Protecting the base of trees

Hey there, time traveller!
This article was published 31/07/2013 (4630 days ago), so information in it may no longer be current.

Our city boasts one of the largest urban forests in North America. Look to the canopy and you’ll find an amazing variety of species.

Species diversity is good for visual interest, wildlife, and pest resilience. Look to the bases of these trees and you’ll find an equally amazing variety of trunk-to-ground situations. Few of them are good for anything, particularly the tree.

If a tree has achieved a height of 15 to 20 metres without tinkering at its base, it probably doesn’t need any assistance at this point. In fact, one of the surest ways of killing a tree is by changing the soil level at the base of the trunk. It’s a slow, painful death.

It’s easy to take a tree for granted. For one thing, it’s big and solid. Surely sturdy enough to roll with some changes. Wrong.

Trees growing in cities are already under considerable stress. Exhaust from vehicles is tough on them. Streets and sidewalks restrict their roots. Most trees like to stretch their roots outward just below the soil surface. Nothing like a strip of compacted gravel covered in pavement to stop that spreading root action. And our lawns and boulevards drain water quickly and efficiently onto the streets and away from the tree’s thirsty roots.

Trees, though, can adapt to adverse conditions — our urban forest is a testament to that. So how about we help them by forgoing the trunk-to-ground improvements?

Piling soil against the trunk of the tree is a very common scenario. Often done in an effort to tidy up, or simply as a place to deposit excess soil, it’s harmful to the tree. Even building up as little as five centimetres against the trunk is bad news.
Piling mulch against the trunk is just as harmful. Give it a five-centimetre berth from the trunk of the tree.

Sometimes a garden makeover involves raising the ground adjacent to a tree. Plan to keep grade changes as far away from the trunk as possible. The drip line is ideal. If the changes have to be close to the trunk, a constructed well with drainage is needed. The well can be filled with small boulders, which allow for good air circulation.

Our trees are precious. Keep them happy and healthy by protecting the existing ground-to-trunk connection.

Carla Keast has a master’s degree in landscape architecture and is a Winnipeg-based freelance landscape designer. She can be reached at contact@carlakeast.com