Adding fertilizer to shredding leaves for overwintering garden and tree beds

Asked November 5, 2016, 2:08 PM EDT

Question 1:

I am shredding the leaves from my trees (maple, oak and ash) to cover garden beds for the winter. I'm told I need to add some nitrogen slow release fertilizer to prevent leaching of nitrogen from the soil. However, I can't find info on the best formulation to use (maybe just urea?) and how much should be applied.

Question 2:

Also, regarding the ash leaves: I'm in an ash borer area (Hamline-Midway, St. Paul) and in June of 2015, Rainbow Tree Care treated my ash tree to protect it. They were recommended to me by a professor from UMN doing a public talk on pollinators and she said they were the very best and that I could have confidence in their work for ash borer. They used the injectable emamectin benzoate at the proper dosage for my large ash tree.

Is the any concern about now using the leaves as a garden mulch, especially on raspberry beds and on beds for pollinator friendly flowers? It's been 18 months since the application, but in the Spring we will get the next injection, and I'm also wondering about using the ash leaves next Fall, which be only 6 months after my Spring 2017 application for the tree.

Any help regarding these two questions would be greatly appreciated. Thanks.


Ramsey County Minnesota

1 Response

My view is that any nitrogen fertilizer added now will be wasted. Nitrogen is very mobile and will probably leach out of the soil before spring. A better plan is to leave the leaf litter on the garden over winter, then add some extra nitrogen fertilizer in the spring when you ordinarily prepare your garden for planting. Blood meal is an excellent source of organic slow release nitrogen, but there are also less expensive non-organic slow release fertilizers. Read here:
http://www.extension.umn.edu/garden/yard-garden/soils/soil-test-interpretations-and-fertilizer-manag...

In answer to your second question, besides the length of time, the pesticide must pass into the tree, into the leaves, then survive the decomposing process of the leaf litter and finally make its way into the raspberry flowers intact. This seems very unlikely to me. Add to that the fact that the contaminated leaves have been diluted (by other tree species) and that the pesticide in question breaks down rapidly by microbial activity and sunlight. I would not worry about this.

This thorough article from Purdue discusses this and other side effect issues of EAB systemic pesticides:
https://extension.entm.purdue.edu/EAB/PDF/PotentialSideEffectsofEABInsecticidesFAQ.pdf