factors: species, siting, and downwind land use.

Use the score sheet below to calculate the risk of spread before you plant.

Use the highest figure applying to your situation.

Choice of species and sites

Some species spread much more readily than others. Spread-prone plantation species such as Corsican pine and Douglas fir should never be planted:

• on or near “take-off” sites (ridges and slopes exposed to strong prevailing winds)
• immediately upwind of undeveloped areas.

Species vary in palatability, which affects how easily they are controlled by stock (see (1b) in the score sheet table below).

Planting design

Two or more rows of less spread-prone species, such as Ponderosa or Radiata pine, planted along plantation edges may reduce the spread of internal trees.

Grazing, oversowing and topdressing

Wilding seedlings are vulnerable to grazing for the first 2 years. Mob stocking with sheep will significantly limit their spread, often to the extent that other control requirements are minimal. Cattle grazing is not as effective.

Spread can be limited by oversowing and topdressing within a 200 m zone of spread-prone trees. This promotes increased grazing pressure on young wildings and helps the tussock grasslands compete strongly with germinating tree seedlings.

Calculate your risk of wilding tree spread from new plantings

Enter your score (the number associated with each option) for each category in the boxes provided and your total score will be shown at the bottom of this form.

Interpreting your score:

• A total of 12 or more = High Risk
• A score of 3 or 4 in "Siting" plus a score of 3 or 4 in "Downwind Land Use" = High Risk
• High risk does not necessarily mean that tree planting is ruled out. A different species, different siting or different downwind land management can significantly lower the risk of spread.

Pinus nigra and Pinus contorta pines spreading near native bush in the Rakaia catchment.

Common wilding species:

• Corsican pine Pinus nigra
• Lodgepole pine Pinus contorta
• Scots pine Pinus sylvestris
• Douglas fir Pseudotsuga menziesii
• European larch Larix decidua

Tree planting can contribute significantly to hill country development and prosperity if wilding safeguards are taken.

Reference: "Wilding Prevention" by Nick Ledgard & Lisa Langer is available free of charge from: Forest Research Institute, Box 29 237, Fendalton, Christchurch.

Acknowledgement: "Calculating Wilding Tree Spread Risk from New Plantings" was prepared by: N Ledgard, NZ Forest Research Institute Ltd.