VINE RECOVERY: No matter what the vineyardist does to prepare the vines for the winter, sooner or later temperatures are going to fall low enough to inflict winter damage. Many factors contribute to the damage caused by low winter temperatures. Improper varieties for the area, undernourishment, disease, overcropping, improper hardening season, the onset and duration of the cold temperatures, and other causes contribute the the vine's inability to survive low winter temperatures. Having brought vines through very low temperatures in the past, here are some of my views on aiding your vines to recovery from winter damage:
The first step to aiding vine recovery from winter damage is to assess the health of the vine and find out how bad the problem really is. It does no good to examine the vines after each cold snap. Wait until the coldest periods of the winter have past. I usually wait until just prior to pruning. This may vary by the year, from March to April. It pays to be patient and assess all the winter damage at once. Once the cold periods are over, you should get out the pruning shears and begin by pruning off a cane that you would normally remove anyway. Begin at the tip of the cane and examine the buds. It should always be kept in mind that the most apical buds are often damaged and these would be pruned away as part of the pruning/training practice. These were still green wood in the fall and were not hardened anyway.
As you go down the cane towards its base, cut through the buds to see if they are a lively green, olive green, or soggy brown and dead. You should also examine the cane wood between the buds for this same appearance. The differences between crisp, lively green, living tissue and olive green or soggy brown dead tissue is quite obvious. If you're still not sure, take the cane and soak it in water at room temperature overnight and this will accentuate the colors of live and dead tissue.
When examining the buds, you should cut sections through the bud with a sharp knife or razor blade. Grape buds are compound buds. There are three actual buds within. A primary bud (which bears the crop), a secondary bud (which is usually vegetative but will form clusters with some hybrids), and a tertiary bud (which is only vegetative). Cut carefully until you get a cross-section through the largest, primary bud. Determine if it is green, olive, or brown. Only lively green primary buds will produce a full crop. Olive and brown buds are usually dead. If the primary bud is dead, keep sectioning through the bud to find the secondary and tertiary buds. If they appear alive, you will get vegetative growth from the bud but will not produce a crop (unless they are from a variety that produces small clusters from the secondary bud). An alternate way to do this is to cut the bud from the cane leaving 1/2 inch on either side of the bud. With a sharp knife or razorblade, make a longitudinal cut through the bud (as seen in the above photo). This should expose all three buds within. You can also see the condition of the wood doing this.
Link to Winter Hardiness Page
If you find damage (dead spots or buds) continue sampling buds down the cane until buds are found that are completely alive. Write down the total number of buds on the cane. Also record which buds are completely dead. Which have primary bud damage, as in the photo above (note that the secondary bud is alive). List which buds have secondary and tertiary buds alive and especially note when live, intact buds are found. The total winter damage can be determined by dividing the number of dead or damaged buds by the total number of buds on the cane.
If all the buds are dead, you have a problem. You have experienced severe winter damage. A trunk assessment should follow to determine if there was injury to the trunk. Look for discoloration under the bark, splits in the trunk, or sap leaking out of the trunk. These are signs of trunk damage. You might have to wait until spring to find out the extent of the damage. Often vines die to the ground but re-sprout from the ground. But if the vine dies to the ground a decision has to be made as to whether to replant of assist in recovery of the vine. I should point out that, in my experience, death to the ground is often common after the first winter from planting. If resprouting occurs after the first winter, I train them as I did after planting. I've found that otherwise hardy varieties don't harden as well as mature vines the first year and lead to more winterkill than will be experienced in coming years.
Recovery is dependent on the amount and type of damage. It is a matter of balancing the pruning levels, leaving plenty of buds, if you are left with some live primary buds. You will have to re-train the vine during the comming growing season. I've found that winter damaged vines often have bare spots along the canes which don't sprout shoots. Other buds further up the cane do have shoots. This leaves the vine in need of training for subsequent years.
Severe damage presents a different challenge. If there are too few live buds left for the vine to grow normally, you get a few, usually weak, shoots that grow from a few buds close to the trunk (if even these). Often they are vegetative, non-producing shoots. If they come from secondary buds from certain varieties, they will produce a few small clusters. The vine's root/shoot ratio is inbalanced and favors the roots. The vines respond by pushing forth latent buds from soil level. These become enormous "bull canes". They are non-productive but can be used to form new trunks for future years. I usually remove the first couple of these that form. They will become the largest bull canes and I want to discourage them. If you remove them, more form but are less vigorous and more what I'm looking for when training a new trunk. In mid-summer, I remove all but the one-three of these that I want to train into future trunks. This allows for more leaf space to aid in recovery during the spring and early summer while channeling the vine's energy into the new trunks during the late-summer and into the fall hardening period.
Anything that sprouts on the above trunk I allow to grow and shape that portion of the vine later in the growing season. At this point I'm interested in vine survival. I usually adjust the number of shoots a couple of times. I determine which shoots are removed by whether the vine is displaying enough health and vigor to allow removal of shoots. If I am removing the shoot as part of training or position of the shoot, then the vine's health becomes very important. I also do summer removal to direct the vine's energy and encourage growth of certain shoots. The objective is to balance the growth and encourage a new framework for the future.
During the growing season position the shoots with the best potential for next years canes. Make sure that they get placed where they get maximum sunlight. This will encourage fruit cluster formation and will aid in obtaining maximum winter hardiness. Follow a good disease prevention program. A healthy vine encourages recovery. In August, I remove some of the basal leaves of the shoots to aid in cane maturation and hardening. Cluster thin varieties that need a balancing of energy. Don't allow over-cropping as this will just weaken the vine and make it more susceptible to winter injury.
Link to Cornell University: Cold Hardiness
Very severe damage means replacement of the entire above-ground portion of the vine or replacement. New vines can be trained from dormant buds that sprout from the soil level. You are starting over. Vines must be trained as from the beginning. Don't expect any production for one to three years. One good thing is that you have a good root system. But is the vine, or variety, worth keeping?
If the winterkill was caused by over-cropping, poor nutrition, poor management, or some other problem that can be fixed, then the vine, or variety, maybe worth re-training. Especially if it was the low temperature of the century that killed the vine. Do it again! But if the vine is totally dead, if the growth coming from sprouts are way too weak, or the variety is just not worth it, then it's time to replant.
Vines can recover from winter injury. It is up to the vineyardist to assess the injury and make pruning and training decisions based on what they find. The overall concept of the trellis system should be kept in mind when tending vines with minor damage. Vine survival must be kept in mind with vines of severe injury. Training and balance must be thought of when making decisions about shoot pruning. Positioning shoots in the sunlight is a must. And when all else fails, replanting decisions must be made.
Ft. Collins Temp
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Neron (a Kuhlmann hybrid) at harvest time in early September, 2003. Hardy, vigorous, productive.