Velutina

Or the more practical solution... Grow lots of different varieties and stick with the ones that grow best. Keep growing these varieties and hope for a sport sucker that improves on the parent.

Or, grow the varieties that show the most promise and work on tweaking the environmental conditions to allow the plant to do better. (ie, different soil types, shade, etc)

One thing i'm planning on doing is setting up a solar collector to heat a 600-1000 gallon water tank. This water can then be used at night to flow through underground pipes underneath the banana plants to keep the roots/corm warm during cold nights. Warm soil will also radiate up and keep the air warmer, especially with a simple PVC temporary greenhouse. I'm not too sure if the bananas will respond positively to this or not, but it might be worth a try. (ie warm roots, cold leaves might cause stress... and warm roots/warm air might cause growth during months you want dormancy)

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Adam