I am a huge euphorbia fan! But not that many types will grow for you successfully in a zone 5. The hardiness ratings seem to jump around from source to source but based on comments from other cold climate gardeners here, I'd stick to E. polychroma/epithymoides ('Bonfire'), E. palustris (swamp spurge, good for damp sites), E. corollata, flowering spurge (native prairie plant) and E. griffithii (a rhizomatous spreader with vivid orange-red bracts). Although sometimes listed as hardy for you, I'd be wary of Martin's spurge, E.xmartinii. At best it will be borderline for you. You can also probably grow myrtle spurge, Euphorbia myrsinites. That can self-seed to the point of invasiveness in my climate but is better behaved in colder ones.

Diamond Frost and other euphorbs in the Diamond series are very tender, semitropical euphorbs grown as annuals. They bloom profusely and continuously from spring through summer and are a great container or bedding plant. I once did a garden design for a wedding that featured this plant and they were stunning! They will like more water than most euphorbs, especially if in a container, but I would still keep them on the drier end of the scale.

Oh thanks for all the suggestions. E. corollata looks especially interesting to me. I was just browsing through Prairie Moon catalogue and they do not have that in the printed version but seed is offered online. I am trying to tell myself I am done with seed ordering for this year due to being out of space for seed starting. I am going to keep it in mind and plan out an order from Prairie Moon for next season though.

I also see Plant Delights offers a named selection. How unfortunate is has the common name of Redneck Baby's Breath though.

Thanks for the photos. Wow really a bright spot in the garden. I might give one a try. I've noticed in big box stores a trend to selling larger annuals for more $$. SO fewer multi packs for a deal. But i keep wanting to give it a try so I just need to see how it does.

In less than full sun, Diamond Frost is loose, tall with bigger leaves.

I love this cute little plant in a lot of settings. The little blooms kind of look like butterflies floating in the air.

Always a bit anxious about US zones - I also grow quite a few euphorbias but fear most would not be hardy in a Z5 garden. E.ceratocarpa, e x pasteurii, e.mellifera...although I also grow e,polychroma and, for my cut flowers, e,oblongata. The last 2 would be hardy for you, Iowa, I think.