Win Cowgill, Area Fruit Agent, Rutgers Cooperative Extension
Foliar applications of Urea nitrogen have been shown to aid fruit size in sweet cherry and increase cold hardiness.
Dr Greg Lang, Michigan State University reported on this work both at the International Fruit Meeting in California and 2007 and repeated the work in 2008.
Here are the details:
The current suggestion to cherry growers is apply two fall foliar applications of low-biuret urea, applied at about 15 to 20 lbs per acre per application.
Spray volumes have been as low as 50gpa up to 250 GPA and with a curtain sprayer. Standard is probably 100 GPA. Dr. Lang reports that they actually got the least amount of marginal leaf burn with the low volume since there was no pooling along the leaf margins.
The nitrogen application definitely increases the flowering spur N going into winter and can improve spur leaf size the next spring. This translates into larger fruit size.
The best timing is in early September, with a second application two weeks later.
In Michigan they did application-timing research with applications made through October but saw the best increase in leaf nitrogen levels from the earlier treatments (September). The September applications were also best for improvement in cold acclimation. Dr. Lang Reports that this is just one year's data, but it was so strikingly consistent that he feels the mid-Sept through mid-Oct is looking more like the best window for Michigan growers. (They usually expect leaf senescence from Halloween through the 2nd week of Nov). In New Jersey we expect the same results depending on how well we have controlled cherry leaf spot and the nutritional status of the tree and leaves.
At the Rutgers Snyder Farm I have applied 20 lbs urea/100 gallons, spraying at tree row volume dilute on both sweet and tart cherries with no foliar burn annually in late September, in 2008-2011. A second application is applied 10-14 days later.
In my cherry research plots at the Snyder Farm I also controlled cherry leaf spot through out the summer and fall months with regular fungicide applications.
I will make my first application of Bordeaux mix (using Copper Sulfate and builders lime) and then saffened with Canola oil, to control Bacterial Canker. I will make one more spay in September and two more in October for a total of four applications.