poinsettia
That depends on where you live. I live in Tennessee and also lived in Iowa and have planted and transplanted Irises in April and also in Fall. Just don't plant when it is real hot and dry.
Cacti would have to be planted in pots and moved indoors during cold, wet weather or planted in a heated greenhouse.
Ferns can be planted outside, in hanging pots, or in wax pots. There are several different species of fern so you must make sure you find one that you would enjoy to have displayed in our around your home.
It's best to plant irises in the spring, but they can be transplanted in the fall. The rhizome (brown part with roots on it) should not be completely covered or the irises will not bloom. They like plenty of sunshine and shouldn't be in a place that doesn't drain well or the rhizomes will rot.
A lot of plants don't like copper. So it can kill some plants
I would use "irises."
Do not allow irises to become too crowded in your garden because the irises will flower poorly or not at all.
Vincent van Gogh painted 'Irises'
William Rickatson Dykes has written: 'A handbook of garden irises' -- subject(s): Irises (Plant), Irises (Plants) 'Irises' -- subject(s): Iris (Plant), Irish
Yes, usually called Vase with Irises.
You can find irises in temperate parts of the world. There are over 300 species.