The most generally garden-worthy and likely to be found on the market is S. praecox. It has been rated Zone 7 by some sources. If the general appearance of the flowers shown here is what interests you S. praecox provides that.
Poetry to Burn, we grow several different Stachyurus, including S. salicifolia. I love the look of the slim green leaves. I recommend the U.B.C.book "The Jade Garden" which has a very comprehensive section about it. We have grown S. praecox for many years without any problem, in a very sheltered woodland on acid soil. So, imagine my horror last winter when a frost turned the evergreen leaves of S. salicifolia black overnight. I thought we had lost it! Not so, new leaves grew and it flowered beautifully. According to the zone map( in the right hand margin) we are in zone 9.
Ron is right, the flowers of S. praecox (or S. chinensis) are really super too.
Zone 7 would get below 0F, so the shrub would be expected to take 7F. Although I may have seen some damage to these in this area after 1990 winter, I'm not sure. We got down to at least 5F, someone in north Seattle claimed 2F. It being a species there may be some variation in hardiness.
With many genera grown as ornamentals the split between taking much frost and not is whether the species or form in question is evergreen. With Embothrium coccineum for instance, even within the same species the "hardy" ones are the deciduous forms. Likewise Eucryphia glutinosa is as much as 10 degrees F. hardier than the others grown, all of these others evergreen.
I have both S. praecox 'Magpie' and S. salicifolia growing outside here in Trenton. I keep salicifolia in a wind protected location just in case. I have had it for the past two years so it has not come under the worst weather possible, but it has done well. The S.praecox 'Magpie' is not protected at all except for afternoon shade due to leaf burn in the summer.
Just glad to see that I am not the only one in the area. Yes, it does keep its leaves through the winter, so far anyway.
I would like to sned you my inventory with you so that we can share information about what we have and have not had success with. I try to keep notes, but with my three year old running around...I am lucky to keep track of where everything is.
Wanted to share a photo of this beauty. I got this plant from Camellia Forest in late Spring. It wasn't so hot out of the box but it's been growing beautifully. It's in a pot with the same mix i use for JM's. I dosed it with nutricote at planting.
I'm very pleased with this plant. I hope to get into the ground in fall.
There's an apparently unlabeled Stachyurus with narrow leaves at the Seattle arboretum. It is now some yards across. Any frost injury that may have occurred over the years is not noticeable in the appearance of the shrub.
Do not remember if the leaves are as extreme in shape as those shown here, but they do seem to be at least semi-persistent. May have to ask about it sometime at the office.