Optimizing the use of nitrogen fertilizers in the production of woody ornamental plants in the field

By: Émilie Lemaire, Agr, M.Sc., Project Manager, IQDHO

Nitrogen is the most important nutrient for plant growth. Soil can only partially meet the needs of plants for this element, and the proportion varies from one soil to another, depending on several factors. Therefore, nurserymen who grow ornamental deciduous and coniferous trees in the field mostly use mineral nitrogen fertilizers (MNF) to meet the needs of their crops.

The use of MNFs generates greenhouse gas (GHG) emissions directly through the denitrification of inorganic nitrogen in the field (N2O), and through the fuel consumption (CO2) associated with the application of the fertilizer. Good MNF management is essential to reduce losses to the environment. Fractionating applications and synchronizing them with crop needs are ways to maximize plant uptake and reduce losses to the environment.

In field grown woody ornamental plants, the most common fertilization practice today is to split the annual nitrogen application into three applications of mineral fertilizer (May, June, July). Fall nitrogen fertilization is still not widely used because of a popular belief that late nitrogen application can interfere with winter hardening of plants. Nevertheless, this approach may be more beneficial in several ways. The tendency is that soil temperatures and metabolic activity of trees are higher in early fall than in early May. These conditions are more advantageous for nitrogen uptake by the roots. The nitrogen taken will be stored for the winter and quickly available the following spring. In fact, for several species, nitrogen remobilization in the tree for growth in the spring occurs before root activity resumes and nitrogen is withdrawn from the soil. The accumulation of reserves is most advantageous for species that have only one rapid growth phase in the spring.

trees growing in a nursery field

Oak trees from the fall fertilization trial.
Source: IQDHO

person measuring the trunk of a tree

Measuring the diameter of the trunk with a caliper.
Source: IQDHO

IQDHO conducted a project to study the effect of early spring and fall mineral nitrogen fertilization on plant growth, quality, nitrogen content and winter survival under Quebec conditions. This project also aimed to evaluate whether improved nitrogen removal applied in the fall rather than in the spring could reduce the current annual application rate and thus reduce GHG emissions. Fertilization treatments were tested on lilacs (Syringa reticulata “Ivory Silk”), oaks (Quercus rubra) and cedars (Thuja occidentalis) for two years.

Data collected did not show that nitrogen uptake and growth were increased following fall (September and October) nitrogen fertilization compared to spring (May). In fact, at the end of the project, there were no noticeable differences between trees in the different fertilization treatments, even for those that received a 50 kg nitrogen (N) per hectare (ha) lower annual rate.

Therefore, the results suggest that nitrogen application could be reduced on certain sites without significantly compromising the yield and quality of some ornamental woody plant species. With the elimination of 50 kg N/ha, nurserymen could reduce GHG emissions by approximately 807 kg CO2e/ha. However, the drought of 2020 greatly affected the growth of all trees and possibly reduced the response of the trees to the various fertilization treatments. Thus, the project will have raised doubts about the usefulness of an annual rate of 150 kg N/ha and demonstrated the potential to reduce GHG emissions by reducing the rate, but results across more sites, species and years will be needed to ascertain the nitrogen requirements of the many woody ornamentals grown in the field.

Nevertheless, the project addressed one of the main concerns of nurserymen and crop consultants regarding fall fertilization. There was no increased winter injury in fall fertilized trees. Therefore, this fertilization method would be safe, at least for these three species.

For more details, see the project’s final report:

See the summary sheet:

“This project was carried out as part of component 4 of the 2013–2018 Prime-Vert Program with financial assistance from the Ministère de l’Agriculture, des Pêcheries et de l’Alimentation (MAPAQ) through the 2013–2020 Climate Change Action Plan.”