When, How, and Why to Fertilize Trees and Shrubs in Your Landscape
With bud break and the return of the gardening season, spring is the perfect time to assess the health of your trees and plan ahead. Are your trees thriving? Do they need fertilization? How can you support the establishment of newly planted trees? These are some of the questions we will explore.
Fertilization is not a cure-all. A tree planted in the wrong location, outside its hardiness zone, or lacking proper care will not be saved by fertilizer alone.
However, fertilization can support growth and vigour in established trees and improve root development in newly planted trees.
Which Fertilizer Should You Choose for Trees and Shrubs?
Fertilizer aisles in garden centres can feel overwhelming. Understanding a few key concepts will help you make the right choice.
Essential Nutrients
Fertilizers are labeled with three numbers, such as 15-30-15.
These numbers represent the percentage of nitrogen (N), phosphorus (P), and potassium (K)—known as the N-P-K ratio. Each plays a specific role in plant development:
Nitrogen (N):
Nitrogen drives plant growth. It is essential for chlorophyll production and protein synthesis. Plants with adequate nitrogen develop rich green foliage and strong growth.
Phosphorus (P):
Phosphorus supports root development, flowering, and fruit production.
Potassium (K):
Potassium plays a key role in regulating plant functions such as disease resistance, cold and drought tolerance, and water balance. In Canadian climates, it is especially important for helping plants prepare for winter.
For trees and shrubs, growth is often limited by nitrogen. Maintenance fertilizers typically follow a 2-1-1 or 3-1-1 ratio (e.g., 30-10-10 or 15-9-12).
Organic or Synthetic Fertilizers?
Organic fertilizers come from natural sources such as compost, manure, or mineral deposits. Nutrients are released gradually as soil microorganisms break them down, improving soil health over time.
Synthetic fertilizers are immediately available to plants and are often more concentrated. They are available as granules or water-soluble products.
While fast-acting, synthetic fertilizers can leach into the environment more easily. For most uses, granular fertilizers are preferred for their ease of application and reduced environmental impact.
Controlled-Release Fertilizers
Controlled-release fertilizers are coated with a resin that regulates nutrient release over time.
Release rates depend on temperature and coating thickness—warmer temperatures increase nutrient availability. Typically, nutrients are released over about 100 days.
In most cases, a single spring application is sufficient for the entire growing season. These fertilizers are easy to use and help reduce nutrient loss.
Should You Always Fertilize Trees and Shrubs?
No—it depends on the tree’s condition and soil fertility.
Think of soil as a nutrient reservoir. While nutrients may be present, they are not always immediately available. Their availability depends on soil biology and pH.
Fertilizers provide readily available nutrients that plants can absorb more quickly.
How do you know what to add?
A soil test is the best way to determine nutrient levels and identify deficiencies.
Fertilizing at Planting (Establishment Phase)
Planting is a stressful stage for trees and shrubs. Rapid root establishment is critical for long-term success.
Avoid fertilizers that are too high in nitrogen, as they promote top growth rather than root development. Instead, consider adding mycorrhizal fungi to improve root performance.
Consistent watering is essential—young plants must not dry out.
Fertilizing Young Trees
Young trees are more sensitive to environmental stress, nutrient deficiencies, pests, and diseases.
Annual fertilization with a nitrogen-rich product can support healthy growth. Always follow label recommendations and choose fertilizers adapted to the plant type—deciduous trees and conifers have different needs.
Fertilizing Mature Trees and Shrubs
Mature trees with well-developed root systems can access nutrients deep in the soil and often require less frequent fertilization.
In many cases, fertilizing every two to three years is sufficient, especially if organic matter is recycled on site.
Exception: Trees weakened by stress (winter injury, pests, or disease) may benefit from targeted fertilization.
If damage occurs in winter, apply a nitrogen-rich fertilizer in early spring before bud break. If damage occurs during the growing season, apply a potassium-rich fertilizer in fall to improve winter hardiness.
Do Native Plants Need Less Fertilizer?
Native plants have evolved over time by gradually adapting to the local climate, soils, and ecosystems of their region. When planted in conditions similar to their natural habitat, they generally require less maintenance and less fertilization than many non-native species.
For example, if you plant a sugar maple in a setting that resembles a natural maple forest, it will already benefit from soil, moisture levels, and microbial life to which it is naturally adapted. This is why choosing native species suited to your region, sun exposure, and soil conditions is often a more sustainable approach—and much easier to maintain over the long term.
When Should You Fertilize?
There are two main fertilization periods: fall and spring.
In fall, after the first frost and before December, soils remain warm enough for roots to absorb and store nutrients for the following season.
In spring, fertilize once the soil has thawed—typically late April in southern Quebec and Ontario—when buds begin to swell.
Stop fertilizing by late July for deciduous trees and late June for conifers to avoid late-season growth that may be damaged by winter conditions.
Also, avoid fertilizing drought-stressed plants. Focus on proper watering instead.
How to Apply Fertilizer
Granular fertilizers—especially controlled-release types—are commonly used.
You will need:
- Fertilizer
- Application instructions
- Gloves
- A trowel or small shovel
Step 1: Calculate the Amount
Follow the manufacturer’s recommendations and estimate the application area.
A simple method is to measure the canopy area, which roughly corresponds to the root zone. For example, a tree with a 3 m spread has an approximate root area of 9 m².
Step 2: Application
For shrubs, apply fertilizer evenly at the base. Move mulch aside before applying, then replace it.
For trees, dig small holes and distribute fertilizer evenly, focusing on the outer canopy where feeder roots are most active.
Repeat after 4 to 6 weeks if needed—unless using a controlled-release fertilizer, in which case a single spring application is sufficient.
Can You Over-Fertilize Trees?
Yes—excess fertilization can cause several problems:
- Excess nutrients can lead to weak, elongated growth that attracts pests;
- High concentrations may cause root burn;
- Overuse contributes to water pollution through runoff.
To avoid these issues, use fertilizers responsibly and favour slow-release products.
By adopting a balanced and environmentally responsible approach, you help protect both your plants and surrounding ecosystems.
Article co-written by Suzanne Simard, horticultural writer and horticultural technologist, and Audrey Rondeau, Biol., M. Env., Production and Development Director at Pépinière Vert Forêt.
