Optimising mini-plug transplanting of Eucalyptus dunnii seedlings.

Abstract

Mini-plug transplanting has only recently been described as a useful propagation method for use in forestry nurseries. Mini-plug seedlings are seedlings that have been pre-cultivated in containers with volumes less than 33-ml. These seedlings are re-planted into containers of a larger volume at a later stage in the growing season. The many advantages of mini-plug transplanting include decreased seedling production costs, improved space use efficacy and the flexibility of producing larger seedlings for harsh field conditions. However, the effect of mini-plug transplanting on seedling quality and root architecture is unknown and may influence seedling survival after planting in the field. This research aimed to optimise a mini-plug transplanting protocol for use in a commercial forestry nursery to limit the development of root deformities and enhance seedling quality in Eucalyptus dunnii seedlings. The effects of mini-plug container type and mini-plug cultivation time prior to transplanting on seedling quality were evaluated. Seedlings were grown in two mini-plug containers, MP288 and MP544, for one, two, three and four months before transplanting the mini-plug seedlings into the final container. In this experiment it was confirmed that the mini-plug container design had an effect on mini-plug seedling quality. Seedlings grown in MP288 containers had lower sturdiness and shoot-to-root ratios compared to MP544-reared seedlings. This was attributed to the lower planting density and higher cavity volume of the MP288 trays. However, MP288 trays increased the frequency of root deformities in the E. dunnii mini-plug seedlings as these trays were constructed out of plastic which encouraged root coiling. The MP544 trays produced seedlings with actively growing, fibrous roots which were attributed to the root pruning properties of the copper oxychloride that the polystyrene mini-plug containers were initially treated with. It is therefore recommended that the MP544 tray should be used for the cultivation of mini-plug seedlings. The age of the mini-plug seedlings also affected transplanting success. Based on the results it was concluded that the optimal age to transplant mini-plug seedlings was between two and three months after sowing. Transplanting one-month-old seedlings tended to increase the frequency of J-roots after transplanting and four-month-old seedlings delayed seedling growth. Once the optimal seedling age and mini-plug tray type to be used for container-to-container transplanting was confirmed, the effects of the propagation method (directly-sown and transplanting) on seedling quality were compared using various containers. Transplanted seedling quality was inferior compared to directly-sown seedlings. This was caused by delayed growth directly after transplanting. The substrate composition that the mini-plug seedlings had been grown in had not been optimised for mini-plug cultivation and may have reduced mini-plug seedling quality which resulted in delayed seedling growth after transplanting. In addition, transplanting increased the incidence of root deformities and using polystyrene containers as the final container for transplanting did not reduce root deformation. The presence of J-roots was believed to be caused by the transplanting process itself. The negative effects of container design observed in the mini-plug containers were also observed in the final container after transplanting. Low plug volumes decreased seedling growth and increased seedling competition due to the higher planting densities. Following the results of the second study, the optimal substrate composition to be used for mini-plug seedling cultivation and in the final container after transplanting was investigated. This investigation confirmed that the substrate composition affected seedling development and growth. In the mini-plug trays, the physical properties of the substrate were shown to be important factors affecting both root development and seedling growth. Substrates with low air-filled porosity values reduced root growth and the negative effects of low air-filled porosity were enhanced when seedlings were cultivated in mini-plug containers. The results also indicated that different substrate compositions were required for the different seedling propagation methods tested and for the different physiological stages in seedling growth. It was recommended that E. dunnii mini-plug seedlings are cultivated using an 80:20 (v/v) mixture of 6-mm pine bark and perlite. For the final container, the recommended substrate was a 90:10 (v/v) mixture of 6-mm pine bark and perlite. The effects of vermicompost leachate, seaweed extract and smoke-water on seedling growth after transplanting were evaluated as many beneficial effects of these supplements have been reported. The use of vermicompost leachate, seaweed extract and smoke-water were not effective in improving E. dunnii seedling growth after transplanting in this study. However, these supplements may have needed to be applied to the seedlings more regularly and at higher concentrations. Further research is required to optimise the concentration and frequency of application to ensure improved seedling growth after transplanting. Although, during the course of this study some improvements to the container to-container transplanting procedure were made, the seedling quality of directly-sown seedlings was still superior. By transplanting mini-plug seedlings a delay in seedling growth was observed and the incidence of J-rooting was increased. Neither, tray type, cultivation time nor substrate composition was able to limit the incidence of J-roots in transplanted seedlings or prevent the delayed seedling growth. For a mini-plug transplanting method to be successful in a commercial forestry nursery, further research is required to overcome the limitations of this method.