An interesting topic and most people that grow peonies will have experienced some frost damage at one time or another. I’ve been looking for some more information about it and especially on what cultivars might fare best.
In a post dated April 5th 2013 by Henry Chotkowski (who posted the original topic here) on the Yahoo Peonybreeders forum, some cultivars are mentioned:
“The horrendous freeze throughout the midwest in ’07 killed all the tree peonies buds that hadn’t opened and most all of the buds from the late Early-season to the early Late-season herbaceous hybrids and lactifloras. The notable exceptions were Rozella, Many Happy Returns, and Prairie Afire, with Rozella the only plant blooming in a field of hundreds of cultivars and Many Happy Returns seemingly none the worse for the circumstances.”
And in the same forum Don Hollingsworth wrote on April 29th, 2016:
“Maybe some will see me as hammering on an unbelievable hypothesis, but I repeat: look to late spring sub-freezing cold events as the cause when short bud covers, distorted petals, dead center structures and / or dead petals of the flowers are seen on plants of a clone which has been observed to make normal flowers in previous seasons. Yes, the extra heat push flower development earlier leading to a vulnerable stage of advancement earlier in the season, thus more likely to be exposed to late cold events. My hypothesis includes that the threshold temperature of cold damage gets higher as flower bud development advances. Further, the very early species, the ones hybridized with Lactiflora cultivars leading to presently valued breeding stock, are thought to have evolved heritable resistance to this cold damage, in order to mature seeds before early summer shutdown (when the snow melt runs out). We see that the spring shoots of early species come out of the ground with flower bud notably advanced, while the Lactifloras tend to avoid cold damage by delaying the advancement of their flower buds until later in the growth of the spring shoots.
So, one of our challenges in selecting for doubled flowers in early hybrids is to retain the heritable cold resistance of their early species ancestry. We see it in the very early flowering Saunders hybrid, ‘Roselette’, one that is notably resistant to late freeze damage, down to +/- 20 degrees F., provided the sun is shaded from striking the buds while the tissues are frozen. We saw this after six successive nights of those temps in 2007, when the flower buds that were shaded by wilted foliage made normal flowers, while those exposed to the sun while frozen were ruined. I have long heard the gardening advice to plant tree peonies where they are shaded from morning sun. In my observation, this can make a difference in the proportion of budded stems which mature into flowers. Among my Suffruticosa cvs., most of the several Chines traditional kinds have damaged flowers year after year. Among my Japanese tps the flowering dependability is much better under my conditions, but certain cvs. typically fail to reach their potential of flowering almost every year. Particularly a problem is ‘Kamada Nishiki”, some years which may make a flower only on a shoot that terminates low in the foliage. Also ‘Tamasudari’, a gorgeous white, when I see a flower – both those named seem to lose their flowers very early in the development, leading to blanks, rather than opening damage blossoms. An early herbaceous example is ‘Sunny Boy’ of which there is some proportion of damaged flowers almost every year, this season I see only two buds developing of 15-20 stems. The two buds that will flower are on shorter stems located on the north side of the bush – had I paid attention to this fact before, I would have been more sensitized to getting out there with a cover for the bush every time a freeze was threatened.”
In other posts from Don Hollingsworth which I cannot locate easily I’ve read that Pink Vanguard does show some frost resistance and is therefore preferred over Blushing Princess as a breeder plant, whilst I seem to remember him also mentioning Many Happy Returns.
That’s not a lot of cultivars of course, but even a few can be good starting points. Many Happy Returns are a no-go here when it comes to using them in hybridizing, I’ve tried a few times without any success. Pink Vanguard has twice shown frost damage the last years here whilst many (though not all) other varieties didn’t show any damage, thus it is surely not completely frost tolerant, perhaps just the prone stage when it was freezing. I’ve grown Roselette, and I don’t recall any frost damage indeed. Never used it in hybridizing (I was primarily using Pink Vanguard). Prairie Afire is a cultivar I’ve not grown yet, but Rozella does seem interesting, although from the plants growing here I’ve not seen any seed. I’ll look closer to that one the following years, I think from all the cultivars mentioned here, it has the best qualities (healthy, fast growing, double, strong stemmed, large flowers).
I’m a bit uncertain as to what species might have some frost tolerance. Mediterranean ones? Roselette comes from three species: lactiflora, tenuifolia and mlokosewitschii. Mlokosewitschii I would not call Mediterranean, it growing in a very small area at the crossroads of three countries: Georgia, Azerbaijan and Russia. Tenuifolia grows in a much wider area, some of which (Serbia, European part of Turkey) might be called Mediterranean indeed. Unfortunately tenuifolia is not one that grows very well here, perhaps Hans Maschke has some experience with frost tolerance of this species and its descendants? When I think of Mediterranean species, these spring to mind: mascula, corsica, broteri, coriacea, clusii, rhodia, parnassica, cambessedesii, kesrouanensis (turcica), arietina, peregrina, officinalis. There are quite a few cultivars that have peregrina and officinalis in them, but are there frost tolerant examples? Also corsica gave us Tranquil Dove, Picotee, Halcyon, I’m not sure they show any signs of being more resistant. If you need large buds from the moment the plants break through the ground, Vanilla Schnapps is a good example, but I don’t think it is frost tolerant. I’m working with some of those species, but it’s still early days. I’d be happy to hear of other cultivars that show some frost resistance indeed.
All the best,