Making a Zinc Dormant Spray

Question came in this week regarding the decision on the timing of a zinc spray.

When should I make a Zinc Spray?
Research by UC Specialist Dr. Scott Johnson at the Kearney Agricultural Center has shown that a zinc foliar spray is the most effective when made during the post harvest period. Applications of zinc should be made to the tree around late October – early November. The idea is to have a leaves still on the tree when the application is made. Dr. Johnson has shown that a roughly 3% of the zinc applied is taken into the tree from a fall foliar spray, but this is dependant upon the source of zinc. Defoliation may occur after the application, but do not worry as sufficient zinc should have made its way into the plant tissues.

Is making a dormant zinc application a bad idea?
If a fall application of zinc was not made and the orchard is suspected to be deficient in zinc, making a dormant application of zinc will increase zinc tissue levels. The percentage of uptake is slightly less, around 2.0-2.8%, and it is harder to get a large amount of zinc into the tree. This is due to the fact that the shoots, buds, and bud scars only account for roughly 3% of the surface area of a peach/almond branch. Therefore the amount of spray landing on the tree surface is significantly less than when the leaves are still attached (Leaves make up the other 97% of the surface area). To counter this, higher rates should be used during the dormant period.

What type of material should be used in a zinc spray?
Dr. Scott Johnson’s (UC Specialist) work has shown that the “biggest bang” for your buck comes from Zinc Sulfate (high uptake, moderate phytotoxicity). There are other formulations that have higher uptake, but have high phytoxicity as well (Zinc chloride, zinc nitrate). Chelates (Zinc EDTA, Zinc Leonardite, Zinc Oxysulfate) have shown to have lower uptake efficiency (bad) and phytotoxicity (good) than zinc sulfate.

I hope this helps!

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4 thoughts on “Making a Zinc Dormant Spray

  1. A fall Zinc Sulfate spray should be applied a few weeks before the trees normally drop their leaves. So, depending on the weather and where you are located, mid fall (Late October) through late fall (November) should suffice. For maximum uptake, the leaves need to be on the tree. After the application, the leaves will turn yellow and fall from the tree.

    If zinc sulfate is used in the season, lower rates may reduce phytotoxicity.

  2. Since only 3% zinc applied foliarly makes it into the tree, why wouldn’t it be more efficient and cost effective to apply through irrigation? – thanks.

    1. Within almond, the mobility of zinc within tissues is low and variable. In this regard, applications of zinc to the soil would need to be large enough to increase concentrations to a point where it would move throughout the plant – in many cases, this rate often becomes toxic to the plant. Another issue with soil application/uptake of zinc is that acidic conditions must exist within the rootzone in order to release zinc from the soil. Depending upon the pH and buffering capacity of the soil, this may be difficult to achieve.

      To compound this problem even further, Nemaguard rootstock, one of the primary rootstocks in almond, has a reduced ability to pick up soil zinc. This is why zinc deficiency is common – even though California soils tend to have adequate zinc.

      Zinc applied foliarly is more efficient than soil applications because it applies the zinc to the tissues that require the zinc, bypassing the issue with low mobility. Please see this presentation of Dr. Scott Johnson’s (UC Pomology Specialist) work on zinc nutrition for further information.

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