In early February 1996, record low temperatures were set in many portions of Iowa. Temperatures of -30 F or below were common, particularly in the northeast where -46 F was reported at Decorah. Although the bitter cold came at a time when most fruit crops should have been at or near their maximum hardiness, such low temperatures approached or dropped below the maximum hardiness of many perennial plant species grown in our climate. For fruit crops grown in Iowa by home gardeners and commercial growers, the maximum hardiness ranges between -25 to -40 F with differences existing between cultivars within a species. Gooseberries, currants and apples are the hardiest woody perennial fruit crops with some of the hardiest cultivars able to withstand temperatures in the -35 to -40 F range. They are followed in declining order by American hybrid grapes and half-high blueberries, European pears, red and purple raspberries, Japanese x American hybrid plums, northern highbush blueberries, sour cherries and American grapes, European plums, apricots, Asian pears and black raspberries, French hybrid grapes, sweet cherries, Japanese plums, blackberries and peaches. Although peach trees can attain a maximum hardiness around -25 F, the fruit buds are killed whenever the temperature drops below 18 F.
At the ISU Horticulture Station where -30 F was recorded, an inspection of apple shoots revealed extensive injury (over 50% browning of the wood) on tender cultivars such as Mutsu, Jonagold and Spigold. Other tender cultivars such as Golden Delicious, Gala and Rome exhibited some injury (20 to 40% browning). At the time of inspection (2/19/96), other cultivars such as Jonathan and Red Delicious did not appear to be injured. Inspection of apple spur buds did not reveal any injury, but the woody tissue at the base of the buds exhibited some browning on the tender cultivars. It is probable that injury will be more severe on the less hardy fruit species.
Because temperatures varied throughout the state during the sub-zero freeze, and different species and cultivars within a species have different capacities to withstand low temperatures, for apples and other tree fruits we are strongly recommending that the trees be inspected by cultivar for signs of winter injury before pruning. Also, because rootstock can influence the hardiness of a tree, a cultivar on different rootstock should be evaluated separately. The trunk is the least hardy above ground portion of a tree, but any attempt to inspect the trunk for damage would cause injury. Therefore, the evaluation should be confined to 1-, 2-, and 3-year-old wood, spurs and fruit buds. This involves slicing through various ages of wood and spurs, and examining the wood (excluding the pith) for browning of the tissues. Injured fruit buds will appear "blackened" in the center. If injury is extensive (over 50% browning of the wood) or if browning in the cambium region (between the bark and wood) is evident, it would be best not to prune that cultivar. This recommendation is based on the fact that winter-injured tissue does not heal-over well, and pruning can result in greater injury through the loss of moisture from the wounds. If the trees survive and appear healthy, prune out water sprouts and dead wood during the summer. Any summer pruning on apples and pears should be delayed until about mid July when there is little chance of fire blight infection.
For summer-bearing raspberries, delay pruning as long as possible so that live canes can be distinguished from dead canes. Where there was snow cover present during the freeze, it is probable that the covered portions of the canes escaped injury while the exposed portions were injured. To adjust for this situation, it may be necessary to leave more, but shorter canes than normal. Under normal conditions, summer-bearing raspberries would be pruned back to 3-5 canes per linear foot for red raspberries; 4-6 canes per crown for black raspberries; and 3-4 canes per linear foot for purple raspberries. Because fall-bearing raspberries fruit on the current year's canes, there should be no problems with winter injury since the over-wintering canes are cut-off at the soil line. Since raspberry canes are biennial and not perennial, the vegetative and flowering balance is not as critical as in other woody fruit crops like apple.
For grapes, inspect the canes and buds for signs of winter injury before pruning. If extensive cane injury is evident, delay pruning until after bud-break and prune back to new healthy canes. Be careful not to break-off the newly emerging buds when pruning or tying the canes to the wires. Where cane injury is not too extensive, bud survival should be evaluated to adjust pruning practices. The procedure for evaluating bud survival is as follows: 1) Collect 100 buds per cultivar. 2) Allow the buds to warm to room temperature. 3) Dissect the buds by making a cross-section cut, 1/3 of the bud depth. Proceed with thin slices until the primary bud is exposed. 4) Evaluate the primary bud for live green tissue. The bud will appear brown if damaged and will not grow. 5) Calculate the percentage bud mortality of the primary buds. 6) If the percentage bud mortality is: 0-15%, prune as usual; 15-30%, retain one or more average canes per vine; 30-50%, double the normal number of buds retained per vine; and 50-99%, delay pruning until after bud break or "double prune" (remove dead wood while vines are dormant, and selectively prune canes after bud break).
Gooseberry, currant and blueberry canes can be inspected for injury. However, because pruning is not critical for these crops, pruning can be skipped for a year if there is any doubt regarding the presence of injury. Like raspberries, the presence or lack of snow cover will influence where any injury may have occurred on the plants.
Strawberries probably escaped injury wherever they remained covered by snow or mulch. If you are concerned about injury and potential yield loss, dig plants by cultivar and split the crown with a knife. Any brown tissue in upper vascular portions of the crown would indicate potential injury.
This article originally appeared in the March 1, 1996 issue, pp. 21-22.