A few weeks ago I saw an article titled “Don’t Plant These Trees in Your Urban Yard“. Before I’d even read it, I guessed one tree that would be on the list: the cottonwood (Populus sp.).
We have two cottonwoods in the backyard, and every year – spring in particular – I wonder what possessed the developers of our mid-70s neighbourhood to plant them. While I enjoy the shade they provide on those 30°+ summer days, it doesn’t offset the many things I dislike about this ubiquitous Prairie tree.
Cottonwoods are a moisture-loving tree, normally found on floodplains and riverbanks. They crave water – and will extend their root systems widely to find it. Our yard is crisscrossed with cottonwood roots – some 10-15 cm in diameter and just under the ground surface, others mere tendrils working their way into the raised garden beds from the bottom up. They’ve stretched perilously close to the foundation of the house, at a corner where water used to collect. Once they find that moisture, they grow prolifically! In the 6 years we’ve lived here, our yard has become increasingly shaded and the veggie garden has not been impressed.
Cottonwoods have thick bark that was meant to protect them from prairie grass fires. They also produce a lot of debris. Windstorms (which we get a lot of down here) easily break small and medium branches out of the trees. In early spring we rake up piles of dead branches, only to be inundated again in late spring as the May/June rainstorms pummel new leafy branches out of the treetops. One of our trees is infested with hard, dark brown galls caused by mites. The round, knobby growths on almost every joint are reminiscent of the inflammation of rheumatoid arthritis. The dogs, however, love them, and there are so many to choose from all scattered around the yard.
When the warmth of spring arrives, the cottonwood buds open and exude a substance stickier than superglue, complete with a bright yellow resin. These sticky buds carpet the lawn (and the dogs, the soles of your shoes, the deck…), leaving a nasty stain in their wake. If you have arthritis, you can make ‘Balm of Gilead’ from the ripe buds – a cooked mix of olive oil and yellow resin to be rubbed into the joints.
Cottonwoods also produce a rooting hormone called auxin, meant to help them colonize new locations via wind or water-borne branches, or where the roots have extended some distance from the main tree. Cottonwood suckers pop up throughout our lawn and in the perennial beds, trying desperately to create a cottonwood grove. Catkin buds fallen from the tree try to root in the gravel around the raised garden beds, looking in vain for some decent soil.
Although cottonwoods make terrible neighbours in suburbia, they are a key species in their native floodplain habitat.
Cottonwoods are like the building blocks of mature floodplains, and are a key component of primary succession in these landscapes. Cottonwood establishment anchors floodplain sediments and can affect river flow paths by reducing the erodibility of the river banks. They’re also well-suited to environments that flood periodically – rather than being wiped out by a natural disaster, they thrive and keep the disaster from worsening by maintaining floodplain stability.
The trouble is, changes in river flow due to dam-related regulation and withdrawal uses such as irrigation has left cottonwood communities in a precarious position along many river systems. A comprehensive approach to water management that includes the water needs of riparian vegetation and aquatic organisms will be important in maintaining and restoring cottonwoods in the floodplain (not suburban!) landscape where they belong.
So out with the cottonwoods – at least in the backyard. Let’s make sure they thrive where they belong – along the river.