By: Brian R. Smith, UW-River Falls
This past winter was truly what fruit breeders call a “test winter,” in that conditions were typically much more extreme than average and if a new prospective cultivar being considered for release makes it through that particular set of conditions, the breeder has increased confidence that once released, that cultivar will have good longevity in a commercial grower’s field. Well, as expected, there were a few of my selections that did not make the cut after last winter, so they are off my “potential introduction” list.
Many growers also sustained massive winter injury in commercial plantings, but why? Not only did we see massive grape, blueberry and strawberry injury but also the same on many tree fruit. The cultivars you are growing are supposed to be winter-hardy aren’t they? Well, that is a loaded question since winter injury is such a complex phenomenon. No two winters have the same set of variables and these variables hit some species of fruit crops more than others depending on the combination. Some newer cultivars released may never have been exposed to a particular combination of variables before. Most winter injury is based on tissue desiccation but here are a few of the weather variables associated with the severity of winter injury: 1. Timing of cold- late fall, mid-winter, late winter?; 2. Intensity of the cold?; 3. How long it lasted; 4. What were conditions like the previous few days before the weather system hit?; 5. How dry was the air mass connected to the cold?; 6. Wind speeds; 7. Abruptness of temperature drop; 8. Depth of snow cover, (if any); 9. Acclimation conditions in the fall ( for hardening off) and the list goes on.
Now let’s throw in a few more plant variables related to winter hardiness. We, as growers are typically tempted to “push the envelope’ and plant cultivars that are maybe borderline hardiness: 1. True hardiness rating of cultivar based partially on USDA Hardiness Zone map (not very accurate but is a guideline); 2. Age of planting; 3. Condition of planting (diseases or insects weakened the plants the previous summer?); 4. Over-cropped this past summer? 5. Soil type and moisture level going into winter; 6. Rootstock used; 7. Plants in too active growth going in to fall/winter (improper acclimation); 8. Nutritional status of planting, and the list goes on. And lastly, let’s look at some farm variables: 1. Topography and planting location (on a hill, even with surrounding topography, low spot); 2. Presence of windbreak and orientation in relation to the planting; 3. Vertebrate pest population that can exacerbate any winter injury-voles, mice, rabbits, deer. Well, yes, the list goes on.
So, what are we to do with all of this information? Basically, at this time of year, there are lots of issues going on with plants trying to recover from winter injury. Winter injury is not typically black and white but a very gray area as many plants do not outright succumb but have varying degrees of different types of injury that are up to you as a grower, to recognize, make decisions on and treat if deemed a solid economic decision. Much of grower decision-making is made “on-the fly” regarding treatment of plantings and whether to keep them, since conditions can become better or worse depending on summer weather, crop loads, pest control, weed control and delayed effects. For example, apples may not die from winter injury until July or August. There may have been compromised vascular injury in the xylem or phloem but not enough to keep the tree from leafing out rather normally. If the tree has a partial crop of fruit and weather turns hot and dry, the tree is now under major stress and the injured vascular tissue cannot keep up with demands and there is tree vascular collapse; leaves turn brown and the tree actually does die. Being aware of these types of possibilities is half the battle.
Many growers question whether pruning should be done on winter-injured tree fruit crops and the answer is “yes”, even at this time of year, despite the fact that it could potentially lead to some disease spread. We are talking about the lesser of two evils. Most winter injury that is complex involves sparse leafing out, indicating vascular damage. Remember, the underlying causes of vascular damage is a partially incapacitated vascular system and the more stress that is put on that system the less likely it is to repair itself and become eventually fully functional. If your trees have either dead branches or those with sparse leafing out at this time of year, they should be pruned back to areas that appear to have normal vigor. Remember, that if the branches have compromised vascular ability, there could also be root damage and the tree cannot support such a large area anymore. Leaving half-dead branches on a tree is open invitation to secondary pathogens moving in and insect infestations such as borers. If you prune back to fully functional live wood (normal-looking leaves), the tree/bush will grow much faster in those areas and will heal over wounds quickly, thus minimizing any potential disease spread. Most pruning is an invigoration process; just don’t overdo it. Make sure you sanitize pruning equipment as often as you can. Plants in general have an unbelievable ability to repair themselves and if given half a chance, they will! Delaying pruning any later than July 1 is not advisable as tree response will be too late and responding vigorous growth will not have the ability to fully acclimate for the upcoming winter.
Even some of the least hardy and risky crops can temporarily recover until the next killing winter. Peaches are not a crop many people grow in WI (with good reason; I do not recommend them for anyone) but there are a few growers. Peaches have high innate vigor and if pruned heavily back to fully live areas, can fully recover if the next winter is mild.
It is difficult to spend money on plants that are not generating income, making it even easier subliminally or otherwise, to ignore the plants. What is the cost, though, of not maintaining them? What would it cost to rip out that block of trees or raspberry planting and re-plant? How long would it take to get back in full production and how much would it cost to maintain that new planting while it is establishing?
Some tips to keep in mind on rehabilitation:
1. There should be no fruit allowed on any winter-damaged trees or fruiting shrubs as this type of stress will hamper their ability to recover in a timely fashion. Recovery can take several years in some cases and again, grower observation of plant response is a key factor in timely and proper maintenance. Allow more fruit retention as trees recover; we don’t want them to become rampantly vegetative!
2. Maintain all normal pest control-insects, diseases, weeds; all of which will delay or prevent recovery of the planting.
3. Employ summer pruning on herbaceous, actively-growing shoots in order to direct growth where needed, whether to reestablish a leader or to balance growth in different parts of the plant.
4. Maintain nutrition but do not encourage rampant growth. Remember, this plant does not/should not have any fruit and does not require the nutrition of a bearing plant. Do not fertilize with nitrogen after July 1.
5. Do not allow drought stress; irrigate on a regular basis as needed.
A special opportunity occurs after winter injury when new tree propagation (budding) is a possibility. In this situation, the top of the tree (scion cultivar) has actually completely died but the rootstock is growing up from the base. This still represents an advantage over replanting next spring because the root system is already established and a new tree can be grown back faster and more cheaply than replacement.
You also have the opportunity to change cultivars. Make sure the dead parts of the tree have been pruned back to where actively growing rootstock shoots are apparent. Thinking ahead about a month, allow maybe two suckers only to grow from the base of the tree; if you are an experienced propagator, leave only one… Once current season’s growth has sufficiently mature buds, (late July/August 1st) we can decide to T-bud or chip-bud that rootstock. T-budding can only be performed on an actively-growing rootstock where the bark is “slipping” and T-budded trees typically do not make quite as strong a union/tree as chip budding but the procedure is faster. I have budded 100’s of trees with either method but chip budding is my top choice as it definitely makes a stronger tree in the long run and does not require that the rootstock be in active growth when the procedure is done. Bud as close to the base of the rootstock as possible. Depending on late summer/fall weather, the dormant bud should be completely healed into the rootstock by October 1st. All budding rubbers should remain in place for the winter but any parafilm used (chip budding) can be removed at or even before that point. Please keep in mind that it is illegal to propagate a cultivar that is still under an active patent but those with expired patents (like ‘Honeycrisp’) are completely legal to propagate yourself. Remember, if your budding is unsuccessful, you can graft those same rootstock suckers next spring (with dormant scion wood collected in February) just as the rootstock buds start to break.This article was posted in Berries and tagged winter injury.