Researching Cherry Fruit Bud Hardiness
Below is a forwarded email from Lynn Long a Hort. Extension agent from the University of Oregon. [Kevin] came across the attached Article written by Lynn and thought it would be good information for the Newsletter. Lynn has given permission to reprint.
Hello, I came across your paper on Fruit Bud Hardiness on Sweet Cherry Trees. I am in Alaska and am past President and Vice President of the Alaska Pioneer Fruit Growers Assn. I have a small Orchard here in Wasilla AK and have been having trouble with my Pie Cherry Trees Winter killing. I have been growing the Evans and Northstar. The Evans are tissue culture so are on their own roots and the Northstar are on Mahaleb root. Mahaleb isn’t hardy here. The tree will live for a couple years then slowly die out so I and others in the group have been experimenting with the Amur chokecherry as a root since it is extremely hardy here in AK. The Evans however keep dieing back so your article kind of explains why, We have often extreme fluctuations in temps through the winter. Last February we had close to +50 and rain then it dropped back to 0 in a weeks time. I have suspected that fluctuation was the cause of the severe dieback I had on the trees. However I have one Evans where I had little dieback on! So I am not sure what to think. I know your article was on Sweet Cherries but am I wrong to assume one could extrapolate the same could be true for the Pie Cherries? Albeit the temp range would certainly be different as Pie Cherries are Hardier. Also, I am wondering if with your permission I could pass your article onto our Newsletter Editor to include in our next issue. I am sure our Members would find the Article very informative.
Kevin Irvin Sundog Orchards
The information that I provided on sweet cherries can indeed be extrapolated to include sour cherries, although as you suggest, the critical temperatures would be different. I don’t know what those temperatures are for sour cherries, however. Jim Nugent from Michigan State University might have that answer as most sour cherries are grown in Michigan. Feel free to reprint the article in your newsletter. Can I ask the name of your newsletter and your circulation number? Thanks.
Fruit Bud Hardiness
Lynn E. Long, Extension Horticulturist
Oregon State University, Wasco County Extension
If severe cold can kill entire trees, it can obviously damage fruit buds. Cold sensitivity in buds varies with the season and weather, which helps to explain how sweet cherry trees in Traverse City, Michigan, can withstand temperatures of -30° F without bud damage, but trees in The Dalles are damaged at -10°F.
Cherry buds become hardy sometime in October. This hardiness capability is largely due to their ability to supercool (cooling below the freezing point of a liquid without solidifying). Once this ability is obtained, buds are capable of withstanding temperatures around -6°F until the period of rest is fulfilled, generally sometime in January. During this time, the buds would not grow even if temperatures were warm.
What happens though if the temperature drops below -6°F? Are the buds automatically killed? Not necessarily. Tree fruit buds have an ability to become more resistant to cold over a period of time. Cold days, when temperatures remain below freezing, cause the bud hardiness to drop. As long as the florets around the buds remain frozen, bud hardiness in cherries will drop about 4° per day. This will continue until the buds reach a hardiness level of about -30°F. If temperatures drop faster than the buds can adjust, then some or all of the flower buds may be killed. At this point, the supercooled liquid freezes with the resultant crystals rupturing the cell walls. On the other hand, if temperatures rise above freezing, the buds quickly lose their added hardiness and return to a hardiness level of about -6°F.
Once rest is satisfied for the fruit tree, sometime in mid-January, the temperature at which buds are damaged rises slowly but still remains near 0°F until just before the buds begin to open. As bud development progresses, the ability to re-harden with freezing is lost. This generally takes place in mid-February. Therefore, if the temperature drops below 0°F after mid-February the buds may not be able to respond and they may be killed as was the case in 1995.
Healthier trees seem to have a somewhat greater capacity to endure cold temperatures. Therefore proper irrigation, nutrition and pruning during the growing season can affect cold tolerance the following winter. With cherry trees, over-watering is probably as serious a concern and a greater problem than under-watering. Likewise, too much fertilizer can cause rampant growth and weak buds. Finally, healthy buds must have light to survive. A good annual pruning is necessary to maintain leaf and flower bud development in the tree center.
[Kevin] contacted Dr. James Nugent at Michigan State University where much of the Sour Cherry Research is done here in the USA (Dr. Nugent was awarded the 2006 Cherry person of the year) on hardiness of Sour Cherry varieties and if he knew of a hardier rootstock to use for them. Below is his initial reply. [Kevin] will pass along any information he gains from Dr. Greg Lang.
Great to hear of your efforts to grow cherries in Alaska. Sour cherries are generally more winter hardy than sweet cherries. Sour cherries evolved basically in the continental climates of Europe and Asia, whereas sweet cherries evolved more in the Mediterranean climate areas of southern Europe and the Middle East. However, there is a lot of variability in hardiness between varieties. Montmorency is more cold hardy than almost all sweets, but it is not as hardy as Northstar. We have introduced three new sour cherry varieties from Hungary to N. America. Two of the three definitely have less cold hardiness than Montmorency (Balaton and Danube). The third, Jubileum, is less tested, but I believe it also is less hardy. A fruit grower in the Upper Peninsula of Michigan did quite a bit of testing and concluded after many years that the only sour cherry variety in his planting that survived and cropped well was Northstar.
As to rootstocks, I’m not so sure what to tell you. Our two major cherry rootstocks are mazzard and mahaleb. I consider mahaleb to have better winter hardiness than mazzard. We have tested mazzard rootstocks that survive well in our climate, but so does mahaleb, so I don’t know what might be hardier. I will say that mahaleb is highly susceptible to phytophthera root rot, so will only tolerate a well drained soil. Soils where mahaleb will survive generally range from sandy loams to sands.
The next cherry rootstock test planting that will be established here at our cherry research station will include (I believe) some new rootstocks brought in from Russia, so I expect they may exhibit superior cold hardiness. I am taking the liberty to forward this to Dr. Greg Lang, our cherry rootstock specialist at Michigan State University. Greg will be better able to suggest potential rootstocks that would improve your chances for success.