I am trying to see what I can push to grow in my climate. It is usually a zone 6, with the occasional zone 5 winter thrown in. I want to try to plant seeds of some citranges, or a poncirus that is slightly less horrible than the others.

Does anyone know what is next on the rung for cold tolerance? Has anyone had exceptional luck with anything other than the normal bitter orange/ flying dragon?

You may want to try some Citrange and some Citrumelo. You'll have to protect them but the branch tips seem to handle a few brief drops to 0F to 5F (-15C to -18C) with calm winds.

Here's my Citrumelo.....low so far was -0.2F (-18C) measured on top of the snow and +4.9F (-15C) measured five feet above the snow. The leaves are really beat up but the branches are likely OK. The Citrumelo did fine last winter.

Citrumelos seem interesting. How old is the one pictured? How is the fruit?

How much protection are we talking about?

Also, could anyone recommend their hardiest poncirus cross?

I'll visit the Rusk tree here this week & see if there is any fruit left.

Here's my protection for the Citrumelo. It's a staked wire cage wrapped in streach wrap with a garbage can cover. The cover is propped a bit to vent. I think you have 0% chance unless you protect. Citrange and Citrumelo are hybrids with Poncirus trifoliata.