Fertilizing winter-injured blueberries

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Some Michigan blueberry fields are showing quite a bit of winter injury to buds and twigs. This raises several questions about cold injury and fertilization. Not all of these questions have been researched adequately, but here are some related facts and thoughts.

Should fertilizer rates be adjusted to account for reduced crop load?

Our answer is not much. It is true that bushes with reduced crop levels may have slightly reduced nutrient demand because less needs to be allocated to fruit, but these amounts are small. Based on reported mineral concentrations in blueberries, each ton of fruit contains about 2.4, 0.2 and 2.0 lbs of nitrogen (N), phosphorus (P), and potassium (K), respectively. Therefore, a five ton crop only contains about 12, 1, and 10 lb of N, P, and K. Even if all flower buds are killed by cold, the impact on plant nutrient demand is relatively minor. If the crop load is only partially reduced, effects on nutrient demand are even smaller. Keep in mind that the N fertilizer you apply this year is only a supplement; most of the N for this year’s growth will come from reserves in the soil and in perennial plant parts. Older bushes recycle most of their nutrients from year-to-year so adjusting rates from year-to-year has limited effect on growth. So our general recommendation is to fertilize as in the past adjusting rates if you have recent soil tests or even better recent tissue tests of plant nutrient levels.

Should bushes be fertilized differently to help them recover from cold injury?

Again, we would say no. Typically, shoots grow more vigorously after flower buds and twigs are injured by cold. The carbohydrate and nutrient resources in the bush are allocated to fewer growing points so shoots may grow longer or more new canes may break from the crown. The effect is similar to that of pruning. We experienced an exception to this several years ago when many Jersey fields experienced severe injury to buds/twigs as well as apparent damage to the wood of older canes. Shoot growth was very weak on older canes with wood damage, and these canes eventually needed to be pruned out. Damage this year seems to be more to flower buds so we can expect normal shoot growth, which will not have as much competition from fruit. We can expect to see shoot growth to continue longer in fields with a light crop and adequate irrigation. We expect that fields in good vigor will set a large crop of flower buds this year and have the potential for a large crop next year.

How can I fertilize so plants are best able to tolerate winter cold?

This question has not been well-researched in blueberries, although there are numerous opinions and some related information in other crops. Hardiness is optimized by maintaining nutrient levels in the sufficient ranges, and by minimizing overall plant stress (drought, over-cropping, diseases, foliar pest feeding). The nutrient most often associated with cold hardiness is nitrogen. Excessive N use has been shown to reduce the hardiness of some fruit trees, and anecdotal observations suggest this is also true of blueberries. The key is to apply recommended rates at the right time. Nitrogen applications after June should be avoided because this may encourage additional flushes of shoot growth late in the season, which may not harden off in time for winter. The bottom line is that following good cultural practices including fertilization recommendations optimizes hardiness. Use periodic leaf analyses and soil tests to monitor plant nutrition and make sure your fertility program is best for your site.

Some people believe that fall applications of K promote acclimation and hardiness. This approach is worth studying, but has not been researched in blueberries. Another interesting idea is applying foliar sprays of urea in the late fall. Sprays have been shown to increase N reserves in the buds of tree fruit crops, and promote growth the following spring. These practices need to be studied in blueberries.

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

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