Q.  I am in the process of building an extension on our house.  Building control have said that I need to cut down my Cordyline australis.  I was wondering what the root structure is like and how far it should be planted away from the foundations of a building?  The tree is 3.5m tall and about 7 years old.  

A.  The root structure of Cordyline australis consists of a large rhizome (food storage organ) and fibrous roots, this below ground structure is in excess of 25 per cent of the whole biomass of the plant.

While roots of cordylines are not considered to be invasive, the shear volume of the underground structure as the plant grows larger could put pressure on surrounding walls and paths and cause structural damage.  Unfortunately Cordyline australis resent their root ball being disturbed and are therefore difficult to transplant.  I believe attempting to relocate a plant that is 3.5m tall would be a waste of time.

Weather you do decide to try a transplant or plant a new tree, a mature Cordyline australis can grow between 12 to 20m in height and that's a very large root mass under the ground, so I would say plant in an area with good light and at least 4 to 5m away from any structure.