When to prune trees in Ontario, month by month

The short version

Most deciduous trees: late autumn through winter into very early spring. The structure is visible with no leaves in the way, the tree is dormant and not spending stored energy, disease and insect pressure is at its lowest, and the ground is firm so your garden takes less of a beating.

Oak: dormant season only. Oak wilt is a fatal fungal disease spread partly by beetles attracted to fresh wounds in the growing season. Do not prune oak in spring or summer if you can possibly avoid it.

Elm: dormant season only, for the same reasoning around Dutch elm disease and the beetles that carry it.

Spring-flowering trees and shrubs: right after they finish flowering. Lilac, forsythia, magnolia, serviceberry, weigela, mock orange. They set next year's flower buds during the summer, so cutting them in winter removes the display you were waiting for.

Summer-flowering shrubs: late winter, before growth starts, because they flower on the current season's wood.

Birch and maple: late summer if you mind the bleeding. Cut in late winter and they run sap heavily. It looks alarming and it is largely harmless to a healthy tree, but if you would rather not see it, wait.

Dead, broken, hanging or hazardous material: immediately, any month. Season is irrelevant once something is already a risk.

Month by month

November to March. The main window. Everything structural on maple, ash, honey locust, linden, hornbeam, catalpa, horse chestnut and the rest. All oak and elm work. Crown thinning, reduction, deadwood, clearance, and structural pruning on young trees. Best visibility of the year and the least stress on the tree.

Late March and April. Tailing off for maple and birch because of sap flow, though it does no lasting harm. Still fine for most other deciduous work before the leaves are fully out. Start watching hedges and shrubs for nesting birds.

May and June. Prune spring bloomers as soon as they have finished flowering. Avoid oak and elm entirely. Nesting season is in full swing, so hedges and dense shrubs get checked before anything is cut. First flush of growth on cedar hardens off toward the end of this period, which is one of the two good windows for hedge work.

July and August. Fine for light corrective work and for anything urgent. A good time for a genuinely overgrown cedar hedge, once the heat breaks. Avoid heavy pruning in a drought or a heatwave: a tree that has just lost foliage cannot cope well with the water loss.

September and October. Late summer and early autumn is the second good hedge window and a sensible time for birch and maple if you want to avoid sap bleeding. Avoid heavy pruning very late in autumn, which pushes tender new growth straight into frost. Good time to plan winter work and to deal with anything you noticed over the summer.

The nesting rule, which is not optional

In Ontario, migratory birds, their nests and their eggs are legally protected. The main nesting season runs roughly April through August, and it applies to hedges and dense shrubs at least as much as to trees.

Practically, that means the hedge gets checked before anything is cut. If there is an active nest, that section does not get touched until the young have fledged, and the rest can be done with a return visit for the remainder.

This is worth knowing as a customer, because it is a reasonable question to ask a contractor. Anyone happy to run a hedge trimmer through a nesting cedar in June is telling you something.

How much, and how often

Timing matters less than restraint. The most common way to damage a tree with pruning is to take too much at once.

On a mature tree, removing a large share of the living crown in one go is a starvation event, and it triggers a flush of weak regrowth in response. If a neglected tree needs a lot of work, staging it over two or three visits a few years apart gets you a better tree than doing it all in one afternoon.

Young trees are the opposite: that is where a small amount of work pays for itself many times over. Ten minutes choosing which of two competing leaders to keep on a five-year-old tree prevents the split trunk that becomes a serious problem in thirty. It is the cheapest tree work anyone ever buys.

For a mature tree in reasonable shape, a look every three to five years is usually plenty, mostly to clear deadwood and deal with anything rubbing or badly attached. More often than that and you are generally cutting for the sake of cutting. Details of what each kind of pruning is actually for are on the pruning page.

Two things not to do, whatever the month

Do not top it. Cutting leaders and limbs back to stubs to reduce height makes a tree more dangerous within fifteen years, not less, because it regrows weakly attached shoots off wounds it cannot seal. There is a whole piece on why topping is so damaging.

Do not paint the wounds. Wound dressings, tar and sealants do not help a tree close a cut and can hold moisture against it. A well-placed cut on a healthy tree is best left alone. The tree will compartmentalise it itself, which is what it evolved to do.