Yucca brevifolia jaegeriana 'Joshua Tree' Delamar Valley, NV

Yucca brevifolia forma jaegeriana also known as the Joshua Tree has proven hardy in Colorado taking -22F several times and many negative teens.
 
Where this seed was collected I had a weather station record -31F once and -25 many times. However humidity is probably 2% during these events. They have done very well in my garden thus far taking -17F and recently -11F.
 
This is one of 3 forms that are grown by us, each exhibit very good cold hardiness. Keeping plants on the dry side in winter is key. Radical drainage helps with this as well. The seed for this was location was collected near the Tonopah Airport and is probably the furthest north location for this plant. 
 
I do not guarantee these in a zone 5/6/7 because everyones growing conditions are different but this would be a good trial plant to grow in your cold outdoor garden. Try to plant outdoor 60 days before 1st hard freeze. Try at your own risk!