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Fertilizing trees, shrubs and perennials

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Staghorn sumac (Rhus hirta 'Bailtiger')
Photo: Jardin botanique de Montréal (Michel Tremblay)

Fertilizing trees

As a rule, well-established trees do not need to be fertilized, because they absorb some of the fertilizers applied to lawns and flower beds nearby. Trees damaged by extreme weather, insect pests or disease are an exception to this rule. They should be fertilized the year following the damage with a high-nitrogen fertilizer (2-1-1 ratio). Apply the fertilizer in spring, just before the buds open. If you decide to fertilize them in the year of the damage, use a fertilizer that is higher in potassium than in nitrogen or phosphorous (1-2-3 ratio). In that case, apply the fertilizer in fall, when leaves change color.

N.B.: Some firms offer tree-fertilizing service. They apply the fertilizers by injecting them in the soil, spraying the foliage or implanting them directly in the trunk of the tree. This service may be useful for seriously damaged or diseased trees.

Fertilizing shrubs

Like trees, well-established shrubs do not need to be fertilized, because they absorb some of the fertilizers applied to lawns and flower beds nearby. Compost or composted manure applied once every three to five years is enough to meet their needs. When they are grown in suitably rich soil, a fast-decomposing mulch is sufficient.

Shrubs damaged by extreme weather, insect pests or disease and those that are pruned regularly will benefit from a high-nitrogen fertilizer (2-1-1 ratio) in spring, just before the buds open. You can also fertilize them in fall, with a fertilizer that is higher in potassium than in nitrogen or phosphorous (1-2-3 ratio). This type of fertilizer should be applied in the year of the damage or the following year.

Fertilizing perennials

Most perennials are happy with one application of compost or composted manure every two to three years. When they are grown in suitably rich soil, a fast-decomposing mulch is sufficient.

To meet the needs of particularly demanding perennials, you can spread compost or composted manure around them once a year or use a natural slow-release fertilizer.

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