Do our nominations have to be plants that we currently grow? Or can they simply be plants that we wish we had?

Also, are we only allowed one nomination, or may we nominate as many as we like (within reason)?

Are houseplants included in this?

I am not qualified to answer your questions. But let's ask @dave.

But the premise of the nomination is "favorite" I would guess that could be in your own, public or other gardens.

Seems like many people have nominated several different plants so I would go for it if you have more than one favorite. Check first to see if your plant has already been nominated and then just add comments which will add "votes" to that plant.

If a houseplant is ever used in a garden setting I would guess it could be included.

There's no restriction on which plants you nominate.