I skipped over the x in the title on first view and was going to mention something about this looking like a hybrid, ditto for 'Arctic Jade' and 'Northern Glow'. How are the nurseries introducing these plants marketing them? Is it for more cold tolerance than Japanese maples for northern growers? Or just more decorative forms of pseudsieboldianum?
Yes, they are suppose to more cold tolerate. Iseli Nursery Jack Forst® Collection Jack Frost® maples are an exciting new collection of small to medium-sized landscape maple trees from Iseli Nursery. This collection of maples has been hybridized over the last 20+ years, using Acer pseudosieboldianum as the foundation for hardiness, combined with the beauty of the branching structure and leaves of Acer palmatum. They have been evaluated and selected to tolerate the dramatic weather shifts in the upper Midwest of North America. The Jack Frost® collection of maples bring a new level of durability to a popular group of landscape trees!
Acer pseudosieboldianum is not a hybrid name, hence no X. If both parents or a dedicated hybrid binomial for the particular cross are known then the name is A. 'Avalanche', without anything else.
The word is that these were crosses of palmatum and pseudosieboldianum that were selected over 20 years for hardiness. I don't know if any of the selections are second generation, or indeed if hand pollination was done at any stage, or simply seeds grown from pseudosieboldianum that "look like they have palmatum in them." I'd suspect the latter, but I have a suspicious nature. I believe, according to the standards, that it is correct to put the × in front of pseudosieboldianum if the other parent isn't known, but whatevs. (Can't be bothered to hunt up the reference, sorry). What bothers me about these, is that even if Iseli grafts them onto Korean Maple understock, no one else will. So no matter how hardy the scion is, the palmatum understock will freeze and the plant will die.
Think of Acer x freemanii - the cross symbol when used for a plant naming combination indicates a species name was created to cover a crossing of two particular species. When no such validly published name has ever been come up with then the presentation is instead the two specific epithets with the x between. As in Acer palmatum x A. pseudosieboldianum. With if there is lack of certainty about one or more of the parents question marks then being included so as to indicate such.
