The Trees of North York and What Goes Wrong.

Maple

Maples dominate North York, and most of them are Norway maple rather than the sugar maple on the flag. Norway maple casts shade so dense that grass gives up beneath it, seeds itself into every ravine, and is listed as invasive in Ontario. It also has a habit of circling its own trunk with surface roots.

Silver maple, common on post-war streets, grows fast and holds weak unions. It is the species we cable most often, and the one that drops limbs in wet snow.

Two maple problems look worse than they are. Tar spot leaves black coin-sized marks across the leaves and costs the tree nothing. Sap running from a late-winter pruning cut is root pressure, not injury. Verticillium wilt is the one that matters: whole branches wilt and die while the rest of the tree looks fine, and there is no cure once it is in the vascular system.

Oak

Oaks are the heavyweights on the older lots in York Mills and Don Mills, and they are the trees with a calendar. Oak wilt was confirmed in Ontario in 2023, first in Niagara Falls and then in Springwater. It spreads through beetles drawn to fresh cuts and through roots grafted between neighbouring oaks.

Toronto asks that oaks not be pruned between April 1 and October 31 for that reason. Red, black and pin oaks die fastest once infected. White, bur and swamp oaks hold out longer. Do not move oak firewood, which is one of the main ways the disease travels between regions.

Otherwise oaks are the most durable large tree in the area. They are worth the cabling, the reduction and the root care that keeps them standing.

Ash

Emerald ash borer has taken most of the untreated ash in the GTA. The larvae feed under the bark on the tissue that moves sugar through the tree, and by the time the canopy thins the damage is done.

The three signs to look for are woodpeckers working high on the trunk, blonde patches where bark has flaked away, and shoots sprouting from the trunk itself. Any two together means a diagnosis now.

A healthy ash with a full canopy can be protected with TreeAzin, injected every two years by a licensed applicator, indefinitely. A tree past about a third dead is not worth treating, and dead ash turns brittle fast, which makes it more dangerous and more expensive to take down the longer it stands.

Spruce, pine and cedar

Blue spruce browns from the bottom up here, and the usual cause is rhizosphaera needle cast, a fungus that takes the older inner needles first. It responds to fungicide timed to new growth over a few seasons, along with pruning for airflow.

Cedar hedges have one hard rule: they carry no dormant buds in bare brown wood, so anything cut past the green stays bare permanently. That single fact separates a hedge that can be brought back over several seasons from one that has to be replanted.

Both suffer at driveway edges, where salt spray browns the road-facing side while the sheltered side stays green. That one-sided pattern is the giveaway that the problem is salt rather than disease.

Birch, elm and the rest

White birch dies from the top down, and bronze birch borer is usually why. The borer takes stressed trees first, so water and mulch through a dry August do more to save a birch than anything applied to the canopy.

The mature elms that survived Dutch elm disease are worth protecting. The fungus still moves through beetles and through root grafts between neighbouring elms, so elms get pruned in the cold months and monitored in summer for a single flagging branch.

Honey locust, linden and ginkgo turn up as street plantings because they tolerate salt and compaction. Linden drops honeydew from aphids onto whatever is parked underneath, which is a nuisance rather than a health problem.

The lesson from the ash

Streets planted with one species lost their canopy in a decade when the borer arrived. The cheapest insurance available to a homeowner is choosing something different for the second tree in the yard, and something suited to the salt, the compaction and the space it actually has.