USDA Zone 6

Zone 6 is one of the best zones for bonsai in the US. Winters drop to -10°F — cold enough for proper dormancy but mild enough that the full range of temperate species does well. Most classic bonsai species originated in climates similar to Zone 6.

You can grow Japanese maples, trident maples, elms, zelkova, and the full range of conifers without heroic winter protection measures. A sheltered spot against a south-facing wall or a simple cold frame handles the coldest nights. The six-to-seven-month growing season gives trees plenty of time to develop.

This zone also opens the door to some borderline species like cork bark elm and certain azalea varieties that struggle in colder zones. You're still too cold for tropicals outdoors, but the temperate selection is outstanding.

Growing on a windowsill instead? USDA zones describe outdoor winter lows, so they don't apply to you — any indoor-tolerant species works in every zone.