Horticultural Extension Bulletin

Pruning Grapevines in Michigan

By:  Gordon S. Howell & R. Keith Striegler
       MSU Department of Horticulture

Pruning is the most important cultural practice in the management of grapevines.   It is done to select fruiting wood, maintain vine shape and form, and regulate the number of buds retained per vine.

During the fall, three important physiological processes occur in grapevines.  The vine must mature its fruit crop, mature its wood to allow overwintering, and store carbohydrates to allow flower differentiation and shoot growth during the following spring.  Each of these vine processes is critical to successful grape culture in Michigan.  In some grape-producing areas of the United States, there is a long period after harvest during which vine physiology can shift from fruit production to the other two processes mentioned above.  Because this is not the case in Michigan, correct pruning is especially critical.

Pruning must be done each dormant season and it directly influences yield, fruit quality, vine vigor and hardiness.   Proper pruning will result in maximum yields of high quality fruit without a reduction in hardiness during the following winter.  Improper pruning will have a detrimental effect on each of these important vine characteristics.  In economic terms, the ability to prune correctly can make the difference between a profit and a loss.

Before you can prune grapevines correctly, you need to understand the fruiting habit of grapevines (Fig. 1).  Current season vegetative growth arises from a bud located at a node.  The vegetative growth continues as a shoot, and in the autumn, shoots mature, become woody, lose their leaves and are termed canes.  Nodes on these canes contain the cluster primordia (developing flowers) from which next season's crop will develop.  Wood that has grown for two years or longer is not fruitful.  This wood can be distinguished from fruitful (one-year-old) canes by the presence of loose, shedding bark.  One-year-old canes do not display this bark.   Grapes are produced as berries on a cluster rachis (stem), and clusters are bone near the base of shoots during the current season.

Pruning Decisions

Method of Pruning
An average grapevine will have 200 to 300 buds on mature canes capable of producing fruit.  If the vine were allowed to produce shoots and fruit from each of these, the result would be a large crop that would not ripen.  The vine's effort to mature the crop would reduce cane maturity, reduce the productivity of buds the following season and decrease the size of the root system.

To avoid this situation, a considerable amount of research was done in Michigan by Partridge and in New York by Shaulis to develop a method of pruning that would give maximum yields of acceptable quality without reducing cane maturity, vine vigor or the following season's bud fruitfulness.  The procedure they developed is called balanced pruning.  Balanced pruning allows the pruner to balance the fruit productivity of the vine with vegetative productivity as cane growth and maturation.  The term used to describe growth and maturation of canes is vine size, which is measured as the weight of cane prunings.

To balance prune a grapevine, estimate vine size and then prune the vine, leaving enough extra buds to provide a margin of error.  Then weigh the one-year-old cane prunings using a small spring scale and a piece of string.   This weight is applied to a pruning formula (Table 1) developed for each cultivar to determine the number of nodes to retain per vine.

For the cultivar 'Concord,' the weight of one-year-old cane prunings is applied to the 30 + 10 pruning formula, with the result that 30 buds are retained for the initial pound of cane prunings, and 10 buds for each additional pound (Table 2).  (Commercially, weighing each vine in a vineyard is not practical.  In practice, the pruner needs to weigh individual vines only until he/she can accurately estimate vine size.  Weighing then becomes necessary only periodically to check the accuracy of the pruner's estimate of vine size.)

Because of the increasingly high cost and scarcity of experienced pruners, efforts have been underway since the late 1960s to mechanize grapevine pruning in the eastern United States.  To date, these efforts have proved unacceptable because commercial mechanize pruning devices lack selectivity.  Consequently, the completely mechanized approach requires much further research.  At present, mechanical trimmings of grapevines followed by hand cane selection and adjustment of bud number  has produced yields and fruits quality comparable to those obtained from vines that were balance pruned to a 30 + 10 formula.  This system of pruning requires vines that are uniform in vine size, cordontrained and shoot-positioned.  The economics of this mixed system of pruning are undetermined.

Selection of Fruiting Cane
The selection of canes with the most fruitful buds is critical to the success of balanced pruning.  The most important factors that influence the fruitfulness of grape buds are exposure of the leaf at that node to sunlight during the previous growing season, cane diameter and the presence of persistent laterals.  Canes on grapevines vary considerably.  Some characteristics of canes have been associated with a higher yield per bud (fruitfulness).  Within a given cultivar, darker colored canes indicate good light exposure during the previous growing season and greater bud fruitfulness.   Buds from these canes will also be more winter hardy.  Small diameter--less than 1/4 inch--canes should not be kept because of their inferior productivity and bud hardiness.  Large diameter is not a serious matter in Concord as long as the internode length does not exceed 4 to 5 inches and produce "bull canes."   Bull canes are poorly exposed, highly vigorous shoots with low hardiness and low productivity. 

The relationship between large cane diameter and bud fruitfulness and hardiness has not been precisely determined for French hybrid and Vitis vinifera cultivars.  Large diameter canes frequently have persistent laterals.  Persistent laterals are side branches on a shoot that mature as shoots and become canes.  The retention of persistent laterals is desirable for all V. labruscana cultivars.  The bud on the lateral is more fruitful than the bud at the node where the persistent lateral has developed.  Do not select canes possessing persistent laterals for tender cultivars, however, because the canes are less cold resistant than medium diameter canes that have no persistent laterals.

When to Prune
Grapevines may be pruned anytime during the dormant season.  Pruned vines are more easily damaged by a cold stress episode than unpruned vines, however, and once vines are pruned, bud number cannot be modified to compensate for losses due to winter cold damage.

Therefore, it is desirable to delay pruning until late winter or spring because delay will give some protection in both cases.   Some growers with large acreages must begin pruning earlier than this to finish before spring bud burst.  In some cases, the hardest cultivars should be pruned first.  Prune hardy cultivars on superior sites before those on poorer sites.   Finish pruning and tying before spring bud swell to avoid considerable mechanical bud damage.

Pruning to Avoid Cold Damage

Pruning Tender Grape Varieties
Bud losses can be expected on tender cultivars every winter in the northeastern United States.  If tender vines are pruned during the dormant season, modify the process of balanced pruning in anticipation of bud mortality.  Prune vines lightly and retain approximately three times as many nodes as the balanced pruning formula indicates.  When the shoots have grown approximately 2 inches in early June, adjust the shoot number, if excessive, to the number called for by balanced pruning, by either cane or shoot removal.

Long Pruning to Reduce Spring Freeze Damage
Spring freeze damage is a significant economic problem for Michigan grape growers.  A pruning technique--long pruning--was developed to provide an inexpensive method of reducing spring freeze damage.  This procedure utilizes the apical dominance of buds on the cane.  That is, the first buds on a cane to begin to grow are those on the tip of the cane.  Buds closer to the base of the cane begin growth later than buds at the tip.

In practice, pruning and cane selection are handled as in normal pruning.  Canes are retained with 10 to 15 more nodes per cane than needed for the training system.  After the date of the last probable spring freeze (May 15-20 in Southwest Michigan), excess buds on fruiting canes are removed and the number of buds per cane for that training system is set (Fig.2).   Bud growth in the spring may be delayed by as much as 14 days with this procedure.   The amount of delay is greatly influenced by the maximum temperature.   Consistent daily maximums above 80 degrees F (27 C) will override the effect, and temperatures approaching this will reduce it.

Fig 2.  Pruning procedure to reduce spring freeze hazard to grapevines trained to the Geneva double Curtain (GDC) or the Hudson River Umbrella (HRU) systems. (A) The number of nodes per cane set at pruning time. (B) Retain all existing nodes on canes selected for fruiting until the last spring frost date (about May 15-20 in Michigan). Set bud numbers after that date.

Pruning and Disease Control

Eutypa Dieback Disease
Deadarm disease has been identified as a serious problem of grape culture since early in this century.  Recent work in Australia, California and Michigan has shown that this disease is actually two distinct diseases, each caused by a different fungus.  These diseases have been named Phomopis cane and leaf spot and Eutypa dieback, caused by the fungus Eutypa armeniacae. Infection by this pathogen arises from spores produced on infected trunks and arms.  The primary point of infection appears to be pruning wounds.  Under Michigan conditions, the greatest number of spores are released by the pathogen during February, March, April, September, October and November.  Spore release primarily occurs after rainfall and can occur at temperatures just above freezing.  The potential for infection is present during most of the dormant season in Michigan.

Research is being conducted on methods of chemically controlling this disease.  A this time, the results are inconclusive.  Contact your county Extension Agent for the latest recommendations.

Growers can reduce crop losses due to Eutypa dieback by using the double trunk system of training.  Remove any infected trunk by cutting it off as close to the ground as possible and disinfecting the cut surface.  Dispose of the infected trunk by burying or burning it.  The full number of buds can be left on the remaining trunk until the second trunk is re-established from a sucker.

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Created: September 25, 1998