Jayco,
I've had Lucifer crocosmia for many years and sometimes it comes back in the spring and other times only some comes back. So for your zone which is colder than mine aggressiveness is not likely a problem.
It doesn't like to be real wet in the winter so soil that drains well is imperative.
Haven't tried emberglow so can't comment on that one but Lucifer can be a showstopper.
{{gwi:230534}}
Flora

I've had Lucifer for about 15 years, and I do have more now than what I was given. Not all that much more, but more. It has been through some very cold winters, and always managed to come back.

I think the one which really spreads is the common orange C. x crocosmiiflora aka Montbretia which is very hard to get rid of in certain climates due to the multiple corms it produces. The picture is not mine but I have an almost identical one I took at the same place. It shows how Montbretia has spread into the wild from gardeners tipping rubbish in the countryside.

Here is a link that might be useful: Montbretia

I'm beginning to find it much more vigorous than I had reckoned. I have an orange one, unnamed, which is vigorous to weedy. It seems to need division often to keep it in bloom. It survives in an abandoned field bed, where it competes quite happily with twitch grass and other weeds.
But C. 'Lucifer', which grew in a nice clump in my centre garden has decided to really spread this year. I am going to have to cut it back severely; it is growing through other perennials nearby. It had behaved well for at least 10 years in the garden and is still very beautiful. But I have a feeling that if I abandoned these gardens, and returned to see what survived in 10 years time, C. 'Lucifer' would have eaten up all available territory.
I originally planted one corm - they were quite expensive then- and it has spread to a clump at least three feet in diameter, with many new shoots emerging a foot or two away.
Our winters are very cold, but snow cover is consistent from November ot March.

I have had mine for quite a few years and moved it three times. I moved it from my garden bed to plant it near the edge of my pond. It was starting to spread so I dug it up, divided it and put some in a pot and buried the pot last year. The rest I threw in the garbage. We will see how it does this year. At least in the pot if it gets too big it will be a lot easier to dig up and divide.

The Crocosmia I have is Lucifer.

I just planted the Lucifer variety last year & only 2 stems bloomed. I've noticed that it has multiplied 3-fold, but I'm anxious to see how well it does & looks. I put it in a trouble spot where I seem to have difficulty keeping all other plants alive in over the first winter.

Thanks, everyone!

Except for flora, no one from a mild climate responded. I had it in a front garden in a rental on the Monterey bay in California. As a last resort I removed all the bedding plants and set up a screen with 1/2 inch hardware cloth and with a shovel ran all the soil through it. I still did not get all the small bulblets, but most of them. By watching very closely in the next two years I was finely through with them. If you want to know if they are aggressive, the answer is yes. Al

Lucifer is also aggressive in Zone 7 in Denmark. But this winter only 1 clump survived(flood)