Grapevine Post-Harvest Nutrition Crucial to Next Year’s Crop

Heavy rains and cloudy days early in the growing season may have delayed maturation of grape crops in some areas of the Midwest, but hot conditions in August triggered heavy crops in time for a somewhat delayed harvest. Stressed grapevines will need a nutritional recharge before the leaves yield to the change of seasons. 

It’s always nice to finish the harvest, but the work is not yet done. We used to   harvest and then forget about it. But we’ve learned that you must prepare those vines for the coming winter and spring. If you put them to bed healthy in the fall and winter, you will have a good start in the spring,” says Ernie Betker.

Betker’s 11-acre Trout Brook Vineyard is near Hudson, Wisconsin. He grows four reds: Frontenac Red and Marquette from the University of Minnesota varieties, and Sabrevois and St. Croix from the Elmer Swenson varieties. In the whites he grows Prairie Star and Frontenac Gris. Many of the grapes go to the Northern Vineyards winery in Stillwater, Minnesota, or to wineries nearby.

“We began harvest in mid-September, with a heavy fruit load. We have seen some drought stress because we didn’t have any rain from the end of July to well into September,” Betker says. “The sugars were moving nicely, though, pushed along by several applications of KDL®. But it was so dry there was no microbial activity in the top 6 to 8 inches of soil. So fall moisture and post-harvest nutrition will be very important.”

Post-harvest nutrition needs have been documented by university and grower research from South Africa to Australia, and New York to the upper Midwest and on out West. As grapes enter dormancy, the maximum crop potential for the next growing season has already been determined, notes a Cornell University Extension horticulture bulletin on bud fruitfulness and yield. (See: http://grapesandwine.cals.cornell.edu/appellation-cornell/issue-10/grapes-101-bud-fruitfulness.cfm)

In a November 2012 factsheet about research for Australia’s Grape and Wine Research and Development Corporation, viticulturists at Charles Sturt University and the Department of Primary Industries note that when leaves remain in reasonable health, with adequate water and nutrients, “continued photosynthesis and nutrient uptake during the post-harvest period allow vines to store carbohydrate and nutrient reserves for use in the next season.”

“Grapevines require a supply of carbohydrates from stored reserves to support new root and shoot growth in spring,” the Australian researchers note. “These carbohydrates are stored as starch, and in a free-sugar fraction consisting mostly of glucose, fructose and sucrose…Starch is stored as granules within the xylem and phloem ray cells, and areas of high starch concentrations can be seen at the whole-tissue level. Starch concentrations in the wood are higher than those of sugars during the growing season, but leading into dormancy some of the starch is converted to sugars to help protect the vine from cold damage during winter. Starch is the main storage carbohydrate in the roots at all growth stages.”

Betker says that in-season tissue analysis determines his foliar post-harvest applications of nutrients to help the vines build and store the carbohydrates and sugars. He works with Larry Shafer, vice president and crop nutritionist at Agro-K Corporation, to interpret and act on test results.

“As soon as we pick we will apply a post-harvest foliar feed of 0-18-25, with zinc and magnesium in a phosphite formulation,” says Betker. “That prepares the vines for next year’s production by giving them a little phosphorus, a good boost of potassium and further zinc along with magnesium. Then even if the leaves are all gone will give the vines 2 quarts per acre of potassium, but in the phosphite form, along with copper. We’ve been applying this combination the past couple of years to help the vines harden off for winter.

“The quality and quantity of our production has increased significantly since we’ve been doing this. I’ve also seen improvement in the condition of the wood in spring when I go to prune. The past couple of years we’ve had good wood to work with in the spring when pruning. The copper and the phosphite help bring that about.”

Betker says his last fall step before he relaxes for the year is a root feeding soil application with BioMax™ micronutrients and 3-18-18. Biomax is a microbial activator containing microbial enzymes plus crucial micronutrients calcium, cobalt and zinc. When incorporated into the soil, the result is a chain reaction of microbial activity and increased nutrient availability to the vine roots. Betker follows that with 5 to 6 ounces of granular potash per vine due to the region’s shortage of potassium in the soil. He repeats the first part of this root feeding again in the spring.

Near Raymond, Nebraska, the fall regimen followed by Eric Nelson at Oak Creek Vineyards, which produces Edelweiss, La Crosse, Dechaunac, and Brianna on six acres, is also guided by tissue sampling at veraison.

“We finished harvest in mid-September and began our spray program, which will consist of probably three sequential applications because you can only expect the vines to absorb so much at one time,” says Nelson. “We had a huge harvest, so our vines are really stressed and we need to ramp up the nutrients going into winter.

“Tissue samples at veraison tell us the deficiencies in nutrients we need to correct for next year, and this year we have a lot of those to address. But I’ve been really pleased with the health of the vines this year. We actually saw some good positive movement on micronutrients when we did the blossom samples, definitely higher than what we experienced previously, which I attribute mostly to the post-harvest sprays in 2012.”

He says all the vineyards in his region had a good harvest but as usual his yield numbers seem higher than the nearby vineyards. This, he says, is encouraging reinforcement that a complete and balanced nutrition program makes sense.

“Magnesium was one deficiency, but we also had some iron and zinc problems,” he says. “We’ve been deficient in boron, too, and that’s a tough micro to manage. But the boron numbers finally are beginning to look good. Actually we now have a block or two where we probably should back off a little on boron.”

Nelson says growers have to look at balancing nutrition in a methodical, scientific way, year after year because needs will differ from year to year and from one vineyard to another.

“You have to develop a game plan. So during the winter we will once again review everything with Larry, confirm what we know based on leaf samples, and work out the applications and schedule needed,” he says. “Other growers want to know what I’m doing, and some say they’ll just copy that, without understanding how and why it works. Every vineyard is totally different. Because I have excess or deficiency of something does not mean that it will be same for you. The program has to be tailored.”

He says this approach has helped him get off the good year/bad year/good year/bad year treadmill many vineyards experience.

“If the vines are not healthy nutrition-wise, you will fall into that cycle where you have a good year but then the vines are so stressed that the next year will be a disappointment. For us this year was better than last year and last year was better than the year before. That’s what every grower is seeking, and vine nutrition is the key. Of course, at some point any vine can only yield so much. I have one variety that may never again yield what it did this year. And sometimes when the yield gets too high, the quality of the grapes can decline quite a bit. You’ve just exceeded that vine’s capabilities. But with the right nutritional balance, you can also achieve an optimal balance of yield and quality.”

Extensive grapevine research underscores the importance of giving the vines potassium during the 30 days after harvest to help support the next year’s peak spring requirements, and the foliar applications Nelson and Betker supply are helping their vines store adequate potassium.

The Australian research also notes that crop load can provide a general indication about the need for post-harvest replenishment of carbohydrates. The Australians found that high yield correlates with higher need for post-harvest nutrient reserve replenishment. Continued photosynthesis is important during the post-harvest period, and along with that comes the need to monitor vines for infections of powdery or downy mildew, “which can considerably disrupt and reduce carbohydrate export from the leaves.”

Here is further information from the Australian fact sheet regarding the mechanisms within the vines for storage and transport of nutrients:

“No more than half of the total stored carbohydrates would normally be used in spring, but more than 80% can potentially still be mobilized. Fine root growth flushes that occur in early spring are most likely supplied from carbohydrates stored in the permanent roots. Localized carbohydrate reserves, and possibly sugars that diffuse from the bleeding sap, are used to support initial shoot development. As phloem function of the canes and older wood resumes in the weeks after budbreak, reserves from more distant parts of the vine are progressively utilized. Transport of reserve carbohydrates to the shoot reaches a maximum at the 8- to 10-leaf stage, and then declines as canopy photosynthesis becomes sufficient to meet carbohydrate demand. Newly assimilated carbon then starts to be exported back to the perennial parts of the vine.”

“The high initial dependence on stored carbohydrates results in a rapid decline of carbohydrate concentrations in root and wood tissue through budbreak and early spring. As the canopy becomes self-sufficient and begins producing surplus carbohydrates, replenishment of reserves will begin in the week or two leading up to flowering. The restoration of reserves may continue in parallel with fruit ripening if yields are sufficiently low, or photosynthesis rates high enough to produce excess carbohydrates for storage. However, under high crop loads, or conditions such as water stress or high temperatures that reduce photosynthesis, reserve replenishment may be delayed until after harvest. With heavy crop loads, renewed mobilization may also occur in the earlier part of the ripening period to aid berry ripening.”

“As early nutrient demand in spring cannot be met by root uptake alone, storage of nutrient reserves prior to dormancy is also required to support shoot growth in the following season. The cycling of nutrients, and thus the vine’s ability to store them and remobilize them again in spring, depends on nutrient mobility within the plant. This is high for the macronutrients nitrogen (N), phosphorus (P), potassium (K), sulphur (S), and magnesium (Mg), but low for calcium (Ca). With the exception of manganese (Mn), which also has low mobility, the micronutrients iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo) and boron (B) have intermediate mobility. Studies with whole mature vines have shown that about 50% of N and P in the new season’s canopy comes from stored reserves. Around 15% of the demand for K is met by reserves, and 5% of that for Mg and Ca.”

“Studies that have targeted carbohydrate reserves … indicate that shoot elongation rates in spring can be slowed by up to 40%, and inflorescence flower numbers reduced by 25%, if carbohydrate reserves are deliberately reduced through defoliation in the previous seasons. Growth responses to smaller fluctuations in reserve concentrations may be less obvious, but reductions in shoot diameter may be seen before reductions in shoot length. High carbohydrate and nutrient reserves will generally encourage strong spring shoot growth and promote reproductive development. …if the aim is to promote strong early canopy development, or to enhance the productivity of run-down vines, then maximizing the storage of carbohydrate and nutrient reserves will assist with this process.”

This article was contributed by the Agro-K Corporation in Minneapolis.  Agro-K has been a pioneer in foliar nutrition and soil conditioning systems for over 35 years.