Fertilization guide: when and how to fertilize in the garden

• Good practices that encourage their presence in the garden
• Why use mycorrhizae?
• Which plants to fertilize?
• When and how to fertilize?
• When and how to fertilize annuals grown in open ground?
• How often should container-grown plants be fertilized?
• When to fertilize bulbs?
• When to fertilize perennials?
• When to fertilize trees and shrubs?
• When to fertilize rose bushes?
• How to fertilize the vegetable garden?
• What you need to know about fertilization 

The ground we walk on and plant in is more complex than it seems. It is made up of rock particles, water, air, organic matter and millions of living organisms, the majority of which are invisible to the naked eye.

These organisms are not only beneficial, but essential for the proper growth of plants and the production of fruits, vegetables and flowers. In addition to transforming organic matter into nutrients that plants can take in, they contribute to soil health and even play a role in pest control.

To survive and multiply, these living organisms need oxygen, water, heat, food and a virtually neutral pH.

Good practices that encourage their presence in the garden

• Add organic matter (compost)
• Fertilize with eco-friendly fertilizers
• Ban pesticides from our garden
• Correct the pH when necessary
• Water efficiently
• Avoid working the soil deeply (certain microorganisms such as mycorrhizae do not tolerate soil disturbance)
• Use mulch
• Add mycorrhizal fungi (beneficial fungi)

Also discover our guide to a more ecological garden.

Why use mycorrhizae?

Mycorrhizae are microscopic fungi that attach to the ends of roots and live symbiotically with plants. The fungi feed on the sugars resulting from photosynthesis and in return allow the plant to absorb more water and minerals, protect it from root pathogens and improve the structure of the soil.

Result: the plant tolerates transplantation better, develops more quickly afterwards, resists diseases and drought better and produces more fruits, vegetables or flowers.

Mycorrhizal fungi are everywhere in nature, but in our gardens, they have rarely survived overworked soil, the use of synthetic fertilizers and the application of pesticides. You can remedy the situation by using a soil mix containing mycorrhizae. A single application is enough for your plant to benefit from it throughout its life.

Recommended products containing mycorrhizae: 

• Organic Seed Starting Mix 
• Organic Vegetable & Herb Mix 
• Premium Potting Mix 
• Premium All Purpose Mix 
• Organic Moisture Mix 

All these growing media also contain nutrients to ensure a good start for your plants and to fertilize them during the first few months for vigorous growth.

Which plants to fertilize?

All plants can benefit from fertilizer applications at different stages of their life cycle, and all will appreciate improved soil conditions. Before planting, incorporate a quality mix into the existing soil.

Bulbs, vegetable plants, perennials, annuals, ornamental shrubs and roses will also benefit from an application of Bone Meal BLOOMBOOST 4-10-0 at planting time and at the beginning of the season, which should be reapplied at the end of the season for bulbs and perennials. Trees and shrubs cultivated for their foliage will benefit from an application of Blood Meal PLANTBOOST 12-0-0 at planting time. 

Read our related article to learn when and how to use bone meal and blood meal to grow healthy, vigorous plants. 

A starter fertilizer such as our ROOT BOOSTER growth stimulant will help all your plants get off to a good start by promoting rapid recovery and the development of a vigorous root system.

Then, heavy feeders can be fertilized every 2 to 4 weeks with a slow-release fertilizer, while less demanding plants will appreciate an annual application of compost at the beginning of the season. Finally, a fast-decomposing mulch should be applied around the base of the plants. Reapplied annually, it will protect the roots from extreme temperatures, help retain soil moisture and enrich the soil over time.

Tip: In northern regions, stop fertilizing perennials in July to avoid encouraging the formation of young shoots that would struggle to withstand the cold. 

Do container-grown plants need to be fertilized frequently?

Plants grown in containers, whether shrubs, flowers or vegetables, usually need to be fertilized on a regular basis. This is because the frequent addition of water causes repeated leaching, which ends up depleting the soil (the nutrients are flushed out with the water through the drainage holes).

Which plants don’t need fertilizer?

Whether grown in the ground or in a pot, some annuals are exceptions to the rule. For example, cosmos, marigolds and nasturtiums flower more when grown in poor soil. As for perennials, you can also obtain better results by avoiding fertilizing yarrow, globe thistles, sages, speedwells, liatris, rudbeckias, baptisias, garden irises, moss phlox and calamagrostis. 

When and how to fertilize?

To be beneficial, fertilizers must be applied at the right time. Fertilizing at the wrong time risks burning the roots, affecting plant growth or interfering with the plants' natural hardening-off process before winter.

Although this can vary from one plant to another and depending on growing conditions, generally fertilize in spring and/or summer when the plants are actively growing. You should always respect the recommended doses, avoid fertilizing during heat waves and stop fertilizing by mid-July or the end of July.

You will find more details depending on the type of plants in the following sections. To ensure you don't forget anything, you can also download and print the suggested reference guide.

Pro tip: For all flowering plants, avoid fertilizers that are too rich in nitrogen (higher first number), which stimulates green growth to the detriment of flowering. 

When and how to fertilize annuals grown in open ground?

When planting: Amend the soil in the flower beds with a garden soil containing a slow-release fertilizer, sphagnum peat moss, which helps retain water, and mycorrhizae, which promote the development of the root system. Distribute a granular fertilizer at their feet.

In season: As soon as they are in flower, lightly fertilize the flower beds every week or two until fall. This will stimulate the growth of plants, meet the needs of the most demanding plants and promote spectacular flowering.

Recommended products:

• Organic-Based Bone Meal BLOOMBOOST 4-10-0 (when planting)
• Organic-Based Garden Fertilizer for Tomatoes, Vegetables & Fruits 3-6-12
• Organic Multi-Purpose Garden Fertilizer 7-3-3
• Organic-Based Multi-Purpose Garden Fertilizer 12-6-6

How often should container-grown plants be fertilize?

When planting: Use a specific potting mix that helps retain water and nutrients. A good potting mix contains sphagnum peat moss, perlite, a slow-release fertilizer and mycorrhizae.

In season: Fertilize plants grown in containers once a week until mid-September (end of July for perennials).

Tip: Never fertilize dry soil. Water the soil before and after fertilizing to ensure the fertilizer reaches the roots. Water at soil level, avoiding wetting the flowers and foliage.

Recommended products:

• Organic-Based Garden Fertilizer for Tomatoes, Vegetables & Fruits 3-6-12
• Organic Multi-Purpose Garden Fertilizer 7-3-3
• Organic-Based Multi-Purpose Garden Fertilizer 12-6-6

IMPORTANT: Take a break during periods of extreme heat, and resume when temperatures return to normal. For more information, read How to have beautiful planters all season long. 

When to fertilize bulbs?

There are two types of bulbs: spring bulbs, which are planted in the fall and will flower the following spring, and summer bulbs, which are planted in early spring and will flower in the summer. In northern regions, spring bulbs will survive the winters without any problem and will return each spring. Summer bulbs, however, must be removed from the ground before the onset of severe cold and must be replanted every year.

As for fertilization, spring bulbs (tulips, daffodils, hyacinths, etc.) will appreciate a light application of fertilizer at planting time in the fall, and should then be fertilized every spring when the new shoots appear. They can also be fertilized immediately after flowering, while the foliage is still green, to replenish the bulb's nutrients.

Summer bulbs (dahlias, gladioli, cannas, lilies) can also be fertilized at planting time, if the soil is poor, at the beginning of active growth, or when flower buds begin to form. Remember that bulbs grown in containers will need to be fertilized more frequently.

Recommended products:

• Organic-Based Bone Meal BLOOMBOOST 4-10-0
• Organic-Based Garden Fertilizer 3-6-12
• Organic Garden Fertilizer for Tomatoes, Vegetables & Fruits 4-4-8

When to fertilize perennials?

When planting: Amend the soil in the flower beds with a garden soil containing a slow-release fertilizer, sphagnum peat moss, which helps retain water, and mycorrhizae, which promote the development of the root system.

In season: Fertilize well-established plants early in the season with a time-release fertilizer such as compost or granular fertilizer. During the season, a fast-acting liquid fertilizer is used to reinvigorate plants that are showing signs of deficiency or exhaustion. Never fertilize after the end of July.

Adding a rapidly decomposing mulch also provides nutrients to your plants while discouraging weeds (leave a clear space at the base of the plant to allow it to expand).

Recommended products:

• Organic-Based Bone Meal BLOOMBOOST 4-10-0 (when planting)
• Organic Multi-Purpose Garden Fertilizer 7-3-3
• Organic-Based Multi-Purpose Garden Fertilizer 12-6-6

When to fertilize trees and shrubs?

Trees and shrubs will benefit from fertilizer at planting time, followed by an annual application of slow-release fertilizer or compost. A fast-decomposing mulch should also be applied around their base. Reapplied annually, it will protect the roots from extreme temperatures, help retain moisture and enrich the soil.

Well-established trees absorb some of the fertilizer from the lawn and surrounding flower beds. If necessary, trees can be fertilized in the spring, just before bud break, or sometimes in the event of damage, in the fall when the leaves change colour.

Applying compost every 3 to 5 years or a rapidly decomposing mulch is usually sufficient. Shrubs that are damaged or have undergone rejuvenation pruning will benefit from an application of fertilizer, ideally before bud break.

Recommended products:

• Organic Multi-Purpose Garden Fertilizer 7-3-3
• Organic-Based Multi-Purpose Garden Fertilizer 12-6-6

Tip: Avoid applying blood meal before and during flowering if you want flowers or fruits. Also avoid using it at the end of the season to prevent encouraging the growth of new shoots that won't have time to strengthen before the arrival of cold weather.

When to fertilize rose bushes?

When planting: Amend the soil with a garden mix containing a slow-release fertilizer, sphagnum peat moss, which helps retain water, and mycorrhizae. In the year of planting, fertilize after the first flowering.

In subsequent years

• Organic-Based Bone Meal BLOOMBOOST 4-10-0 (when planting)

In May, use a nitrogen-rich formula such as our Organic-Based Multi-Purpose Garden Fertilizer 12-6-6. In early July, use our Organic Multi-Purpose Garden Fertilizer 7-3-3.

For more information, read How to plant roses.

How to fertilize the vegetable garden?

When planting: For your vegetable plants, choose a quality organic potting soil containing sphagnum peat moss, organic fertilizer and mycorrhizae.

In season: For an optimal harvest, apply an organic-based garden fertilizer every 3 to 4 weeks when growing in the ground, or every two weeks when growing in containers.

Recommended products:

• Organic-Based Bone Meal BLOOMBOOST 4-10-0 (when planting) 
• Organic-Based Garden Fertilizer for Tomatoes, Vegetables & Fruits 3-6-12
• Organic Multi-Purpose Garden Fertilizer 7-3-3
• Organic-Based Multi-Purpose Garden Fertilizer 12-6-6

For more information, read 12 tips for a more productive vegetable garden.

What you need to know about fertilization 

Avoid fertilizing: 

• Immediately after planting or transplanting (except with a root stimulator like ROOT BOOSTER).
• When the soil is very dry; water first!
• During periods of extreme heat; temporarily stop fertilizing and resume when temperatures return to normal.
• When plants are wilted, stressed or diseased; address the problem first, then resume fertilizing when the plant shows signs of active growth.
• When the foliage is already yellowing and drying out, especially for spring bulbs.
• In the fall in northern regions for all perennials, except for spring bulbs, which can benefit from a light application of fertilizer at planting time. 

What type of fertilizer to choose: 

• A balanced fertilizer or a nitrogen fertilizer at the beginning of the season or when you want to promote foliage growth (excess nitrogen can hinder flower and fruit development).
• A fertilizer richer in phosphorus and potassium when you want to encourage flowering and fruiting for all types of plants, including vegetables.
• Well-rotted compost for a light soil amendment.
• Choose a liquid or water-soluble fertilizer for a quick effect.
• Choose a granular fertilizer for a slower effect. 

Good fertilization practices to adopt: 

• Apply fertilizer to moist soil.
• Ideally, fertilize early in the morning or late in the day.
• Always water after fertilizing.
• Avoid direct contact between the fertilizer and roots, stems, flowers or leaves, even when watering.
• When it comes to fertilization, it's better to underfertilize than overfertilize. Always follow the manufacturer's recommendations. 

To make sure you don't forget anything, click here to view, download or print our reference sheets!