They’re popular, but poplars not suited to the city

Hey there, time traveller!
This article was published 08/06/2003 (7236 days ago), so information in it may no longer be current.

I am constantly amazed why urban property owners put up with the inconveniences, nuisances and potential hazards of having one or more large poplar or willow trees near their houses on small lots. I have spent my professional life observing and studying trees in Canada and around the world. Only on the Prairies is the notion that the positive features of a large shade tree overshadow its negative traits.

Now I do have a fondness for very large trees, and will go out of my way to find them, but I draw the line when my professional assessment puts “large tree” and “hazardous” in the same sentence. I recently advised a property owner to remove a 20-metre-high northwest poplar tree. It had a number of large vertical cracks in the trunk, including a wide lightning crack on one side of the trunk leading to a hole at the base.

The ground around this tree near the base of the split was also cracking up towards the house. The owner was justifiably concerned as the tree had quite a sway in the wind. I have a device called a resistograph that is able to give me a graphical print-out of the condition of the wood inside trees: solid, checked, decayed, open cavity or a combination of these features. This 76-centimetre-diameter tree had a large hollow core with remnants of decayed heartwood, and its only means of support was a doughnut-like ring 12 cm thick just inside the bark. This tree was extremely hazardous.

Most planted poplars and willows around homes and parks in towns and cities in southern Manitoba are types of hybrid poplars and willows; that is, they have been developed as a cross between two or more species or varieties of poplar or willow trees.

Poplars and willows have three common features:

* they are both in the willow family.

* they both grow very quickly on the Prairies.

* they both develop many structural and disease problems as they get older much earlier than any other kind of tree.

Over the course of my 33 years as an urban forester and arborist, I have received more complaints about these two types of trees than any other. It is very common for people to refer to the hybrid poplars as cottonwoods owing to the cotton-like fluff they produce.

The magnificent native cottonwood is by far the largest and most massive of all Manitoba trees, and unfortunately a number of them do find their way on to small urban yards. The varieties of poplars and willows that have been available over the last 60 years were developed for farm shelterbelts, not for urban yards. However, their rapid growth characteristic made them a natural choice as shade trees for the hot prairie summer. These trees quickly become decadent within 15 to 20 years of planting. In Winnipeg, there are many 50- to 60-year-old poplars and willows that are dropping large limbs or have become so badly decayed that the trees topple over.

How do these trees get so decadent at an early age? In compacted urban clay soils, poplar and willow tree roots tend to grow just below or at the surface of the soil. They have a tremendous need for water so the roots come near the surface to take advantage of any rainfall or irrigation. When these trees are young, their bark is quite thin. Casual lawnmower impacts can open the bark and establish entry points for various diseases that infect these species. Wood-rot disease is a major disease in poplars and willows. In addition, the roots that show up at the surface get damaged from lawnmower activity. Openings or wounds in the surface roots are a major source of root decay. Thin bark in poplars also makes them susceptible to early winter frost. Frost boils on the trunk were once a very common feature in many varieties of poplar trees.

Trees have a survival mechanism to limit the spread of wood decay into healthy wood. A special wall of cells compartmentalizes the decay; that is, the cells seal off the decay area from the healthy area. If the tree is continually wounded, wood decay will enter the once protected healthy area of wood. Repeated damage, especially to the roots and the lower trunk, will ultimately result in almost complete wood decay of all wood inside the tree trunk.

Structurally, the tree has virtually nothing to hold it up, and it will collapse. Remarkably, I have seen large poplars support themselves with only a half inch of solid wood inside their bark. Eventually they collapse. Prior to this collapse, the tree will let its owner know it is under severe growing stress. Trees that have been stressed by root and lower trunk wood decay will start to produce smaller and fewer leaves that lose their dark green colour, especially near the top of the tree’s crown. With time, the crown has a distinct thinned-out look as leaf density is dramatically reduced. Portions of the branches in the crown start to die, leaving very noticeable dead branches. Many poplars often collapse before the branches show signs of death. Willows do the same thing as poplars but they tend to drop large branches rather than having the whole tree collapse.

There is a current trend, at least in new Winnipeg neighbourhoods, in planting narrow upright poplars such as ‘Tower’ and ‘Prairie Sky’ poplars in small yards near the property boundary. Often, the plantings are only a few metres apart. The competition between these trees will also stress them as they age. In 30 to 40 years, or less, these trees will start dropping very large upright stems. The damage can be considerable. Rows of these upright poplars are sometimes planted fairly close to swimming pools. Both poplar and willow roots will aggressively seek out the moisture around a pool and in weeping tiles around houses, especially during hot, dry weather. In Winnipeg, poplars have a reputation as being the bad neighbour tree. In my 14 years as the city forester in Winnipeg, I received hundreds of complaints about privately owned poplar trees affecting neighbouring yards.

If you are an owner of a poplar or willow tree growing in a typical urban yard, you might want to think about the value that tree has for you and your neighbours.

Mike Allen, a consulting urban forester and certified arborist, owns and operates Viburnum Tree Experts. He can be contacted for consultation by calling 831-6503 or by e-mail at viburnumtrees@shaw.ca You can also mail questions to Mike Allen c/o Newsroom, Winnipeg Free Press, 1355 Mountain Ave., Winnipeg, Manitoba, R2X 3B6.