In the heart of summer, trees, shrubs, conifers, and perennials are still lush and green and may appear to be actively growing. Yet, as August approaches, an important change begins to take place. Even though fall may still seem far away, hardy plants in Eastern Canada are gradually beginning to prepare for the cold season.
For this reason, it is generally recommended to reduce fertilization of deciduous trees and shrubs, particularly heavy applications of nitrogen, starting in August. At this point, the goal is no longer to stimulate or support growth, but rather to allow plants to complete their seasonal development and prepare properly for winter.
But what exactly happens to plants as summer draws to a close? Let’s take a closer look.
What Role Does Nitrogen Play?
Nitrogen is one of the main nutrients plants need. It plays an important role in chlorophyll production and in the development of leaves and new shoots.
In spring and early summer, stimulating this growth is generally desirable. The days are long, temperatures are favourable, and plants have several weeks ahead of them to develop their new shoots.
By late summer, however, the situation is different.
A heavy application of nitrogen-rich fertilizer can encourage a plant to continue growing or produce new vegetative growth just as the growing season is beginning to wind down. These young tissues may not have enough time to fully mature before cold weather arrives.
By August, Plants Are Already Preparing for Winter
Woody perennial plants do not wait until the first frost to begin preparing for winter. The process starts much earlier.
As the days become shorter and the season progresses, many trees and shrubs gradually slow their growth. New shoots stop elongating, buds begin to develop, and tissues produced during the growing season gradually become firmer. In trees and shrubs, this process is known as shoot maturation or hardening and includes the lignification of young shoots. These tissues gradually become more resistant and better prepared for cold temperatures.
Excessive or late-season nitrogen fertilization can interfere with this natural transition.
Photo: Red Maple in August. A: Buds for the following spring; B: Current-year shoot undergoing lignification; C: Previous year's stem. (TreesOnline.ca)
Why Do Young Trees and Shrubs Require Special Attention?
This precaution is particularly important for new plantings.
When a young tree or shrub has just been planted, the main objective should be to encourage successful establishment rather than trying to achieve maximum above-ground growth as quickly as possible. This is why fertilizers formulated for root development typically contain more phosphorus than nitrogen. Phosphorus supports root development, while nitrogen primarily encourages the growth of new stems and foliage.
Keep in mind that during the first year after planting a tree or shrub, proper watering is generally much more important than fertilizing, particularly during the hot, dry periods of late summer.
What About Conifers?
The same principle applies to conifers such as spruce, pine, fir, cedar, and larch.
Although they may not display seasonal changes as visibly as deciduous trees, conifers must also complete the maturation of their new growth before winter arrives.
Starting in July, heavy nitrogen fertilization is therefore generally not recommended. For newly planted conifers, it is preferable to pay close attention to soil moisture so they can continue establishing their root systems without experiencing water stress.
Should You Stop Fertilizing Completely in August?
Not necessarily.
It is important to distinguish between reducing nitrogen fertilization intended to stimulate growth and assuming that every type of fertilizer automatically becomes harmful as of August 1.
The effects of fertilization depend on several factors, including the type of plant, the fertilizer used, its concentration, the soil, and growing conditions. Fertilizers also contain different proportions of nitrogen, phosphorus, and potassium.
The simplest recommendation for home gardeners is therefore to avoid heavy applications of nitrogen-rich fertilizers intended to trigger a new flush of growth toward the end of summer.
In an established garden, trees and shrubs do not necessarily need to be fertilized routinely. Suitable soil conditions, the addition of organic matter, and organic mulch all contribute to maintaining healthy growing conditions.
What About Perennials?
For perennials, the general principle is similar: as the growing season draws to a close, the goal is no longer to encourage abundant vegetative growth.
However, nutritional requirements vary considerably from one species to another. Some perennials are naturally heavy feeders, while others grow very well in relatively poor soils.
Excessive fertilization can even have undesirable effects on certain species, including overly tender growth, less compact plants, or stems that become weak and flop under their own weight.
Once again, more fertilizer does not necessarily mean a healthier plant.
What Should You Do Instead of Fertilizing in August?
Late summer is an excellent time to focus on the care that will truly help your plants finish the growing season in good condition.
Keep an Eye on Watering
Pay particular attention to recently planted trees, shrubs, and conifers. Keeping the soil adequately moist supports root development and helps reduce water stress.
Maintain a Layer of Mulch
Mulch helps retain soil moisture, reduces competition from weeds, and protects roots from significant temperature fluctuations.
Avoid Unnecessarily Stimulating New Growth
Late summer is a time for maturation rather than accelerated growth.
Observe Before Fertilizing
Unusual colouring or weak growth does not automatically mean that a plant needs fertilizer. For example, problems related to watering, drainage, soil pH, soil conditions, or root establishment can produce similar symptoms.
In Summary: In August, Support Your Plants Rather Than Stimulating Growth
In spring, we often focus on giving plants a strong start. By late summer, our role gradually begins to change.
For trees, shrubs, conifers, and many perennials adapted to the climate of Ontario and Eastern Canada, August marks the beginning of an important transition toward fall. Reducing applications of nitrogen-rich fertilizer helps respect this natural cycle and avoids unnecessarily stimulating late-season growth.
For young plantings in particular, it is better to focus on proper watering, good mulch management, and conditions that promote healthy root establishment.
Sometimes, the best thing you can do for a plant is not to encourage it to grow more, but simply to give it what it needs to finish the growing season successfully.
Article written by Suzanne Simard, horticultural writer and certified horticultural technologist, and Audrey Rondeau, Biol., M. Env., Production and Development Director at Pépinière Vert Forêt, with editorial support from AI.
Related article: Fertilizing Trees and Shrubs in Eastern Canada
