Wednesday, October 19, 2016
Fertilizing Trees in the Fall. 10.18.16
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.
Growingagreenworld.com: "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
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 vintageprintable.com)