can I put yews in cast iron urns, zone 6?

Answer 1

I don't think I would. The metal will conduct cold right to the root zone. I know, in the winter the ground freezes anyway... but in a pot it freezes solid, not just from the top, as in garden soil.

Yew (Taxus) depending on the species isn't super cold tolerant.

My general rule of thumb for container gardening in colder areas is; don't spend more on the plants that you want to try to over winter, than you would spend on annuals in the same pot. If they make it, great! if they don't, well, you haven't lost that much in the effort.

Good luck
I hope that this helps

Answer 2

As long as there are drainage it should be fine. I use an old cast iron kettle that has a hole that rusted through the bottom and I use it to plant in.