Bonsai root systems for city trees

We see pictures of trees falling over during wind events and assume this is natural but look at the pictures and you will see large canopies and trunks with very small and shallow root systems. Of course, they blew over—the leverage on the canopy was transferred to the unnaturally small root system.  

So how is that a tree can get to be so large but have so few roots? How can it support all those leaves— especially during the heat of summer and the cold of winter. Look to the north of Canada into the boreal forest and you will see tall but narrow conifers which grow slowly in almost impossible conditions. In a city a tree will try to do the same, especially if it can find water and it has aerated soil. It will grow slowly but will try to survive. 

Bonsai is an example of how we learnt to grow trees with tiny root systems by supplying all they needed and pruning them to keep the roots and top growth in balance. The Chinese royalty wanted to be able to transport their gardens from palaces and homes to other places and so created this skill many hundreds of years ago. By observing nature, we see this is an inherent ‘skill’ that most trees have—they adapt to the growing circumstances they find themselves in when they first put out roots.  

On the cliff face of the Niagara escarpment are White cedars, that normally have a life expectancy of between one hundred and two hundred years, but it has been shown, through cores taken from the center of those trees, that many are twelve hundred years old. They grow very slowly, as moisture is only available in early spring, which means a very short growth period and then they quickly shut down growth to get through the remainder of the year. But these trees do not fall during strong winds because their root systems are deeply anchored into the rock face. 

When a tree grows from seed, it first puts down roots—deep roots and shallow roots—each having a different priority to meet. Unfortunately, once the first deep roots are stopped from growing deeply, they ‘bonsai’ and instead grow into the available space, which for city trees typically planted in pits isn’t very much. This means that as a city tree gets taller, its roots are not able to grow deeper and so the roots can't counterbalance the leverage forces of strong winds, particularly when the winds swirl or come from unfamiliar directions.  

The public sees these images of large trees laying on houses and across streets and are left with the idea that, because they are large trees, their root systems are normally shallow. They are shallow because of the limited growing space and because the roots were pruned in the nursery before being planted in the city site. This root pruning is creating a Bonsai effect for the tree which therefore adapts to that ‘message’.  

If we want large trees, it is essential to plant young trees whose root systems have not been trained to stay small by root pruning and confined to grow into small spaces in pits. When we think of normal root systems as being small then of course city trees will blow over in strong winds and have shortened lives. Healthy roots, healthy trees. Healthy trees, healthy cities.