Below‐ground morphology of Cordyline australis (New Zealand cabbage tree) and its suitability for river bank stabilisation

Abstract

Observations and measurements on the below‐ground characteristics of the New Zealand cabbage tree, Cordyline australis, from river bank environments near Christchurch, New Zealand, revealed a unique structure of peg‐like rhizomes and fine spaghetti‐like roots. By age 25 years, root depths reached 1.75–2.00 m, root spread reached 3.00 m, and below‐ground root biomass, including the rhizome, exceeded 50 kg or 38% of the total tree biomass. Fine C. australis roots of diameters 0.6–3.8 mm had mean tensile strengths in the range of 26.7–17.5 MPa, 30% less than those of most willow (Salix) species. The pullout resistance of five c. 8‐year‐old self‐seeded Cordyline australis trees ranged between 5.57 and 14.2 kN. In terms of the parameters assessed against published information for willows, it appears that C. australis falls short on both growth rate and tree anchoring parameters (tensile strength and resistance to pullout) for use as a river bank protection plant in all rivers. However, when grown with other native riparian colonising plants such as flax (Phormium spp.), river bank protection may be comparable, especially in low‐order streams with silty soils and lower hydrodynamic forces.

Keywords:

• Cordyline australis
• cabbage tree
• biomass
• rhizome
• root tensile strength
• erosion control
• biodiversity