How flooding affects trees: Water in moderation

Tree Canada


Tornadoes, wildfires and flooding – extreme weather events exaggerated by climate change – have recently touched and continue to touch countless communities across our country with devastating effects. Having personally experienced the aftermath of the tornadoes last fall and the effects of flooding this spring, I think we can say that climate change is not coming, it is officially here. It affects us all greatly, and it affects the trees – at a time when we need them more than ever – as you’ll learn below.
– Michael Rosen, President, Tree Canada

As a teenager, my son had a saying, whether original or borrowed I don’t know (the saying, that is), which went something like “All things in moderation. Especially moderation.” It would seem Mother Nature took that to heart, and dispensed with moderate rainfall and snow melt this spring. If not her, then maybe it was Creepy Uncle Climate Change. At any rate, the resultant flooding has been heartbreaking to observe.

While I am of course sensitive to the anguish of those people affected by the record-high waters, as an arborist I cannot help but think about the suffering trees as well.
Flood water impacts trees in many ways, one of which would be literal impacts, such as when objects entrained in flowing water scrape against tree trunks. That kind of injury is obvious, as well as relatively uncommon and typically not too severe. What really harms trees is a shortage of oxygen in flooded soils.

Soil pores are what allow oxygen to passively reach tree roots. This is the main reason tree roots are so shallow: 90% in the top 25 cm and 98% in the top 45 cm. It is also why adding fill to raise the grade over a tree’s root zone causes stress, and often leads to the tree’s decline starting two to five years later. Very few tree species are adapted to extreme low oxygen conditions.

Many of us have seen photos of the semi-tropical bald cypress happily growing in swamps. Bald cypress has evolved structures called pneumatophores, which enable them to channel air to their roots so they don’t suffocate. Our trees have no such adaptations however and can’t hold their breath for long.

The extent of root damage wrought by flooding depends on many factors. If it is still the dormant season, plants can forgo oxygen longer. Soils are cool and root respiration rates are commensurately low. Severity of flood damage also depends on a tree’s health before the event.

Soil type matters. If a site is sandy, it will drain faster once the water recedes, as compared to a heavy soil. Sand also naturally allows oxygen in more easily. Trees on clay or silt soils will be more acutely stressed.

The length of time roots are under water is critical as well. Two or three days may not cause undue harm, but if it goes a week or more, most species will suffer grave injury. In part, flood tolerance depends on genetics – some species can handle being inundated better than others.

In cases of a week or more of flooding, trees like red maple (Acer rubrum) and silver maple (A. saccharinum) fare better than sugar maple (A. saccharum), for example. River birch (Betula nigra) will suffer less than paper birch (B. papyrifera). Pin oak (Quercus palustris) can handle sodic conditions much better than red oak (Q. rubra). Eastern cottonwood (Populus deltoides) is another tree that can hold its water. Black tupelo, also called black or sour gum (Nyssa sylvatica) is fine with a couple weeks of water-soaked roots. Willows (Salix spp.), American larch (Larix laricina), and northern catalpa (Catalpa speciosa) are other flood-tolerant trees.

Hickories (Carya spp.), black locust (Robinia pseudoacacia), black walnut (Juglans nigra), eastern redbud (Cercis canadensis), Colorado spruce (Picea pungens), as well as all fruit trees, are more likely to come to harm when surrounded by water for a week.

Symptoms of flood stress include chlorotic, wilting, undersize, or curling leaves, a sparse crown, early fall color (as compared to others of its species), and branch-tip dieback. Depending on all the factors discussed above, such symptoms may occur the first season, or they may take several years to manifest.

After things dry up a bit, most people affected by this year’s flood will understandably be quite busy with more pressing things. When the time comes to think about the trees, one of the more important ways you can help is to do no harm – a very important point! Do not park, drive, or stage materials within the root zone (two times the branch length). After having been submerged, a tree’s root zone is vulnerable even to modest activity and such conditions can destroy soil structure and compound tree stress exponentially.

Consider hiring an ISA Certified Arborist to assess your tree(s) and to also potentially aerate the root zone through pneumatic soil fracturing, vertical mulching, or other treatments.

I wish all the best to those suffering from high waters. May you have a more moderate summer.


Back to all articles