I vaguely remember seeing this on the box. Did he use something like tamarillo as the rootstock? I don't remember.
A good rootstock for hardiness would be the Mountain Kangaroo Apple or Solanum linearifolium. It survives winters here so it is very hardy indeed. Another one worth considering would be the Goji Berry or Lycium barbarum. It is supposedly hardy to -20°C!
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Ray
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