The beaver is the largest rodent in North America. In Ontario, they have been known to eat almost every tree and shrub species available but they prefer poplars and willows.

How they harm trees

  • Girdle and kill trees by eating the bark.

What you can do

Beaver dams help create wetland and reduce erosion. They also benefit rivers and streams and the fish population. These good aspects make their presence desirable despite the damage they do.

Reduce beaver activity by:

  • wrapping wire mesh around the base of the tree; remember to loosen the mesh every year to prevent the mesh from girdling the tree
  • spray tree trunks with products that beavers don’t like to taste or smell -these can be found at your home and garden centres
  • choose to plant trees that beavers don’t care for, like evergreens and hardwood

Beavers like running water. Consult professionals to construct culverts to reduce the water flow.

Trapping is not effective. No matter how many are trapped and removed, others will take their place.

Bees are important pollinators and honey producers. Wasps and hornets are considered beneficial insects because they feed on a large number of insect pests.

How they harm trees

Bees, wasps and hornets nest in trees but don’t typically damage the tree.

What you can do

Bees, wasps and hornets are important for the environment.

If you see a nest on a City-owned tree, call 311. Don’t disturb the nest! The City will investigate. Unless the nest threatens public safety, the nest won’t be removed.

If you see a nest on your own property, don’t disturb the nest! Contact a professional to help with removal.

Keep bees, wasps and hornets away by:

  • removing food littered on the ground
  • making sure garbage cans are tightly covered
  • ensuring you don’t have standing water, like ponds or birdbaths

Galls are unusually-shaped formations that grow on plants including trees. Galls are caused by bacteria, fungi, mites and more. Wildlife like birds and butterflies can help control the spread of galls. Plant flowers and other plants that attract them.

How they harm trees

Most galls don’t harm trees and are only cosmetic.

Black knot of cherry is destructive and can kill a tree.

What you can do

Galls should be tolerated since they don’t cause problems for the tree.

Branches can be pruned:

  • make branch cuts far away from the gall
  • disinfect tools between each cut
  • make sure pruning won’t affect tree health

Girdling roots are roots that grow around other roots or around the main stem of a tree.

How this harms trees

Girdling roots restrict the movement of water and nutrients within a tree.

What you can do

  • If you don’t have a lot of space to plant, choose plant species that are less likely to have girdled roots.
  • When planting, inspect the root ball and straighten out any curly roots.
  • When planting, prune out small curly roots – this damages the tree but is necessary for the overall health of the tree which may die otherwise.
  • After four to six years, prune out visible girdling roots – this damages the tree but is necessary for the overall health of the tree which may die otherwise.
  • Don’t pile too much mulch around the tree since this reduces the oxygen the roots get. Roots will grow around one another trying to get more oxygen.

Species that commonly have girdling roots:

  • Norway maple
  • Sugar maple
  • Red maple
  • Green ash
  • Honey locust
  • Littleleaf linden

Mechanical injuries are one of the most common causes of damage to trees in an urban environment. Even the smallest of damage can allow pests and diseases to enter the tree. Trees can be damaged by::

  • lawn mowers
  • vandalism
  • construction
  • deep planting
  • injections of substances
  • soil compaction
  • improper mulching (e.g. “volcano” mulching)
  • choking
  • flowerbeds
  • improper pruning

De-icing salt used in winter to provide safe road and walking conditions can cause significant damage to trees and other vegetation.

How salt harms trees

  • Directly, when airborne spray damages the leaves, needles or branches.
  • Indirectly, when the roots absorb too salt from the water in the soil.

Some tree species are very sensitive to salt:

  • sugar maple
  • red maple
  • white pine
  • oaks
  • lindens
  • basswood
  • crab apple
  • cedar
  • Scotch pine
  • dogwoods

What you can do

  • Use calcium chloride or calcium magnesium acetate as a de-icing salt, rather than sodium chloride.
  • Reduce the amount of de-icing salt by mixing in other abrasives like sawdust, sand or ash.
  • Remove snow by shovelling instead of using de-icing salt.
  • Plant trees away from areas that are salted.

You can also choose trees that are tolerant of salt:

  • Ohio buckeye
  • horse chestnut
  • silver maple
  • Austrian pine
  • black and honey locust
  • Colorado spruce
  • white ash
  • birches
  • cottonwood
  • staghorn and smooth sumac
  • yew

Water is vital to trees. Without water, a tree cannot maintain its physiological functions and will die.

How water affects trees

Too little water:

  • leaves might wilt, droop or turn yellow or brown
  • leaves might brown at the edges
  • the tree will become vulnerable to pests and diseases

Too much water:

  • roots can rot and prevent growth
  • soil can erode and the tree could fall

What you can do

  • Water trees – usually once every seven to 10 days.
  • Young and newly planted trees need more watering until their roots are established.
  • Some soils hold water better than others:
    • clay soil holds water longer, so water trees slowly and less often
    • sandy soil doesn’t hold water very long, so water more often
  • Water during the early morning.
  • Don’t water the leaves unless the species specifically requires it.
  • Place organic mulch in a ring 7 – 10 cm deep around trees to help preserve water. Keep mulch away from the bark, since it can cause decay.

Wood decay is a process of wood disintegration that is caused by fungi or other micro-organisms.

How it harms trees

Wood decay affects the structure of a tree and can kill it. Trees with decay could drop branches or the whole tree might fall.

In an urban environment, this can cause damage to property or to members of the public.

What you can do

  • If you notice splits, cracks, increase leaning of a tree, or fungi/mushrooms on the trunk:
    • City-owned trees, call 311
    • private trees, call a tree expert
  • Don’t fill cavities since this can make decay worse and hide it.
  • Don’t put paint on cuts/wounds since it can make decay worse.
  • Avoid mechanical damage.
  • Don’t prune unless necessary.
  • If you see vandalism on a City-owned tree, call 311.
  • Call a professional if you need advice.