Frequently Asked Questions for Pruning Grapevines 

By: Annie Klodd, University of Minnesota Extension

  Pruning season is almost upon us, this article references common grapevine terminology and requires the reader to have basic knowledge of grapevine pruning. If you are new to pruning, take some time to familiarize yourself with grapevine structure and the basics of pruning.

Does it matter when I prune during the dormant season?

  If you would like to do some light pruning before the first big snowfall, feel free. If you prefer to wait for sub-zero temperatures to prove something to yourself, that is fine too! Just wait until the vines are fully dormant and the leaves have fallen before you start pruning.

  Pruning in the coldest months has an upside. It minimizes the risk of diseases infecting the pruning cuts. When temperatures dip below about 35 degrees F, fungal diseases of grapes are not actively spreading. If you prune early, it is best to do a “long prune.” Leave extra length on each cane in case a severe cold event causes bud damage later on. Additionally, leave a couple of centimeters of wood past the last bud – cold, dry wind can desiccate the wood at the pruning wound.

  Late pruning in March and April is certainly more comfortable than deep winter pruning. However it also means you are pruning when fungal pathogens are more active. Be aware that fungal trunk disease pathogens are more active during the spring and will readily enter the wood via those pruning wounds.

How do you decide how many buds per spur to leave during pruning?

  The general rule of thumb is to cut each cane down to a 2-3 bud spur. This assumes that all buds are healthy and that the cordon contains one spur every 3-5 inches. If this ideal scenario does not exist on a vine, you can either alter the number of buds per spur or the number of spurs per cordon to account for imperfections.

  In cold climates, winter damage usually kills some percentage of the buds on a vine. It is helpful to estimate the percentage of dead buds and adjust your pruning to make up for bud loss.

  To measure bud mortality, take a representative sample of canes throughout the vineyard and dissect their buds with a razor blade. The color of the bud’s interior indicates whether they are alive or dead. Buds that are green inside are healthy and will grow into shoots. Buds with brown interiors have died.

  Remove between 20-50 normal canes from throughout the vineyard. Bring them inside to room temperature for 24 hours. With a sharp, slim, and clean razor blade, carefully slice off the tip of each bud in the first and second positions along each cane. The bud contains 3 parts – the primary, secondary, and tertiary sections. The primary is the middle and largest section, and it produces the most fruitful shoots. Record whether the primary section of each bud is green or brown (alive or dead), and repeat this with 100 buds. If 10-15% of primary buds are dead, do not adjust your pruning. If 20-40% are dead, leave about 25% more buds than you typically would. If 40-60% dead, double the number of buds you keep. If more than 60% are dead, do minimal pruning, leaving 5 buds on each spur.

  Very long spurs are cumbersome – the longer the spur you leave, the higher the chance that only the buds at the very top will break. This is due to a concept called apical dominance. Excessively long spurs also creep up out of the regular fruiting zone, interfering with the structure of the vine. To avoid spur creep while still leaving extra buds, you may instead leave a higher number of 2-3 bud spurs.

Should basal nodes or “non count buds” be accounted for during pruning?

  In cold climate hybrid grape growing, yes. In Vinifera vineyards, no.

  A basal bud is the bud at the base of the new spur wood. In other words, it is located at the point where the 1-year old part of the spur meets the 2-year old wood.

  When it comes to cold hardy hybrid grapes like Marquette and Frontenac, the basal buds are usually fruitful. In fact, they can sometimes be the most fruitful bud on the whole spur. Most of them will carry two cluster per shoot. But this is not the case on vinifera and French-American hybrids, where the basal buds are just vegetative. While the traditional recommendation, which arose from Vinifera vineyards, is to not count the basal bud during pruning, this recommendation is revised for cold climate hybrids where they should be counted. If you leave a basal bud plus two more buds, you will have up to 3 shoots per spur.

Some of my vines are getting old, and I notice that parts of their cordons are missing spurs and canes. What should I do?

  We call these empty spots along the cordon “blind wood.” Blind wood happens when old spurs die, and no new buds form from the cordon to replace them. One thing that causes blind wood is winter injury, so it is common in cold regions. Winter injury accumulates over time, so older cordons tend to have more blind wood than newer cordons. In cold climate grape growing, we recommend replacing cordons once they start showing blind wood.

  To replace a cordon, first find a healthy new cane that is growing from the base of the cordon or the middle of the vine along the wire. Lay the new cane down alongside the existing cordon, and tie it to the wire. Clip off the end of the cane where the wood is very skinny so that only the healthiest wood remains. If the old cane is totally unproductive, remove it at this time. If it is still producing some fruit, you have the option of leaving it in place and growing the new cordon alongside it, as long as the vine is vigorous enough to support both.

What if the whole vine has died back to the ground level?

  Extreme dips in winter temperatures sometimes kill the entire aboveground part of the grapevine, including the cordons and trunk. This is even more likely if the vines are stressed going into the winter, either from drought or wet feet. Rest assured that even if the trunk and cordons are dead, the roots are usually still alive and can re-grow a new vine.

  If you grow “own-rooted” (non-grafted) vines like University of Minnesota cold hardy hybrids, you can re-grow the vines from suckers rather than planting new vines. Start by cutting the dead or dying trunk back to the ground level. If suckers are present, choose 1-2 of them to become the new trunks.

If the vine was particularly vigorous before it died, you may want to keep extra suckers so that the excess energy from the roots has somewhere to go. The extras can be removed during or at the end of the growing season.

  After selecting the suckers that will become your new trunks, prune them back to the point where the wood becomes thicker than pencil diameter. The reason we do this is because the thinnest wood is the least productive and has a high chance of dead buds. Keeping only the healthiest wood helps those vines produce vigorous new trunks and cordons. There is no need to cut the cane back to a 2-bud spur. For example, if the first 4 feet of the cane are healthy and thicker than a pencil, then make your cut 4 feet off the ground.

Should I prune out small pieces of dead wood like old spurs?

  If time permits, pruning out dead spurs is a good idea. They can harbor spores of diseases like powdery mildew and phomopsis that re-infect the vines in the coming season. Pruning out dead wood is one good non-chemical disease management tool.

What is the liquid coming out of the pruning cuts when I prune in the spring?

  That liquid is sap! This is a sign that the vines are exiting dormancy. Sap runs through the vines as the soil warms, so that the buds can start actively growing. It is time to wrap up the pruning as the vines “wake up.”

What is a bull cane?

  A bull cane is an exceptionally thick, long cane with very wide spaces between buds. They grow more aggressively than regular canes, often growing into the next vine and beyond. Rather than being round, these canes take on a subtle oval shape. They tend to be less winter hardy and less fruitful than normal canes, so they should be removed. If possible, do not use bull canes to establish new trunks. Bull canes tend to grow if the grapevine is too vigorous, such as when a vigorous variety is grown on rich, moist soil.

How do you remove tendrils from the wires?

  My best advice for this is: Only have as many wires as you need to trellis the vines. More wire means more tendril magnets. For example, on a Single High Cordon trellis system only one wire is necessary. Do not string any other wires lower down.

  For a deeper dive into these Frequently Asked Questions, watch our recorded webinar from the Cold Climate Fruit Webinar Series:

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