Managing orchard nutrition during an early spring

Wise use of ground-applied fertilizers can help manage orchard nutrition during the 2012 early spring weather conditions.

Early spring growth this year raises several questions about nutrient management in orchards. First, when should spring nitrogen (N) fertilizers be applied? Typically, N is applied either in a single application as growth begins in the spring or a split application where half is applied when growth begins and half is applied in June after the crops size is known. The second half of the split can be reduced or skipped if cropping is low due to poor return bloom or set, or frost damage. Split applications will be very useful this spring because we are heading into a prolonged period of high risk of frost damage. Apply 30 to 50 percent of your typical seasonal rate in the next few weeks, then wait a month or so until the risk of frost is diminished. If the crop level is good, consider applying the rest of your N.

A second question is whether nutrition can improve the frost tolerance of fruit trees. There are no hard and fast recommendations, but there are some interesting observations. Foliar sprays of urea may enhance frost tolerance. Some work in Israel indicated that peach branches sprayed with low biuret urea (10 percent w/v) three days before freezes sustained less injury to flowers than non-sprayed branches. Foliar urea sprays (2 percent) also reduced freeze injury to avocado leaves. These plants were not deficient in N, so urea did not seem to correct an inherent N shortage. How urea promoted cold tolerance is not clear. We could not find any other instances where spring sprays of N or other nutrients enhanced the tolerance of temperate fruit crops to frost.

A related but different effect was observed in New York studies in the 1990s. Pre-bloom sprays of boron, zinc, and urea were applied to Empire and McIntosh apple trees that were injured by mid-winter cold. These nutrient sprays often increase cropping, and the authors thought sprays may have supplied needed nutrients to the flowers that were not able to obtain enough naturally due to cold injury. These treatments helped cold-injured trees rather than protected trees from cold injury. Whether sprays applied prior to spring frost injury would result in similar effects is an open question. Growers interested in testing foliar nutrient sprays should make sure they leave appropriate, untreated check trees so they learn from their efforts.

In summary, be conservative in N rates this spring, as the risk of losing fruit to spring freezes is high. You can always apply more N, but you can’t take away what is already down. Spring nutrient treatments are unlikely to affect frost tolerance if trees have been adequately fertilized to this point. Apogee growth regulator treatments can be used post-bloom to tame apple and pear trees with excess vigor, but this is an expensive option.

Dr. Hanson's work is funded in part by MSU's AgBioResearch.

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