Nitrogen is an essential nutrient for plants, playing a key role in their growth, development, and overall health. It is involved in the formation of proteins, nucleic acids, and chlorophyll—making it critical for plant productivity.
Although nitrogen makes up about 80% of the atmosphere, plants cannot use it in this form. So where—and how—do they get this vital nutrient?
In agriculture and horticulture, nitrogen fertilizers are commonly used to support plant growth. In natural ecosystems, nitrogen becomes available mainly through the decomposition of organic matter and the activity of soil microorganisms.
However, some plants have the remarkable ability to form symbiotic relationships with soil bacteria that allow them to fix atmospheric nitrogen. These are known as nitrogen-fixing plants. In these cases, adding nitrogen fertilizer is often unnecessary.
Do All Trees Fix Nitrogen?
No. Nitrogen fixation is a specialized process limited to certain plant species. These plants form symbiotic relationships with bacteria that convert atmospheric nitrogen (N₂) into forms plants can absorb, such as ammonium (NH₄⁺) or nitrate (NO₃⁻).
How Does Nitrogen Fixation Work?
Large populations of bacteria live in the soil around plant roots. In nitrogen-fixing species, specific bacteria form colonies inside small nodules on the roots.
This relationship is mutually beneficial: the plant provides sugars produced through photosynthesis, and in return, the bacteria convert atmospheric nitrogen into a usable form.
This process is called symbiosis, as both organisms benefit. When root nodules break down, nitrogen is released into the soil, naturally enriching it.
(Image source: Wikipedia)
Why use nitrogen-fixing plants?
Ecological benefits
- They improve soil fertility by naturally enriching it with nitrogen;
- They reduce the need for synthetic fertilizers and limit water pollution;
- They support biodiversity and help restore degraded environments.
Economic benefits
- They reduce production costs by lowering fertilizer inputs;
- They can improve crop performance over time.
Which Nitrogen-Fixing Trees and Shrubs Grow in Quebec?
In Quebec, several tree and shrub species can fix nitrogen. Most are pioneer species, meaning they are among the first to establish on disturbed sites.
Because they are low-maintenance and adaptable, they are excellent choices for restoring degraded soils or creating resilient landscapes.
Nitrogen-Fixing Tree Species
Speckled Alder (Alnus rugosa)
| Hardiness Zone | 2a |
| Type | Small tree or shrub |
| Sun Exposure | Full sun |
| Height | 20–33 ft (6–10 m) |
| Growth Rate | Fast |
| Soil | Adaptable; prefers moist, slightly acidic soils |
| Uses | Naturalization, windbreaks, riparian zones, soil stabilization |
Siberian Peashrub (Caragana arborescens)
| Hardiness Zone | 2a |
| Type | Small tree or shrub |
| Exposure | Full sun |
| Height | 2–6 m |
| Growth Rate | Fast |
| Soil | Sandy to gravelly; neutral to alkaline; low moisture |
| Uses | Informal or trimmed hedge, mass plantings, naturalization |
Kentucky Coffeetree (Gymnocladus dioicus)
| Hardiness Zone | 4b |
| Type | Large tree |
| Exposure | Full sun |
| Height | 18–21 m |
| Growth Rate | Very slow |
| Soil | Loamy; moderately moist; neutral to alkaline pH |
| Uses | Specimen tree, street planting, and mass plantings |
Honey Locust (Gleditsia triacnathos)
| Hardiness Zone | 4a |
| Type | Medium-sized tree |
| Exposure | Full sun |
| Height | 18–20 m |
| Growth Rate | Fast |
| Soil | Sandy; acidic to alkaline |
| Uses | Specimen planting, mass plantings, and street planting |
Eastern Redbud (Cercis canadensis)
| Hardiness Zone | 6b |
| Type | Small tree or shrub |
| Exposure | Full sun, partial shade, or shade |
| Height | 5 m |
| Growth Rate | Slow |
| Soil | Loamy soil, preferably rich and calcareous |
| Uses | Mass plantings or specimen planting |
Russian Olive (Elaeagnus angustifolia)
| Hardiness Zone | 2b |
| Type | Small tree |
| Exposure | Full sun |
| Height | 10–11 m |
| Growth Rate | Fast |
| Soil | Sandy and loamy; low moisture; neutral to alkaline; tolerates poor soils |
| Uses | Specimen planting or mass plantings |
Nitrogen-Fizing Shrubs Species
Sea Buckthorn (Hippophae rhamnoides)
| Hardiness Zone | 2b |
| Type | Shrub |
| Exposure | Full sun |
| Height | 2–4 m |
| Growth Rate | Moderate |
| Soil | Sandy; alkaline; medium moisture; tolerates poor soils |
| Uses | Hedges, mass plantings, and naturalization; wind-tolerant; edible small fruits |
Green Alder (Alnus crispa)
| Hardiness Zone | 1a |
| Type | Oval-shaped shrub |
| Exposure | Full sun |
| Height | 3–5 m |
| Growth Rate | Fast |
| Soil | Sandy to clay soils; acidic, neutral, or alkaline; high moisture; tolerates poor soils |
| Uses | Hedges, mass plantings, and naturalization |
Silverberry (Elaeagnus commutata)
| Hardiness Zone | 3a |
| Type | Shrub |
| Exposure | Full sun |
| Height | 2–4 m |
| Growth Rate | Fast |
| Soil | Sandy or gravelly; acidic, neutral, or alkaline; low moisture |
| Uses |
Specimen planting, hedges, mass plantings, naturalization, slopes, and embankments Avoid small gardens, as it spreads aggressively by suckering |
False Indigo (Amorpha fruticosa) 
| Hardiness Zone | 3b |
| Type | Dense, bushy shrub |
| Exposure | Full sun or partial shade |
| Height | 3 m |
| Growth Rate | Moderate |
| Soil | All soil types; low to high moisture |
| Uses | Specimen planting, mass plantings, naturalization, and difficult sites |
Silver Buffaloberry (Shepherdia argentea)
| Hardiness Zone | 2a |
| Type | Large shrub with a bushy, upright habit |
| Exposure | Full sun or partial shade |
| Height | 1–6 m |
| Growth Rate | Moderate |
| Soil | Sandy; alkaline; medium moisture |
| Uses | Hedges, mass plantings, and naturalization |
Canada Buffaloberry (Shepherdia canadensis)
| Hardiness Zone | 2a |
| Type | Bushy shrub |
| Exposure | Full sun |
| Height | 1–4 m |
| Growth Rate | Slow |
| Soil | Sandy; acidic, neutral, or alkaline; low moisture |
| Uses | Hedges, mass plantings, and naturalization |
How to Use Nitrogen-Fixing Trees and Shrubs in Landscaping
Here are a few practical ways to integrate nitrogen-fixing species into your landscape:
Soil Improvement
- Restoring poor soils: Use species such as seabuckthorn (Hippophae rhamnoides) or Siberian peashrub (Caragana arborescens);
- Example: Plant them on degraded sites to stabilize soil and improve structure.
Gardens and Landscapes
- Vegetable gardens: Integrate nitrogen-fixing plants to improve soil fertility naturally;
- Example: Combine buffaloberry (Shepherdia canadensis) or false indigo (Amorpha fruticosa) with other plantings.
Hedgerows and Windbreaks
- Mixed hedges: Combine nitrogen-fixing shrubs with other species;
- Example: Use alders, peashrub, and seabuckthorn to reduce erosion and improve soil.
Forest Garden Systems
- Biodiversity: Integrate nitrogen-fixing plants into layered plantings;
- Example: Combine canopy trees, fruiting shrubs, and nitrogen-fixing species for a resilient system.
Why Nitrogen-Fixing Plants Matter
In summary, nitrogen-fixing trees and shrubs enrich soil naturally by converting atmospheric nitrogen into plant-available forms.
They improve soil fertility, reduce dependence on synthetic fertilizers, and support more sustainable gardening and agricultural practices.
They also contribute to biodiversity, help restore degraded landscapes, and create more resilient ecosystems.
Incorporating nitrogen-fixing plants is a simple and effective step toward healthier, more sustainable landscapes.
Article co-written by Audrey Rondeau, Biol., M.Env., and Suzanne Simard, horticultural writer and technologist.
