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!