Discussing cold tolerance of persimmons, we tend to focus on absolute low winter temperatures. While this variable is no doubt important, it seems we need to pay at least as much attention to episodes of less severe cold in spring.
This article, forwarded by a friend, demonstrates the extreme importance of the state of a tree’s acclimation to cold, which depends on recent ambient temperatures, which depend mostly on the season of the year. Simply put, cold tolerance tend to be much greater in winter than in spring.
In the growing region in Korea where this research was performed, winter temperatures drop to a minimum ~ -10 C / +14 F, equivalent to U.S. FDA Z8. At those temperatures in December/January, the trees tolerated roughly -20 C / -4 F, equivalent to U.S. FDA Z6. But in this same region in late winter / early spring, temperatures drop only to a (warmer) minimum ~ 0 C / +32 F. At those temperature, the trees tolerated only -10 C / +14 F.
So all of the three varieties tested (Fuyu, Taishu, Romang) seem capable of surviving winter lows characteristic of U.S. Z6 (~ -5 F). But once late winter / early spring temperatures get warm enough that the daily low is consistently at/above freezing, all the trees would be damaged by any drop to 10-15 F or lower.
Against this backdrop, variations among varieties in sensitivity to absolute low temperature seem of secondary importance to most growers in the northern / central U.S. What may matter much more is how quickly each variety becomes de-acclimated when exposed to warming temperatures. Romang, for example, was the most cold-tolerant vs severe (-5 F) winter lows but least cold-tolerate vs milder (+15 F) spring lows.
Do any of those factors negate the main point that persimmon trees are much more sensitive to cold in spring, after they have begun to de-acclimate to cold? Or the related point that a variety that is relatively less sensitive to cold than others may be quick to come out of dormancy and therefore relatively more sensitive to cold in spring?
Those are valid concerns. I am working on establishing everything that should survive -10F or colder for my 6b location.
Then comes the spring test… Hopefully most will do ok. Here in kansas we get big swings, worse than most eastern locations, so anything that is not late vegetating will be damaged eventually.
And it will be a trial that might be worthwile compiling results for others after we get enough years of mature testing to get a good idea of averages(5+ years?).
While I am testing hardy kaki, at this stage, I am only recommending hybrids to my local friends, in order to err on the side of caution and increase their chances of success.
All species that harden off to survive winter become more tender as they approach bud break and beyond. Based on my experience, I believe that it is much more often that cold spells in early spring kill apricot trees than winter lows. Cambium injury for other species, even apples and certainly J. plums also seem most frequent during spring cold snaps in my region. Z6, south NYS.
The literature is full of warnings about trying to grow species capable of surviving extremely cold temps in regions with very stable weather conditions (where winters stay cold and seasons turn gradually and dependably) but tend to be killed by the more wild fluctuations of temps experienced in the U.S.
@alan – My related experience is mostly with figs but it mirrors yours with apples. From what I’ve observed, the “cold hardiness” of well-acclimated common figs varies little across varieties and probably protects the trees roughly to 0-5 F. I’ve had unprotected, well dormant trees survive lows of 3 F - 15 F without material damage. But trees in the process of de-acclimation (e.g., bud break) can be killed by cold snaps to 20-30 F.
Smith is a common fig variety that is (a) delicious and (b) sufficiently early-ripening to be suitable for the northern U.S. Z6-7. But in my experience it breaks dormancy so easily that it may often suffer severe damage in an early spring frost. I wouldn’t be surprised if this is the problem with some cold-sensitive persimmons.
p.s. More sophisticated growers may say, “Well, duh!” Yet I encounter numerous compilations of data on the cold tolerance of persimmon varieties where the only data point is the winter low temperature. It is as if the grower (1) notices the dead tree, then (2) looks back at winter records to find the absolute low, then (3) infers that absolute cold killed the tree. There is almost no systematic assessment of the conditions in spring. One exception is commentary on the variety Ichi Ki Kei Jiro, which is reported to break bud late, which may account for its relative hardiness in Z6-7.
Research dollars go to commercial fruit production and growers will grow fruit where it can be grown profitably. If a region has weather that supports commercial growth of any given fruit, we leave it up to growers to find and exploit it.
It would be nice if some university botanist took an interest in this issue and it could lead to some genetic engineering that allowed me to grow Fuyu persimmons in my climate in S. NY. If human beings have a future, that will probably be in it.