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Winter Cold Inury to Grapevine Canes and TrunksWhat to do with injured trunks |
Deciding what to do when trunks are injured
The extent to which you will decide to modify standard viticultural practices because of potential cold damage to trunks depends upon how often you expect trunk injury to occur. Many important grape producing regions suffer trunk injury one or more times per grape grower generation. Colder parts of Champagne or Germany may expect such injury as often as once in 10 years. In spite of this, the growers take few special precautions. When vines are injured they either train up new vines from buds near the ground or replant the vineyard. The injury just doesn't happen often enough to justify the cost of special precautions or practices.
In colder areas, trunk injury may happen more often, and special practices such as multiple trunks are used. In the long run there is only one thing to do with an injured trunk, replace it with a new trunk. Injured trunks never fully recover and will have a negative impact on the vine and vineyard as long as they endure.
Thus the decision is not whether to replace, but when and how to replace the damaged trunks.
If the vine is dead, there is little to decide except how to replace the vine. If the vines were grafted, then a replacement, grafted vine should be used. If the vineyard is self-rooted, most often the best practice is to produce a new vine by layering in a cane from an adjacent vine.
If the vine has multiple trunks and only one has been injured, then the proper decision usually will be to remove the damaged trunk , the grower can depend upon the remaining trunk(s) to produce a normal crop. He/she should select a "sucker" or water sprout originating near the ground to form a replacement trunk. (REMOVING DEAD TRUNKS CAUSES LARGE PRUNING WOUNDS WHICH MAY SERVE AS ENTRY POINTS FOR EUTYPA ARMENICA, THE CAUSATIVE AGENT OF EUTYPA DIE-BACK DISEASE. SPECIAL PRECAUTIONS SHOULD BE USED TO AVOID EUTYPA INFECTION.)
The problem is deciding what to do when a trunk is not killed, but is injured and there are no "spare" trunks in place to use. Then questions arise.
The insights gained from the previous section on how damaged trunks recover provides the answer to these problems. Retaining the old trunk temporarily is almost always the best idea. Removing all above ground growth from a mature vine which has a mature root system results in very prolific shoot growth from the base of the vine. These strong growing shoots are so vigorous they rarely mature well, and they are very difficult to train and to spray for disease protection. Even should the retained trunk collapse during mid-summer, retention usually helps to counter the excessive growth of suckers. If it doesn't die, it may produce a small to medium crop which is always good.
Here is the sequence of actions to take when trunk injury occurs in your vineyard.
First, retain a full complement of buds on the vine, and make sure any low growing spurs, canes or buds which might be used to form needed replacement trunks are retained.
In June, around the time of grape bloom, the vascular cambium of sound trunks will be fully reactivated (reactivation starts near the developing buds, and proceeds down the trunk; the base of the trunk is not usually reactivated until just about bloom). At that time go through the vineyard and make decisions about trunks. If you come to a suspicious vine (irregular shoot growth, unusually strong sucker growth, trunk cracking and callusing) make a small cut into the trunk at the effective winter ground height (snow on the ground may have raised the effective height). You will find one of three conditions:
The appropriate action for these three situations are:
1. No discoloration/bark slips the trunk is probably not injured, use your normal viticultural practice.
2. Discoloration/bark slips the phloem was injured, but a vascular cambium has reformed. This trunk will not fully recover. It should be flagged and replacement trunk(s) trained. The flagged trunks should be removed during winter or the following spring.
3. Discoloration/bark does not slip the phloem and perhaps the xylem were damaged, and the vascular cambium has not regenerated. This vine may well collapse during the growing season. If possible, two or more shoots should be trained up as replacements. If the old top dies, the extra shoots will help prevent excessive shoot growth. The old trunk should be flagged and removed as with example two.
You can see the critical decision regarding winter injured vine trunks is not made in winter, but in summer when the extent of injury may be better evaluated.