Farming is all about timing. Some of the things we need to do are time-flexible like pruning, while others are time-sensitive like applying a spray for powdery or downy mildew. Missing a spray to control mildew can leave you with a problem for the rest of the season. Monitoring the nutrient status in vines is also a “time-sensitive” activity.
Bloom And Veraison Petiole Sampling
The best way to find out what fertilizers you might need to apply is to analyze “petiole” (leaf stem) samples at bloom time and again at veraison (berry softening). Bloom time is important because insufficient levels of micronutrients at bloom can have a season-long effect on fruit quality. Veraison is important since at this time there is a major change occurring within the vine (the beginning of fruit ripening). This involves the uptake and remobilization of macronutrients (potassium in particular).
For vinifera (European/California) varieties, research work has been completed on both bloom time and veraison vine growth stages to establish desirable levels for individual nutrients. For the labrusca and hybrid varieties, critical nutrient levels are established for veraison with research underway for desirable levels at bloom time.
When sampling at bloom time and veraison, it is important to use a consistent approach each year.
• Vineyard Location(s): If a vineyard is uniform in growth, you can select one location. If one area has consistently better growth than another, you may select two areas to sample. For each area being sampled, select specific rows that are representative and use the same rows each season.
• Timing and Petiole Position: For the bloom time sample, the vines would ideally be at 70% to 80% bloom (50% to 100% bloom is OK, too). If you are ahead of or behind this timing, it should be noted for each sample. At bloom, the petioles sampled should be the ones opposite the first grape cluster. For a veraison sample, the sample should be obtained at the beginning of berry softening with the petiole obtained from a mid-cane position. A total of 70 to 80 petioles are sufficient for each sampled area. When taking a sample, snap off the petiole and blade from the cane, then discard the leaf blade and save the petiole.
• Varieties and Rootstocks: Don’t mix varieties. While critical levels of some nutrients are the same across varieties, there can be significant differences. Research during the last 10 years has also identified significant differences in nutrient uptake among rootstocks. Therefore, don’t mix rootstock samples.
• Sample Handling: Petiole samples should be placed in a paper bag and labeled with the location, row number, growth stage, and date. The samples should be delivered or shipped to an agricultural laboratory. If there is going to be a delay in delivery, the paper bags should be left open to air-dry. Do not use plastic bags since this can result in molds growing on the green tissue.
Interpretation Of Results
There are a number of laboratories across the U.S. that can perform an analysis of your samples. The Extension service (farm adviser) offices in each state typically maintain a listing of agricultural laboratories. You should not use an environmental laboratory since their fees are higher than ag labs.
You will want to run a “complete” analysis which will include the macronutrients (nitrate-nitrogen, total nitrogen, total phosphorus, potassium, calcium, and magnesium) plus the micronutrients (sodium, boron, zinc, manganese, iron, and copper). Other micronutrients might be included depending on the area.
When you submit your sample, tell the lab that the analysis is for grapes, the type of grape (vinifera, labrusca, or hybrid), the variety, and the growth stage (bloom or veraison). Based on these sampling parameters, the lab should print the critical levels for each nutrient across the bottom of the report. It is important to ask the lab if the critical levels that they include in your report are based on university research or their personal experience. Unfortunately, sometimes labs develop their own set of critical or desirable levels for individual nutrients and do not indicate the basis of these numbers. It is not that the values that a lab uses are not appropriate, but you should be aware of their source. It is also a good idea to use an “independent” lab, not one associated with an ag chemical dealer.
Survey Sampling Versus Problem Diagnosis
A good vine nutrition monitoring program would include “survey” sampling at bloom time and veraison every year. In addition, there may be noticeable differences in vine growth or leaf symptoms within a vineyard that appear to be due to a nutrient deficiency or excess. Obtaining “symptom vs. non-symptom” or “good vs. poor” growth samples can frequently let you know if the problem is nutritional or not.