Guava, Oxknee and American Arnica

I am in Boston, MA - Zone 6. What is the growing zone for guava?? Can it winter inside in zone 6? How about oxknee? And American arnica?

Guava survives outside in zones 9b to the tropics. It does very well as a house plant as long as you keep it on a sunny window. If you wish to summer it outdoors, you have to expose to full sun gradually, to avoid burning the tender leaves.
Oxknee is hardy in zones 8-9 and must also be overwintered indoors in zone six.
American arnica is perfectly hardy in zone 6 since it grows from zone 5-8. You can overwinter it indoors if you can keep it cool.