Root structure of Cordyline, and remarks about C. indivisa

I visited North Island of New Zealand in year 2000 as the guest of the most gracious Keith Boyer.  We drove around the island and visited a park near New Plymouth.  There I observed the true Cordyline indivisa.  Since then I have been obsessed with the goal of obtaining a plant.  I did purchase one from Cistus nursery but it was killed by a gopher.  ( Thomomys bottae )

After that I was not able to find a replacement.  There is a lot of plant material sold in California labeled as C. indivisa but it is never the true species.  I started to grow it from seed, but suffered repeated crop failures.  Now years later, I have succeeded in growing a few plants to a planting size.

  (They are sensitive just after germination of the tiny seeds and become much more stable and easy the larger they grow.)

So far so good.  I have a lot of risk from gopher intrusion from the surrounding, untended properties.  Now, any plant that I really care about I install in a stainless steel gopher basket.  I was just about to do this with the Cordyline when a dim memory returned about a "tap" root in Cordyline.  How can a monocot plant have a tap root? 

A brief internet search showed this result.  https://www.tandfonline.com/doi/pdf/10.1080/0028825X.2005.9512995

So I cut out the bottom of the basket, leaving only the sidewalls as protection when I planted my second attempt at C. indivisa.

  Here is an no ID Cordyline that I removed, clearly showing the root development.  The orange tape marks the soil line.