Hi Iswald, welcome to this corner of the country and to ATP!
I'm a PNW coastal moldie with no clue about Idaho growing conditions. But I'm going to guess that you have extremes of heat AND cold, and drying winds.
Are you thinking about ornamental trees, evergreens, stately oaks or ... ?
I think the most important single factor after "will they live here and be healthy?" is "What do you like?" For example, if you don't like lots of bamboo, this won't help you at all. (And I don't really know if bamboo is likely to survive in Idaho. They might not like drying winds.)
If I was trying to fill in two 1/4 mile rows without going broke, I would look for a very cold-hardy clumping bamboo, and then split the original patch every 4-5 years. I'd also split each new patch every 4-5 years, until I had enough clumps started along the driveway that they would fill in within a few years.
If you really can split each clump four ways every five years, and transplant the extra three clumps, it adds up relatively
quickly, compared to waiting for oaks to "fill in" a long row.
By year 15, you're a bamboo nursery.
But Zone 6B gets down to −21 °C or −5 °F in an AVERAGE year.
I wonder how cold it gets in an unusual year, say one that only occurs every 20 years? That might be a good temperature to choose for "cold hardiness" of trees that might be a 50-year or 80-year investment.
1 clump of bamboo
4 clumps (Year 5)
16 clumps (Year 10)
64 clumps (Year 15)
256 clumps, or continuous rows (Year 20)
I searched the ABS site for clumpers cold hardy down to -10F with minimal damage and found four species within the genus Fargesia.
This is my little cutie:
from Home Depot, 2009 and 2014