Is it a good idea (or at least not harmful) to fertilize large spruces and...

Asked March 30, 2015, 3:37 PM EDT

Is it a good idea (or at least not harmful) to fertilize large spruces and pines? These trees are about 25 years old. Last year a house was built next door. Therefore these trees have been stressed by having some of their roots severed and/or compacted. Some of these trees were treated with cambistat last fall. A proposal from a tree company recommends doing "maintenance rootcare fertilization with 3 macronutrients and 8 micronutrients." If fertilizing the trees would benefit the trees is it better to it in spring or fall?

Hennepin County Minnesota

1 Response

Thank you for the question. Generally, mature landscape trees in Minnesota do not need additional micro nutrients. We recommend doing a soil test so that you know exactly what is needed. Here is the link to the University of Minnesota Soil testing lab with instructions on how to submit a test: http://soiltest.cfans.umn.edu/. Perhaps your tree care company already did this.
The very best time to fertilize is in spring as soon as the ground is workable and before tree growth begins around mid May.
You mentioned some of the trees were treated with the growth inhibitor Cambistat last fall. This can be beneficial to the trees in increasing their resistance to environmental stress and fungal diseases. Maybe this was done to help them withstand the nearby construction. I'm not aware what effect fertilizer has on trees treated to reduce growth mass. Logically, it seems growth is being controlled by one chemical and stimulated by another. I don't know if this is a good thing and would check with your tree care company.
Here is a link to the University of Minnesota Publication all about tree and shrub fertilization:
http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/tree-fertilization-guide/
Thank you for contacting Extension.