I looked around for hardiness advice regarding Eremerus here in Sweden, thinking that the northern parts of Sweden could well be a reasonable match for Canadian conditions. Always difficult to look at the USDA zones and find a good match, because zone by zone the Swedish climate usually is harder on (most) plants. My zone here for instance technically is USDA zone 7b, but that should for actual plant hardiness be more like a zone 5-6 from what I understand, yet of course that depends a lot on local conditions and what the plant is sensitive for during winter.

What I found was that the Eremerus hybrids and E. stenophyllus that I grow are only rated as hardy (according to our Swedish zone system) for the southern parts of the country and along parts of the coast but E. robustus is considered much, much hardier growing well far north. Probably this would be the best species to try for a cold zone. Still growers here in the south sometimes have problem over wintering it. From what I understand this has to do with drainage/winter moisture that can be a real killer.

Eremerus robustus naturally grows on gravelly slopes, so I think a raised bed with lots of gravel would increase hardiness, especially if it could be protected with a raised piece of glass or plastic(with good airflow under it) during autumn rains as to keep the ground a bit dry. When the ground freezes properly one should switch to a dry mulch instead. Of course this is mostly speculation on my part, so take it for what it is, but still these are methods that are fairly common and often can increase hardiness considerably for many perennials.

As a side note E. robustus naturally grows in areas that has real winters, but spring comes fast with no real setbacks, this makes them sensitive to late night frosts after they start to grow. For me E. robustus is a very new plant, but I already find it interesting comparing how it grows with the older Eremerus (well they are only a few years old) in my garden. So far the growth point seems to expand a lot more during the winter than the rest of them. This makes me believe that it may well need more protection from late frosts than the others I grow.

Yes, excellent drainage is important for these plants.

The cold zones are rated differently by different countries.
Our zone 4a is not necessarily the same as a USA zone 4a becuase the Canadian scheme takes in more than just temperature.

I like to place plants which require good drainage on slopes.

For now, I will just try to get them through the spring and summer in large pots.

That's interesting about the Canadian zone system, Caroline. With so many factors in play regarding plant hardiness it can only be a good thing to consider more than temperature. It's not uncommon for plants to be sold around here as Canadian hybrids and that is used as a sales argument, meaning they should be hardy also for the Swedish climate!

I hope the Eremurus will do really well for you in pots and that they will bloom the first year . Some, but not all, of the Ruiter hybrids I planted (with a reasonable root size) bloomed the first year from a spring planting, but took a break the second and bloomed again the third year.

William, did you plant your Ruiter hybrids in spring or fall?

Neal, I planted 6 of them in the spring. I found them cheap in a grocery store and they were supposed to be E. stenophyllus, but turned out to be a mix also including Ruiter hybrids. In this case I didn't mind at all

I also tried one planted in the fall, bought from a "proper" source, but that one was much smaller and took two years to bloom! To be fair it was a good price on that one as well, so couldn't really ask for more.

Edit - of course not all 6 were Ruiter hybrids. Some were E. stenophyllus as they were supposed to be.

That is promising news for Caroline's spring planted roots, sounds like there may be blooms this summer

Somewhere on the web, I came across the idea that they should be planted in spring, in colder climates,--- and in the fall in warmer climates.

Lois Hole in her book on perennials says they can be planted in spring or fall; I would suspect in our colder climate spring planting would be preferable. I've heard of a lot of gardeners on the Canadian prairies having good success with eremerus despite their zone 5 rating. I'm thinking their preference for a more sheltered location and hence micro-climates has a lot of bearing on that. Let us know how you make out with your potted eremerus Caroline; I am very interested in giving them a trial here as well. 😀

I will keep everyone posted on these Ruiter Hybrids.
I think some varieties might be more hardy than others , also.

Well the main shoot on E. robustus rotted during the winter, so can't really share any info regarding how early it starts to grow in comparison to the rest of them . It looked okay, put I touched it as I was looking for slugs and it was like a balloon without air. We did have a very wet winter, but unsure if that was the cause or not.
The good news is that there are several new small shoots forming from one of the side roots . Unfortunately this plant also seems to be a slug magnet so already lost a few of the shoots. It's a serious slug year over here .

Sorry to hear that William