I had the cultivar 'Colossus' in my yard in Minneapolis and it did well. Nice plant. They do have at least one at the Arb.

After two or three tries, I now have a M. sieboldii that is hardy. The tree (now 10ft x 8ft) was planted in 1996, so it has been through the normal cold winters of zone 4a before the climate changing began. Although, there were one or two years when it died back some over a winter. The 2012 late spring frost severely killed early foliage, so I only got the sporadic mid summer blooms this season, rather than the big flush in early summer.

Pics from 2011, see here:
http://nargs.org/smf/index.php?topic=274.msg12885;topicseen#msg12885

http://nargs.org/smf/index.php?topic=274.msg10402;topicseen#msg10402

Thanks ginkgonut and leftwood. I am planning to check out the sieboldii at the arboretum next year. Your pics are beautiful leftwood. I was really blessed by a chance to live in England for 1 year, I traveled all over seeing in person the many gardens I had read about for years.

At Hidcote there was one of these trained to hang over a woodland path so that one could look up into the flowers, it was so beautiful. I want to try the same here in NE Iowa.

I saw sources for the Colossus, but I'd rather have smaller blooms with the deeper red stamens.

What was/is the soil ph you grew them in? My soil is slightly alkaline, but I read that they should tolerate it.

Mine grows in rich clay, pH 6.8-7.

Don't know the pH of the soli, but it was very sandy. I think it did scorch a few times during dry summers.

I think there's a huge one at Whispering Gardens nursery in Cottage Grove, iirc. It's been a while since I was there, but I seem to recall that it was not a stellata or one of the others I would expect to see.