Preharvest thoughts for Vermont vineyards

By Terence Bradshaw

Pardon my absence these past few week- a trip to beautiful Istanbul, where I presented some of our research from the past couple of years on cider apple management, and both wrapping up and starting a new course have kept me a bit distracted. Grapes in Vermont vineyards are some of the best quality I’ve seen in recent years, as that dry summer facilitated low disease pressure and fabulous growing conditions. Veraison has come and gone, hopefully all who need it have bird nets up and are now starting to arrange the logistics of moving that fruit into ready fermenters (or customers’ baskets, for the table grape growers out there).

As harvest approaches, it is critical to track ripening in the vineyard to best achieve the juice chemistry values to optimize wine quality. Sampling should be methodical and regular (at least weekly, or more often as harvest approaches). Generally a 100 berry sample is sufficient to ascertain general ripeness. Berries should be randomly selected, but should come from a diverse distribution in the vineyard: from all parts (top, shoulder, bottom) of the cluster, and from clusters well-distributed through the canopy. Fruit can be collected into a plastic bag, crushed lightly, then a corner of the bag cut off to squeeze juice into a sampling jar. Juice should then be assessed for pH, sugars, and titratable acidity (TA). Remember that for most popular cold climate grapes, TA is a primary determinant for ripeness; for reds (Frontenac, Marquette), a target TA of 1.5% or lower is preferred; for whites, 1.2% should be considered the upper end, although La Crescent may frequently have higher values. Ideally, all grapes for winemaking should have TA below 1%, but that is not always possible for the cultivars that we grow. Work with any wineries you plan to sell grapes to to determine their preferred juice chemistry levels before harvest.

Dr. Imed Dami at Ohio State has a good fact sheet on evaluating grape ripeness at: http://ohioline.osu.edu/factsheet/HYG-1436

Sugar level and pH are easily evaluated with simple tools (a refractometer and pH meter, respectively) available from most winemaking supply outlets. TA test kits are typically available from the same vendors. A procedure for measure TA through titration is available at: http://www.extension.iastate.edu/wine/titratable-acidity

It’s not too late to perform petiole sampling, which is the best method for assessing nutrient status in plants. I’m copying my instructions below from a previous post, as that information is still relevant:

Petiole sampling
Growers may ascertain overall nutritional status in their vineyards at two times through petiole analysis: bloom and 70-100 days post-bloom. Now is the time for late (post-veraison/pre harvest) petiole sampling for plant nutrient status. Dr. Joe Fiola form the University of Maryland has posted some good recommendations in his latest Timely Viticulture newsletter:

· Grape petiole analysis is recommended along with soil samples and visual observations as part of a complete nutrient management program.

· A three year cycle of sampling all of the varieties in a vineyard is typically recommended.

· Tissue/petiole analyses reveal the actual nutrients in the vines.

· Spring tissue sampling is a good time to sample, as you can make nutrient adjustments to the vineyard that will influence this year’s crop quality.

· Nitrogen status is best evaluated with tissue sampling not soil sampling.

Some specifics on sampling:

· Each sample should be less than 5 acres; less if there are major changes in soil or topography

· Sample different varieties separately. Samples should represent plants that are planted on the same soil type and are of the same age, variety and rootstock.

· Vines should represent that portion of a block that is maintained under the same cultural practices, i.e. fertilizer, irrigation and vigor control practices. For example, irrigation blocks are not to be combined with non-irrigated blocks even if they are on the same soil type.

· Do not sample vines on the border of the block or near dusty roads.

· For the late-season sampling period, sample the petiole of the most recent- FULLY EXPANDED leaf (NOT the one across from the first blossom cluster as during bloom).

· About 75-100 (depending on size) petioles are needed as they are typically smaller at the end of the shoot.

· Gently wash petioles with water and gentle detergent, pat dry and place in OPEN paper bag (lunch, #6 size) to dry for a few days.

· The closest analytical lab for grape petiole analysis is Dairy One, which is affiliated with the Cornell Nutrient Analysis Lab. It is recommended that you contact them before you send any samples to confirm that recommendations will be sent along with the analysis and to confirm costs.
Video about petiole sampling: https://www.youtube.com/watch?v=8EHbojLfXek

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