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What Comes After Grape Harvest

After grape harvest ends, there are a handful of key tasks grape growers can do to get the vineyard and equipment into shape before winter.

With the exception of equipment cleaning, the rest of these tasks are on an “as needed” basis and will not be necessary for all growers. These include one final fungicide spray if needed, fall weed management if needed, and fall fertilization if the soil test indicates a need.

Late-Season Disease Management

Scout the vineyard for visible powdery mildew or downy mildew on the leaves. Unless you have visible infections, you are done with fungicide responsibilities for the season and can start winterizing the sprayer (info below).

For those with active powdery mildew or downy mildew, one last fungicide spray to dry up infections will help prevent premature leaf drop and potentially reduce infection next season.

Read Post-Harvest Disease Management of Grapevine Powdery and Downy Mildew for information on what and when to spray.

Fall Weed Management

If possible, take the time to remove mature weeds that have produced seeds, to reduce future weed outbreaks.

Fall herbicide applications are optional and based on the weeds in the vineyard. They may or may not have any impact depending on what weeds are present.

Horseweed/marestail, a winter annual weed to look for in the fall (Photos by Annie Klodd)

Winter annual weeds are weeds that germinate in the fall, form a rosette, go dormant over the winter, and then continue growing in the spring.

Vineyards with lots of winter annual weeds like horseweed (also called marestail), pennycress, and shepherd’s purse may benefit from a fall herbicide application. Include both a preemergent and postemergent in the tank to extend the effectiveness of the application to un-germinated winter annual weed seeds.

Three other scenarios in which fall herbicides could be useful include:

  • Killing perennial grasses in the rows (not always necessary; mature vines compete well with perennial grasses)
  • Killing Canada thistles by spot-spraying with glyphosate
  • Cutting and spot spraying the stumps of woody perennial weeds like sumac, buckthorn, and honeysuckle. Fall is the best time to do this.

If your vineyard does not have many winter annual weeds, perennial grasses in the rows, or woody perennial weeds, then fall herbicides are not very useful because:

  • Annual weeds are already senescing on their own.
  • Annual weeds are often too large and slow-growing at this point to be effectively killed with herbicides
  • Most preemergent herbicides applied now will no longer be active in the spring. Jed Colquhoun, weed scientist at UW-Madison, says that the persistence of preemergents from the fall to spring is unpredictable at best, and that “it’s like paying for the whole dinner and only getting the side dishes.”

If needed, which herbicides to use? An entire article could be written on herbicide options for the fall, and maybe I will do that for next week. For now, please refer to the Midwest Fruit Pest Management Guide, page 198-200 to compare herbicide effectiveness on different weed types and species.

Fertilizing Vineyards in the Fall

Similarly to fall herbicide applications, fall fertilizer applications may or may not have any impact depending on the soil nutrient situation in the vineyard.

If your most recent soil test showed a phosphorus (P), potassium (K), or secondary nutrient deficiency, fall is an appropriate time to apply them. However, you can also wait until the spring and apply them with spring nitrogen.

Soil fertilizer applications are not needed unless your soil test shows a deficiency. That underscores the importance of taking soil tests every 3-5 years.

Click here to see what constitutes a “deficiency” for each nutrient. The table is also copied below. This is based on multi-state UMN Soils research led by Dr. Carl Rosen, soil specialist and is based on MN soils and cold climate grape varieties.

No nitrogen: Unless you are using compost or manure, there is no good reason to apply nitrogen fertilizer in the fall. Grapevines do not take up nitrogen once they go dormant. Unused nitrogen is quick to leach out of the soil. So, nitrogen applied in the fall is going to leach out of the soil with rain and snow, wasting your money and polluting nearby waterways.

Additionally, do not apply post-harvest nitrogen to grapevines that are still growing. This could stimulate new growth at a time when the vines need to be focusing on senescing and moving their carbon and nutrients to the roots for winter storage. Putting on new fall growth interferes with this process and reduces winter hardiness.

Cleaning and Storing Equipment

Sprayer experts recommend winterizing your sprayer and thoroughly cleaning all nozzles and other sprayer parts prior to winter storage.

Jason Deveau and Tom Wolf co-manage a non-profit resource called Sprayers 101. Visit this website to find a plethora of information on sprayer operation, including cleaning and storing.

Read Wolf’s recent article, End of Spraying Season Checklist for instructions on how to winterize and inspect your sprayer. Then dive deeper with Clean Your Nozzles.

Wait to Prune Until Winter

While pruning when the temperatures are above 40 degrees is lovely, pruning must happen when the vines are fully dormant.

Theoretically, one could do some light (long) pruning in December. The problem is that December is too early to predict whether a winter “polar vortex” is coming. Therefore you may end up accidentally pruning off too much and not have enough “insurance” buds left in case of an extreme cold event in January or February. The most I would do, if anything, in December is a very long prune that still leaves each cane at least 6 buds long. Only do that if you think it would save you time or labor. — By Annie Klodd, University of Minnesota Extension Educator – Fruit and Vegetable Production 

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