Oct 27, 2020
Postharvest vineyard management: Preparing vines for winter

Preparing vines for winter begins between harvesting and the first frost. Helping grapes survive the cold period will help timely feeding, treating pests and diseases and it is the best time for vineyard floor management.

Grapes grown in cool-cold climate regions face many environmental challenges. The most prominent are cool and short growing season and cold winters with extremely low temperatures that can negatively impact grapevine health through dormant bud kill, destruction of vascular tissues and in extreme cases, complete vine loss.

Michigan’s cool-climate viticulture regions provide a challenging climate for European (V. vinifera) cultivars. These varieties vary in their tolerance to cold; however, most are considered cold-tender, meaning temperatures between 5 and -8 degrees Fahrenheit are capable of inducing widespread vineyard damage. This, in turn, requires expensive vineyard renewal or replanting. Outside of appropriate vineyard site and cultivar selection, growers must ensure cultural practices applied during summer do not further exacerbate susceptibility of vines to cold injury. Through proper management, vine stress can be minimized through ensuring optimal accumulation and storage of non-structural carbohydrates in buds and woody tissues.

After harvesting, it is time for the last management of the vineyard before the vines reach the dormancy phase. Since each vineyard has a different growing and canopy management status, it should be handled according to its particular requirements, but there are some general guidelines for post-harvest management of the vineyard. The guidelines allow grape growers to prepare the vines for rest, as well as to ensure the reproductive development and shoot growth in the following vegetation season is fine.

Postharvest fertilization

While it might appear like nothing happens to the grapevines after the grapes are harvested, the truth is different. After harvesting, the vines will begin to distribute resources; from the soil, the grapevines will take up nutrients and minerals, and through the mechanism of photosynthesis, they will establish carbohydrate reserves and store them in permanent wood structures—roots and trunks. The post-harvest phase is also one of the most critical times for absorbing nutrients, as these reserves of carbohydrate are used by the vine for respiration during dormancy and for fueling new growth in the following season.

In other words, the vine relies solely on stored carbohydrates from bud burst to flowering, so bud break, bud fruitfulness, early shoot and root growth, flowering and even fruit set are related from the post-harvest period to those stored carbohydrates.

According to research conducted by Bennett et al. in 2005, vegetative growth and yield are directly related to post-harvest management of grapevines. In the following season, adequate post-harvest recovery is therefore crucial for the productivity of high-yielding grapevines.

Nutrients are lost when grapes are taken out of the vineyard and not recycled back into the soil like leaves or canes, so it is really important to replace those minerals. One of the critical times for the absorption of nitrogen is the period between harvest and leaf fall but at the same time nitrogen fertilization in late fall may reduce cold hardiness since it promotes the growth of the grapevine and delay acclimation. Nitrogen fertilization can be applied based on the result of soil analysis to keep the canopy as active as possible.

Post-harvest vineyard fertilization helps vines to get all the necessary nutrients for spring growth and winter dormancy. Fall is the best time to apply compost, right after harvest but before the ground freezes. Compost needs to be incorporated in the soil profile, therefore nutrients and soil microbes will have time to be integrated into the soil before winter and they will be available to the vine in the following spring.

A successful fertilization program requires regular soil and tissue analysis and a record of soil texture and characteristics, crop load, pruning weight and previous fertilization. Active and green leaves are necessary to produce and store. Indeed, more carbohydrates in buds/woody tissues can enhance cold hardiness and minimized the risk of winter cold damage. Thus, keep the canopy as much as possible. Fertilizer should be applied closer to harvest in cooler climates where the post-harvest period is much shorter than in warm and hot climates, whereas fertilizer application can be applied in early autumn.

Depending on the circumstances of the individual vineyard, fertilizer can also be applied in installments or as a foliar application. In addition to nitrogen (N), phosphorus (P) and potassium (K), depending on the result of soil and tissue analysis, calcium and zinc can also be included in the fertilizer.

After grape harvesting, it is also a good time to carry out soil analysis but not tissue analysis, as this gives grape growers time to prepare a good fertilization plan for the coming season as well as to provide a proper amount of nutrients to the soil. Sufficient soil moisture should be available for the vines to obtain all the required nutrients and minerals from the soil, so post-harvest irrigation, depending on the climate, weather and soil texture, may be needed. Although irrigation may not be important in cooler climates as the rain does the job, post-harvest irrigation in mild to hot climates is always essential.

Postharvest disease management

After harvest, protecting vines from trunk diseases and crown gall should be the major focus. Other items like clearing the vineyard floor may also reduce black rot and Phomopsis by removing plant debris. Crown gall (caused by the bacterial pathogen Agrobacterium vitis) is a widespread and devastating disease, particularly in cool-climate regions in the world. Unfortunately, there are no synthetic chemical treatments for controlling crown gall of grape. A protective practice that can be used to reduce winter injury is “hilling-up”, which is mounding soil over the graft union in the fall to insulate it from exposure to low temperatures.

Soil needs to be carefully removed (so not to cause mechanical damage) in spring once all pruning-related activities have ended to avoid scion rooting. Plowing snow from inter rows can offer similar protective benefits. However, unlike soil, snow is unpredictable in its occurrence and duration. Fertilization with K2O in place of nitrogen fertilizers also improves vine resistance to cold. For more information about managing crown gall in Michigan, please see this Michigan Grape Facts sheet on “Managing Grapevine Crown Gall.”

Postharvest insect pest management

After harvest, there are few economic insects of concern in Michigan. Sanitation of dropped berries and prunings later in the winter can help keep pest levels down with additional benefits for disease control.

Postharvest vineyard floor management

Fall is the best time to control perennial weeds and apply residual herbicides to vineyard floors. After harvesting, the most important step is to perform thorough scouting of your vineyard and prepare the list of weeds that are problematic in your vineyard.

Fall is a good time to take action for controlling woody perennials such as poison ivy, Virginia creeper, wild grape and tree seedlings (poplar, maple). At this time, perennial plants translocate carbohydrates towards underground plant parts such as crowns, rhizomes and flashy root to reserve food for starting growth during the following season. The application of systemic products such as glyphosate is appropriate as it will translocate to kill below-ground parts and inhibit growth during the upcoming season. For effective results, these perennials should be treated before leaves senesce. However, extreme care should be made at the time of application so that glyphosate does not come in contact with grapevines. Glyphosate absorbed by leaves and bark moves within the vines and can show significant injury symptoms during the following season. In most cases, these woody perennials may need to be removed manually from the grapevines.

At this time, the risk of grapevines injury due to herbicide exposure is less as vines are shedding leaves and not growing actively. Fall is the best time for applying preemergence herbicides. Fall applications of preemergence herbicides should be made before the soil freezes. Preemergence herbicides such as Alion (indazaflam), Princep (simazine), Solicam (norflurazon), Casoron (dichlobenil), Kerb (pronamide), Goal Tender (oxyfluorfen), Prowl H2O (pendimethalin), Chateau (flumioxazin) and Matrix (rimsulfuron) may be applied in fall. It is also advisable to mow or use a hedge trimmer to remove the large or dead vegetation under the vines before applying any preemergence herbicides because this will help the residual herbicide penetrate into the soil that is essential for herbicide activation and control of emerging weed seedlings. “Weed management in vineyards starts with fall herbicide application” from Michigan State University Extension provides more information on fall weed management in vineyards.

Conclusion

Vineyards could be quickly neglected after harvest and focus solely on the vinification process. At the end of the growing season, an insufficient amount of fertilizer and water will result in a low level of stored carbohydrates, a lack of macro-nutrients, and, consequently, a maximum risk of winter cold damage. This can lead to uneven bud break, poor and non-uniform shoot growth, and in extreme cases, complete vine loss. Once the chlorophyll in the leaves begins to break down, the leaves change their color and lost their function to produce carbohydrates. Therefore, with effective pests and disease control, be sure to keep the canopy green and healthy as long as possible. That way, before the leaf drops, the vines will be able to produce adequate carbohydrate reserves. Effective post-harvest vineyard management is crucial in preparing the vineyards for rest and fruitful growth in the spring.

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