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New Zealand Journal of Forestry (1972) 17(1): 61–73
©New Zealand Institute of Forestry

Research article
Conditiong of radiata pine seedlings by undercutting and wrenching: description of methods, equipment and seedling response

J.C. van Dorsser and D. A. Rook

Unless Pinus radiata D. Don seedlings are conditioned to withstand transplanting, survival rates can be low. A suitable conditioning treatment consisting of undercutting and wrenching and periodic lateral pruning has been developed at the Forest Research Institute, and this is being applied in most of the larger nurseries in New Zealand. Formerly this was done manually by spade; now a reciprocating undercutter / wrencher drawn by a tractor cuts the seedlings' roots below the surface and a lateral pruner of self-aligning coulters on a tool bar trims vertically between the rows. After being undercut, seedlings are wrenched every 1, 2, or 4 weeks, and lateral pruned every 4 to 6 weeks. Seedlings must be undercut during the growing season, and timed to give a conditioning period of at least 8 to 10 weeks before planting. The effect of such undercutting and wrenching is that shoot growth virtually cases, whereas root growth continues at the same rate, producing a plant with a compact mass of fibrous roots, a high root-shoot ratio, and a hard, woody stem. Carbohydrate levels are also increased.
Although in some nurseries restraints are imposed by soil, climate, and weather, well-conditioned planting stock can be obtained throughout the year by regulating sowing dates and timing of undercutting. Seedlings conditioned in this way were subjected to a range of test conditions in the field and laboratory, e.g., resistance to root exposure, heat desiccation, cold storage, transplanting into a hot, dry climate. Results indicate that the efficient root system induced by the conditioning regime was the main factor responsible for the higher survival rates; but other factors (e.g., woodiness of shoot, and carbohydrate reserves) are probably also important.

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