Wednesday, October 19, 2016

Fertilizing Trees in the Fall. 10.18.16

This post is a lot more dry than usual.  I have wondered whether it is beneficial to fertilize trees in the fall.  For the past few years I have done so, and do not see harm occuring from that practice.  In addition, the fertilized trees do seem to have a good burst of growth in the Spring, more than trees that I have not fall-fertilized. 

Different websites give different advice about fall-fertilizing of trees.  For the most part, these sites refer to deciduous shade trees.  Much of the research that has been done, relates to conifer forestry.  "many experts now consider late fall, or about a month after the first killing frost, to be the ideal time for applying fertilizers. We now know plants utilize nutrients throughout the year in different ways."  The site further states, that when trees are dormant, their roots absorb nutrients and apply them to root growth, disease resistance, and storage of nutrients for Spring.  I don't know if those claims are supported by science, but they make sense.

According to the University of Minnesota extension, early Spring is the best time.

Gregory Forrest Lester (Ohio) states that Fall fertilization is essential.  Again, the rationale is to prepare for Spring.  There aren't a lot of sources regarding fall fertilization of trees.  Via google scholar, there has been some specific research, that may or may not apply to yard shade trees.

Annals of Forest Science.  Related to seedlings of red pine,  "Results suggest that fall fertilization of red pine seedlings can help render desired target height in the nursery, while maintaining or increasing cold hardiness levels."  Benefits were seen for number of needles. concentration of nitrogen in shoots and roots, and cold hardiness parameters.

Western Journal of Applied Forestry.   Related to Douglas fir seedlings, fall fertilization increased nitrogen concentrations in the seedlings.  There was no difference in root growth or cold hardiness.  It did not appear in this project that fall fertilization had much effect.

Annals of Forest Science.   Regarding a species of oak,  "early fall fertilization promotes nutrient loading of P(hosphate) in Holm oak, with significant effects on root growth potential and field growth by means of a phenologically earlier development and a higher aboveground biomass."  and in the discussion it was noted that "six months after planting, fall fertilized plants showed higher shoot biomass, higher proportion of new leaves, and faster development, producing leaves earlier compared with unfertilized plants."

 Southern Journal of Applied Forestry.  Regarding fall fertilization of one year old longleaf pine, there was "substantial overwinter dry weight gains and increases in nutrient content and concentrations" especially for nitrogen.  Based on their research, they conclude that fall fertilization "offers a means of increasing seedling size and nutrient reserves prior to out-planting on the relatively infertile sites where seedlings are normally established."

 Journal of American Society for Horticultural Science.  Regarding field-grown peach trees, there was not a benefit for fall fertilization vs. spring fertilization, for peach production or tree growth.

Journal of Agriculture.   Regarding shade tree fertilization researchm up to 2002.  The authors state that nitrogen usually appears to be the most important nutrient, and note that "studies conducted with (labeled) nitrogen
showed low N(itrogen) uptake during the dormant (leafless) season, bringing into question the practice of dormant-season N applications."  However, they also note the inadequacies and limitations of the research that had been done.

Canadian Journal of Forest Research.   Regarding more Douglas fir seedling research, one month after fall fertilization,  "Total nitrogen concentrations increased 1 month after fertilization, remained stable throughout winter, and tended to decrease or remain stable just before budbreak."

My conclusions:  A lot more work is needed to determine whether fall feeding benefits, doesn't benefit, or harms trees, and in what situations.   There does appear be benefit in some situations.  Nitrogen sources are more likely to be beneficial, and unless a deficiency is seen, fertilizers that contain signicant amounts of other major nutrients are usually probably not useful.  If one is interested in pee-cycling,  that seems like a reasonable approach as long as it is not overdone.  A liter of "liquid gold", diluted to 4 liters, could be applied over an area of about 10 feet by 10 feet, to a tree with a drip line about 8 feet in diameter.  It's not rocket science, and I would not do so if the soil salts are high. 

(All images are public domain, via


  1. Interesting stuff. I've never fertilized my trees, although I've peed on them many times! (I live in the country where nobody sees me.) I suppose it's time to start saving my urine for the winter garden. It's a hassle (and stinky)--and I'm not sure it's worth it, but it serves a dual purpose: I don't flush as often which saves water--soft water to boot, meaning I don't have to regenerate my water softener as often. Oh, the joys of gardening!

  2. Randy, it is interesting that the evidence for or against fall fertilization is so limited. At the least, there is no evidence for harm, especially if reasonable amounts are used, and not the wrong kinds. It does make sense that you would not fertilize in the summer or early fall, which could stimulate growth that doesn't have a chance to mature and protect against freezing. If soil is already high in phosphorus or potassium, you don't want to add those.

    I need to check on where the nitrogen goes in a home septic system with drainage system. I think it goes into the soil, and since it's concentrated, is more likely to go to water table than if used to fertilize trees. But I don't know that for sure.

  3. Its always good to dilute urine for fertilization; it all depending what you are fertilizing. For vegetables, I would put it in the compose pile. For mature trees, just directly pee cycle. If worried about the smell, you can water afterward and none will be wiser:-) Sometimes it does attract flies.
    I dug up some live oaks seedlings and bamboo at my Dad's property in Louisiana; pack them in a suitcase and haul it home, plus some acorns to germinate. Flood damaged the whole entire town.

  4. Lance, I agree with you about diluting. Also, discretion is important. I found that in the dog yard, watering takes care of the odor from them, too.

    I'm really glad you were able to preserve some of your Dad's Louisiana oaks and bamboos. The few plants I have from my own dad are like gold to me. Or diamonds. That is most of why I took scion from my dad's ginkgo tree to graft onto my local ginkgo seedlings. I wanted my Dad's tree in my yard.